.GN6TISM RNIA. 'SAN DIEGO l> UNIVERSITY OF CALIFORNIA. SAN ut III Illllllllllllllllllllllllllll III I I I I II 1; 3 "l 822 00559 6341 BY THOMAS DUNMAN ^QYised and Completed by IAPMAN JONES, E 1C. ECS. p WILLIAM E.ECKAHT, WILLIAM E.ECKART, LINAR WILLIAM E.ECKART. A SHORT TEXT BOOK OF ELECTRICITY AND MAGNETISM. BY THOMAS DUNMAN. REVISED AND COMPLETED BY CHAPMAN JONES, F.I.C., F.C.S., Member of the Physical Society of London, &c. With 165 EXPLANATORY ENGRAVINGS and DIAGRAMS. SECOND EDITION. WARD, LOCK AND CO., LONDON, NEW YORK, AND MELBOURNE. 1889. PREFACE. THE subject matter of this volume appeared originally as a series of Chapters in the "Universal Instructor." Before, however, its issue was completed, the lamented death of the author rendered it necessary for the work to he carried on by other hands. In preparing the Chapters for publication in a separate form, a few changes have been made to render it more convenient for the student and more suitable as a text book. The work has been somewhat enlarged to fit it as a guide to candidates for such examinations as those of the Department of Science and Art, but no attempt has been made to restrict the subjects dealt with to those enumerated in the syllabus of any examin- ing body. The Chapters on " Recent Applications " were written and published when Electric Lighting and allied subjects were en- gaging the attention, not only of experts, but of the general public, hence this part has a greater prominence than it would appear to rnerji at the present time. CONTENTS. CHAPTER I. PEF.LIMINARY EXPERIMENTS IN FRICTIONAL ELECTRICITY .. 1 CHAPTER II. THE GOLD LEAF ELECTROSCOPE CHAPTER III. INDUCTION BY FRICTIONAL ELECTRICITY 18 CHAPTER IV. DISTRIBUTION OF FRICTIONAL ELECTRICITY 18 CHAPTER V. ELECTRICAL MACHINES 23 CHAPTER VI. CONDENSATION OF FRICTIONAL ELECTRICITY 83 CHAPTER VII. SPECIFIC INDUCTIVE CAPACITY 46 CHAPTER VIII. ATMOSPHERIC ELECTRICITY 94 CHAPTER IX. ELECTROMETERS 52 CHAPTER X. ELECTRICITY OBTAINED BY CHEMICAL ACTION ... ... ... 67 CHAPTER XI. GALVANIC BATTERIES AND ELECTROLYSIS 70 CHAPTER XII. MAGNETISM . . ... 89 CHAPTER XIII. ESTIMATION OF ELECTRIC CURRENTS AND RESISTANCES ... 110 CHAPTER XIV. ACTIONS OF CURRENTS ON EACH OTHER AND BETWEEN MAG- NETS AND CURRENTS VOLTAIC INDUCTION . 131 Tiii. CONTENTS. PAGE CHAPTER XV. ELECTRO-MAGNETS AND COILS ... ... 155 CHAPTER XVI. VOLTAIC ELECTRICITY (continued) ... ... 163 CHAPTER XVII. VOLTAIC ELECTRICITY (continued) 169 CHAPTER XVIII. VOLTAIC ELECTRICITY (continued) 174 CHAPTER XIX. VOLTAIC ELECTRICITY (continued) 180 CHAPTER XX. RECENT APPLICATIONS 184 CHAPTER XXI. RECENT APPLICATIONS (continued) 189 CHAPTER XXII. RECENT APPLICATIONS (continued) 194 CHAPTER XXIII. RECENT APPLICATIONS (continued) 199 CHAPTER XXIV. RECENT APPLICATIONS (continued) 205 CHAPTER XXV. RECENT APPLICATIONS (continued) 211 CHAPTER XXVI. RECENT APPLICATIONS (continued) 216 CHAPTER XXVII. RECENT APPLICATIONS (continued) 222 CHAPTER XXVIII. RECENT APPLICATIONS (continued) 227 CHAPTER XXIX. RECENT APPLICATIONS (continued) 235 ELECTRICITY AND MAGNETISM. CHAPTER I. PRELIMINARY EXPERIMENTS IN FRICTIONAL ELECTRICITY. Advantages of a study of electricity The necessary materials Necessity for having apparatus perfectly dry and clean Preliminary experiments Other simple experiments The facts arrived at by the above experiments Origin of the word electricity Early progress of the science All substances not electrically the same Electrics and non-electrics Conductors and non- conductors Relative conductivities of various bodies Insulating substances Action of electrified bodies on each other How to make an electrical pendulum Shellac varnish Experiments with the electrical pendulum Two kinds of electricity Further experiments Vitreous and resinous elec- tricities Use of the terms positive and negative Theoretical explanation of the foregoing facts. ONE of the most important and interesting branches of physical science is undoubtedly that of Electricity a science which has shared, per- haps, more than any other in that great advance in general scientific knowledge which the present century has seen. It is, moreover, a science which specially commends itself, inasmuch as it is one which may be experimentally studied with apparatus, in general, of the most simple and inexpensive kind ; and there are but few, even of its more elaborate experiments, which may not be performed by the in- telligent student at a moderate outlay of money, if only that outlay be coupled with the expenditure of a little patience, tact, and ingenuity. One of the great advantages of a study of natural science, beyond the important fact that it gives us a more comprehensive view of the phenomena of nature than we should otherwise 'possess, is that it brings into play that faculty of per- Advantages of Bonal observation, which is too often allowed to lie electricity dormant, and which no other branch of study calls forth in anything like an equal degree. In such studies as those of history or grammar, we are necessarily taught to accept facts upon authority, and from that authority there can be no appeal; but the student of .physical science is taught to verify, as far as possible, for himself 2 ELECTRICITY AND MAGNETISM. every fact which is brought before him by actual experiment, to test it by the evidence of his own senses, to know it of his own know- ledge. In this spirit we intend conducting our readers through the interesting and pre-eminently experimental science of electricity; and, as we shall give full instructions for the performance of experi- ments and for the construction of the necessary apparatus, we sincerely trust that they will not rest satisfied with merely reading the accounts of the facts and phenomena which it will be our duty to describe, but will endeavour to verify them for them- selves by performing at least the more simple of the experiments by which they may be illustrated. The apparatus necessary to begin with is of such a simple character, that it may be found ready to hand in almost every household. A few pieces of brown paper, a lath, a stick I ne necessary .... . , materials. * sealing-wax, a piece of glass rod, a sheet of foreign letter paper, a clothes-brush, a piece of flannel, a piece of silk, and a piece of india-rubber, these are the only materials with which we must be furnished in order to commence our studies in frictional electricity. <> Supplied with these simple requisites, let us see how by a judicious use of them we may learn a few fundamental facta of interest and importance. Before we begin, however, it will be neces- for having sar y to assure ourselves that our materials are quite dry apparatus and clean ; and it will be as well to state, once for all, perfectly dry ^bat in all our electrical experiments it will be necessary to have our apparatus perfectly dry (in most cases even warm), and perfectly clean, if we would ensure success. We must remember that experiment is the language by which we question nature, and we have no more right to address her in slipshod and careless terms than we have to employ uncouth and ungrammatical speech in our conversation with those around us.. It is therefore essential that we should perform all our experiments carefully and neatly, never being satisfied with just making a thing do, but continu- ing our efforts until we have attained complete success. Being then assured that our materials are in proper condition, let us proceed to make use of them. First it will be necessary to balance the lath in such a manner that it will turn readily upon its centre. The best way to do this is to get an egg, put it with Preliminary -|. g narrow en( j U p wa rds in an egg-cup, and carefully balance the lath upon it. Another plan is to take a Florence oil flask, and place it neck downwards in an empty bottle, and then balance the lath upon the bottom of the flask. Having in either of these ways balanced your lath, now rub your stick of ELECIRIC1TY AND MAGNETISM. 3 sealing-wax briskly with a piece of warm flannel, and bring it near one end of the lath ; the lath will be attracted towards it, and if you are careful you will succeed in making the lath follow the sealing-wax, so as to perform a complete rotation upon its centre. Bring your excited stick of sealing-^ax near some small scraps of paper, feathers or any light substances, and they will be attracted to it. Now take your glass rod, warm and rub it with warm silk, and bring it near the lath ; and, as with the sealing-wax, the lath will be attracted. Any light materials will also be drawn towards the excited glass, as they were towards the excited wax. In just the same manner warm brown paper rubbed with a clothes- brush, or even with the hand, and foreign post paper other gimple rubbed with india-rubber, will be found to exhibit this experiments, power of attracting light substances which are free to move. A very amusing experiment with the excited brown paper is to hold it over the head of a person whose hair has not received too liberal an allowance of pomade ; when it will be seen to attract towards it the individual hairs, which will literally " stand on end, like quills upon the fretful porcupine." If either the excited brown paper, or the foreign paper rubbed briskly with india-rubber, is held near a wall and let go, they will fly to it, and stick there for some minutes before falling. These are but a few of the- instances of simple attraction which we may obtain from our materials, and we might have obtained similar results with other equally simple substances, -rh e facts arrived Enough, however, has been seen to show the truth of at by the above our first statement of fact which is, that certain sub- experiments. stances, uch as sealing-wax rubbed with flannel, and glass ruloed with silk, have the power of attracting light bodies. More than two thousand years ago it was known to the Greeks, that amber when rubbed with a woollen cloth had this mysterious power conferred upon it, and the Greek word for amber being elektron, substances when they possess this power are said to be electrified ; while to this power of at- tracting light bodies, exhibited by amber and other substances when excited, the term electricity has been given ; moreover, as this elec- tricity is produced by friction, it is, to distinguish it from electricity produced in other ways, termed frictional electricity. For more than two thousand years the discovery that amber rubbed with cloth attracts light bodies remained an isolated fact, until about the year 1600 Dr. Gilbert of Early progress Colchester, physician to Queen Elizabeth, showed that the power thus conferred upon amber by friction might also in the 4 ELECTRICITY AND MAGNETISM. same way be conferred upon other bodies, such, as sulphur, glass, and sealing-wax; and it is to Dr. Gilbert that we are indebted for the term electricity. One of the earliest results of electrical research was to show that all substances are not alike with regard to their power of exhibiting All substance* electrical excitement, and it was at one time thought not electrically that some substances, such as the metals, could not the same, under any circumstances be electrified. If, for instance, we substitute in our previous experiments a metal rod for the sealing-wax or glass, we shall find that no . amount of rubbing will produce in it the least elec- trical excitement, so long as it is held in the unpro- tected hand. This fact gave rise originally to a classification of sub- stances into two groups, one comprising those which exhibit electrical excitement when held in the hand and rubbed, and which were termed electrics, and the other comprising those which under similar conditions did not exhibit electricity, and which were accordingly termed non-electrics. This classification has, however, been shown to be erroneous, for if we hold a metal rod, one end of which is sur- rounded by vulcanized india-rubber, in the hand, we shall find that by rubbing the other end we can electrify it just as successfully as we can glass or sealing-wax. The true explanation of this curious fact is that bodies differ from each other in the rapidity, with which the electricity generated upon them by rubbing passes from the part rubbed to the nonconductors ther P arts > and 8O from them to otner Dodies witn ' which those pails are in contact. In some bodies, as the metals, the electricity generated rapidly diffuses itself over the whole mass of the electrified body, and passes away to surrounding bodies, while in others as wax or glass this diffusion is extremely slow. The former condition is expressed by saying that the body is a good conductor; the other by saying that the substance is a non- conductor. It must, however, be clearly understood that there is no such thing as an absolute non-conductor ; all substances conduct elec- tricity more or less rapidly, and when we speak of a conductor we signify a substance possessing this power in a very high degree, while by a non-conductor we signify a substance which conducts very feebly. With this qualification we shall, in accordance with common usage, Relative con- em ply the terms conductor and non-conductor ; and ductivities of the following list of substances, arranged in the order various bodies. O f their conductivity, the best conductors being placed at the top of the first column and the worst at the bottom of the second, will be found useful : ELECTRICITY AND MAGNETISM. ft Metals. Metallic Oxides. Gas Coke. Ice at 250. Graphite. Fats and Oils. Solutions of Acids. Caoutchouc. Solutions of Metallic Gutta Percha. Salts. Dry Air, Gases, and Vapours. Metallic Sulphides. Wool. Water. Ebonite. Metallic Salts (solid). Diamond. / Linen. Silk. J Cotton. Glass. | Hemp. Wax. ' Paper. Sulphur. Alcohol. Resins. Ether. Amber. Dry Wood. Shellac. Dry Ice. Paraffin. From this list it will be seen that the metals are the best con- ductors, and that paraffin is the worst. It will also be noticed that water occupies a high position in the list, and is therefore a good conductor of electricity. This will explain the necessity for having all our electrical apparatus perfectly dry for, if it be damp, the moisture conducts away the electricity as fast as it is generated, and little or no electrical excitement is rendered apparent. i v Substances which are non-conductors are often useful to prevent the escape of electricity from a good conductor, and this is the philosophy of electrifying a metal rod by holding it in a sheet of vulcanized india-rubber or caoutchouc, whic our list shows us is a fairly good non-conductor. Thus held, the metal rod when rubbed becomes electrified, and the elec- tricity generated cannot escape from the rod into the hand, because of the non-conducting caoutchouc which intervenes ; whereas, when the metal is held in the naked hand, the electricity generated by friction at once escapes through the body, which is a good conductor, and therefore none is found upon the rod. When a non-conductor in this way intervenes between an electrified substance and other con- ducting substances, the electrified body is said to be insulated, and the non-conducting material is then termed an insulator. Hitherto we have only dealt with instances of the simple attraction of unelectrified substances by those which are e ^ ec- Action of electri- trifled ; now we must turn our attention to the action fl e d bodies on of electrified bodies upon each other. each other. In order that we may do this effectually, it will be necessaiy to ELECTRICITY AND MAGNETISM. construct our first piece of electrical apparatus, which is represented in fig. 1. Upon a wooden stand is fixed a glass rod, the extremity of which is drawn out fine and bent into a kind of hook. This is accomplished How to make * JV holding the glass in the flame of a gas-burner or an electrical spirit-lamp until red-hot and soft, when it can be bent pendulum. to any desired shape. It is best to make the hook at some little distance from the tapering end of the glass rod, and the piecebeyond can afterwards be broken off by making a mark first with a tri- angular file ; the glass rod can be fixed in the stand by resin or glue. To make the glass a better non-conduc- tor, it should be coated with a thin layer of shellac varnish. From the hook is suspended, by a silk thread dipped in melted paraffin, a pith ball or small feather. This simple piece of apparatus forms what is known as the electrical pendulum. For those who wish to save time and trouble the stand may be dis- pensed with, and the silk thread and pith ball suspended directly from a gas-pipe or any other projection. As the shellac varnish mentioned above is a very useful material, it may be as well to give here instructions for preparing it : Get some shellac (orange shellac is the best), just cover it with methylated spirit, and leave it to stand for about twenty- four hours, shaking it occasionally. Then pour off the clear fluid and treat the undlssolved lac (if any) with a little more spirit. The varnish is applied in the ordinaiy way with a brush, and dries immediately. Having constructed your electrical pendulum, touch the pith ball with an excited glass rod ; it will first be attracted by the rod and Experiments then re P ellecl b 7 ^ wWle > if an excited stick of sealing- with the electri- wax be brought near the pith ball, the ball will be cal pendulum, attracted. In like manner, if the ball be electrified by touching it with the excited sealing-wax, it will afterwards be repelled by the excited sealing-wax and attracted by the excited glass. Jf, moreover, you make two electrical pendulums, and electrify the pith ball of one with excited sealing-wax and the pith ball of the other with excited glass, the two balls upon being brought near will be powerfully attracted towards each other, while if both pith balls be Shellac varnish. ELECTRICITY AND MA QNETISM. 1 similarly electrified they will repel each other. Fig. 2 shows us the condition of the pith balls in one case, and fig. 3 their condition in the other. From these experiments it seems clear that the electricity which is developed on glass by the friction of silk, differs from that generated on sealing-wax by rubbing it with flannel. Two ^f B of It seems equally clear that each kind of electricity re- " pels the same kind, but attracts the opposite kind. These experiments may be varied in a great number of ways. For instance, we may electrify a stick of sealing-wax and suspend it by placing it in a small wire stirrup, hung from some support by silk tape, and then bring another excited Furtlier e*peri- stick of sealing-wax near it, when we shall find that the suspended one will be repelled, while, if an excited glass rod be brought near, the suspended sealing-wax will be powerfully attracted. Again, if we electrify a sheet of warm brown paper by rubbing it with the hand or with a brush, and then with a sharp knife cut it into strips, it will be found on lifting the strips from the table that they will diverge from each other, being, of course, similarly electri- fied. In the same way an electrified sheet of foreign letter-paper may be cut into strips and made to form a divergent tassel. These experiments show that ladies similarly electrified repel each other, while bodies oppositely clectrijied attract each other. To this we may add that it is generally said that bodies which exhibit no electrical excitement possess equal quantities of the two kinds of electricity, which are mingled together so as to neutralize each other ; while, by rubbing, this equilibrium is disturbed by the separation of the two electricities, one going to the rubber, the other to the substance rubbed. At one time it was usual to call the electricity developed on glass 8 ELECTRICITY AND MAGNETISM. by friction with silk vitreous electricity, and that developed on wax Vitreous and ^7 friction with flannel resinous electricity ; but these resinous electri- terms are now disused, as it is found that the nature cities. o f t^e electricity produced on any substance depends upon the material with which it is rubbed, and also upon other con- ditions, such as temperature, etc. We may, for instance, get from glass the same electricity as we do from wax, if the glass is ground and rubbed with flannel instead of silk ; or even by holding an ordinary glass tube in an alcohol flame it becomes negatively electrified. It is therefore usual to employ instead of vitreous the term positive. tfse of the terms an( ^ i ns tad of resinous the term negative; the former positive and usually being denoted by the sign + (jplvs), and the negative. j a tter being usually denoted by the sign (minus). In this signification we shall use these signs and terms throughout these chapters. In the following list, if any two of the substances named be rubbed together, the one nearest the top of the list will be found positive to the one below it. Thus, if glass be rubbed with silk the glass will be positive and the silk negative ; if gutta-percha be rubbed with fur, the latter will be positive, the former negative. This list must be taken, however, as only approximately correct. + Fur Cotton Flannel Bilk Ivory The hand Quartz Sulphur Wood Vulcanized caoutchouc Shellac Ebonite Resin Caoutchouc Metals Gutta-percha Glass Gun-cotton With regard to the theory of electricity, no thoroughly satisfactory explanation of electrical phenomena has yet been given, but it is usual to adopt the so-called " two-fluid theory" the lan- expUmatioifof ua S c of wnicn we have employed so far. It must, the foregoing however, be thoroughly understood that this theory or facts. hypothesis is rather an expression than an explanation of the phenomena of electricity, and must by no means be regarded as a final and thoroughly satisfactory solution of their various features, but should rather be looked upon as an hypothesis which is tempo- rarily made use of until some more comprehensive and trustworthy theory shall take its place. ELECTRICITY AND MAGNETISM. CHAPTER II. THE GOLD LEAF ELECTROSCOPE. How to make a proof -plane How to use the proof -plane How to construct an electroscope How to charge an electroscope Theoretical explanation Experiments with the electroscope Experiments to illustrate conduction- Meaning of the experiments. FOB the detection and further elucidation of the elementary facts of electricity it will be necessary for our readers to construct two pieces of apparatus : the first of these is what is called a proof-plane, the second is known as a gold-leaf electroscope. The best way to make a proof-plane is to get an ebonite penholder (or a piece of fiat varnished glass), and attach to it with shellac a circular or elliptic disc of cardboard covered with metallic paper, as in the illustration (fig. 4). The use of the proof-plane is to take samples of electricity from an electrified body. The metallic paper with which the disc is covered, being a good conductor, readily accepts electricity from any electrified body with which it may be brought in contact ; and the ebonite or varnished glass handle, being a non-conductor, the electricity is unable to escape from the disc, and we are thus enabled to convey it to the electroscope for examination, in order that we may determine the kind of electricity with which the body is charged. The construction of a gold-leaf electroscope is a somewhat longer process, but is nevertheless simple and the materials How to con- inexpensive. For it we require a large glass flask, which utruct an elec- must be thoroughly clean and dry; a cork, through toscope. which a hole must be bored, and into the hole about an inch of narrow glass tubing fitted. In addition, we shall require a straight piece of brass wire about nine inches long ; a small disc of zinc or brass, from an inch to an inch and a half in diameter ; and some Dutch metal or gold leaf. Provided with these, fill the small piece of glass tube with resin or 10 ELECTRICITY AND MAGNETISM. shellac, or with a mixture of both ; then solder your metal disc to one end of the wire, making a hole in the centre of the disc in which to fix the wire. Having done this, warm the wire and push it through the resin in the glass tube of the cork of the flask, leaving some two or three inches of the wire, and also the disc, above the cork, and therefore outside the flask. Next bend the other end of the wire into a hook, and attach with gum a leaf of Dutch metal on to each side of the hook ; place the cork with the gold leaves, etc., in the flask, and your electroscope will be complete, and will be similar to the more elaborate one shown in fig. 5. The cutting of the leaves of Dutch metal, and attaching them to i^ the wire, will require some care, ^xX but the exercise of a little patience ^' r^Z, w *^ soon overcome any temporary [& jfr difficulties which may stand in the ^^/Hf wav of ultimate success. It is best ^r to cut the Dutch metal with a large pair of shears (so as to make only one cut), holding it between tissue paper. Having constructed the electro- scope, the next thing is to get accustomed to its use. It may be charged in one of two ways. By holding in contact How to charge wkh the digc an an electroscope. electrified body, such as excited glass or wax, or a proof-plane which has been mo- mentarily in contact with an excited body, the leaves will be seen to diverge ; and by bringing a similarly electrified body to the one just used near the electroscope, the leaves will be still further repelled by each other, showing that in this case the electroscope has been charged with the same electricity as that with which the charging body was endowed. The rationale of this method of chai-ging an electroscope, which is known as charging it by "conduction," is as follows: The excited body being placed in contact with the disc of the elec- trosc P e becomes for the time being an extension of the disc. The disposition of the electricity is at once affected, and it distributes itself over the disc, wire, and leaves of the electroscope and the object in contact with the disc, as if these were ELECTRICITY AND MAGNETISM. 11 all one, and the electric charge of the object is shared by the elec- troscope according to the rules of the distribution of electric charges. Thus the gold leaves diverge, being mutually charged with the same Bort of electricity as that of the electrified object used. The second method of charging an electroscope is by " induction." In this case the electrified glass rod is held near, but not touching the disc of the electroscope, when the leaves will diverge. While it is thus being held, the disc is momentarily touched with the finger, and immediately afterwards the rod is withdrawn ; the leaves will then first collapse, and afterwards diverge. The explanation of this is as follows : When the glass rod is held near to the electroscope the positive electricity is repelled and the negative attracted by it. When the finger touches the electroscope disc the positive electricity escapes, and the negative, set free by the' subse- quent removal of the rod, diffuses itself, and the leaves diverge with negative electricity. Thus you can charge an electroscope with excited sealing-wax: negatively if by conduction ; positively if by induction. If the insula- tion (by which the escape of the electricity is prevented) be perfect, the electroscope will retain a charge for some hours. The gold-leaf electroscope forms a most useful and instructive in- strument in the hands of the student, as by its aid he may prove for himself the accuracy of a very large number of the statements which it will be our duty to make. We have already made one or two such statements ; and it will be good practice, and also a good test of the trustworthiness of the electroscope, to at once set about proving them, as may be easily done by the following interesting and simple experiments. First charge your electroscope, in the manner described above, with positive electricity that is to say, hold near, but Ex eriments not touching it, a stick of sealing-wax which has been with the excited in the usual way by friction with flannel and electroscope, then, previously to removing the wax, touch the disc of the electro- scope with the finger ; the gold leaves will then diverge with positive electricity. Now bring the flannel rubber near the electroscope, and it will be seen that the leaves will diverge still more, showing con- clusively that the rubber is charged with the same kind of electricity as are the gold leaves. Now, we have agreed to call that kind of electricity which is developed upon sealing-wax when rubbed with flannel negative elec- tricity ; therefore the electricity of the flannel must be positive. Hence we have proved the truth of the statement that when elec- tricity is produced by friction, both the rubber and the body rubbed 12 ELECTRICITY ANJJ MAGNETISM. become electrified, the electricity developed on one being of the opposite kind to that developed upon the other ; it can also be proved that the quantities of electricity thus produced are always equal, but more complicated apparatus is required for the purpose than it falls to the lot of the average student to possess. It may, however be approximately determined by exciting a small stick of sealing-wax with flannel, and then laying both together on the disc of the electro- scope. It will be found that so long as both rubber and wax remain the leaves will not diverge, but if either be removed they will at once become charged with electricity ^osrtu-e if the wax, or negative if the rubber be removed. This experiment undoubtedly tends to show not only that both rubber and body rubbed are oppositely elec- trified, but also that the quantities of each kind of electricity are the same, because when they are both allowed to exert their influence upon the electroscope at the same time, they balance and therefore neutralize each other. The above experiments may be varied in an almost endless variety of ways, but always with the same result. When testing only small quantities of electricity, it will be found best to first charge the electroscope with the electrified body under examination, and then to test the electricity of the gold leaves by bringing near an electrified glass rod or stick of sealing-wax. When larger quantities of elec- tricity are being examined, first give a feeble charge to your electro- scope of either positive or negative electricity, and then bring near the body whose charge of electricity is to be tested. In all cases remember that repulsion is the only reliable test; a neutral body will cause the leaves to collapse by discharging them ; an electrified body alone can cause them to diverge. Another interesting series of experiments with the electroscope ia Ex eriments ^ ie * Uowing : Provide yourself with two or three to illustrate yards of fine copper wire, and the same length of ordi- conduction. nary a^ s $k COI &. First attach one end of the wire to the metal rod of the electroscope, and wind the other once or twice round the rod. Then, standing as far from the electroscope as the wire will allow you, excite the glass rod with the silk rubber and slip the coil of wire to the excited part of the rod ; immediately the leaves will be seen to diverge. Next substitute for the wire the silk cord, and proceeding as before, it will be found that no amount of rubbing will cause the leaves to diverge. Finally, net the cord, and now it will be seen that the electroscope is as readily charged as when the wire was employed, or as when the rod itself was brought into actual contact with the electroscope. What do these experiments mean? Obviously, that copper is an ELECTRICITY AND MAGNETISM. 13 excellent conductor, silk a bad conductor, and water a very good conductor of electricity. In the same way, by testing Meaning of the various substances, we may arrange them in order, experiments, according to the difficulty or ease with which they allow electricity to pass along them. CHAPTER III. INDUCTION BY FRICTIOXAL ELECTBICITY. Experimental illustrations A less costly form of the above apparatus Varia- tion of foregoing experiment Theoretical explanations Further experi- ments to illustrate induction Explanation of foregoing experiment A cheap electrophoros Principle of the electrophoros A better form of electro- phoros. WE have hitherto been mainly studying the simple facts of the attrac- tion and repulsion of electrified bodies, and of the communication of electricity from one body to another by actual contact, or, as it is termed, by conduction. We have now to study the manner in which neutral bodies may be influenced by electrified substances at a dis- tance, which operation is known as induct ion. One example of this method we have already had, in the second mode of charging our electroscope, it having been necessary to so far anticipate the present subject. A very easy method of illustrating induction is given in figure 6. AB is a brass cylinder with rounded ends, supported upon an insulating stand consisting of a glass rod covered with 14 ELECTRICITY AND MAGNETISM. shellac varnish. At each extremity is a small wire fixed into the cylinder by one end, and having suspended from the other a small pith ball (a i). Near to this insulated conductor is brought an elec- trified metal ball, C, similarly insulated to the conducting cylinder. We will suppose that the metal ball C is charged with positive elec- tricity ; it will then be seen that the pith balls, which hung vertically, are both repelled by the cylinder. Tt can easily be shown, by means of the proof-plane and the electroscope, that the end A of the cylinder AB is endowed with negative electricity, and that the end B is endowed with positive electricity. It may also further be shown that the ends of the cylinder are more powerfully electrified than the other portions, and that midway between the two ends there is a so-called " neutral line," which exhibits neither positive nor negative electricity. Now let c be removed, and the pith balls will instantly fall. The electrified body does not add anything to the cylinder nor subtract anything from it ; and when removed, the insulated cylinder returns to exactly the condition that it was in before the experiment. Instead of merely one cylinder, several might have been employed, and the pith balls of each would have diverged as in the case just supposed ; but the divergence of the pith balls of the first cylinder would have been greater than that of those upon the second, and so on the amount of electrical disturbance getting gradually less as the distance from the electrified body increased. Moreover, when only a single cylinder is employed, it will be seen (as represented in the figure) that the divergence of the pith ball nearest to the electrified ball is greater than that of the ball at the opposite end. Instead of employing brass as the material for our ball and cylinder, Alesscostl ^ e same resu -^ 8 ma J be obtained with cheaper and form of the more easily manipulated materials. A wooden ball above covered with gilt paper or tin-foil will do quite as well appara a. ^ & raeta ^ one . ^^ & wooden cylinder, or even a hollow cardboard one, similarly covered, will supply the place of a brass cylinder. In order to cover the ball with the gilt paper or tin-foil, the latter must be cut into narrow strips and carefully pasted or glued on. If a hollow cardboard cylinder be used, two wooden balls covered with metal paper and made to fit into the ends of the cylinder will answer very well for its rounded extremities. If the glass stands are not available, both electrified ball and conducting cylinder may be suspended by silk threads, in which case it will be best to dip the threads in melted paraffin wax. A variation of the last experiment is shown in the next diagram (fig. 7). As before, AB is an insulated conducting cylinder, and c an ELECTRICITY AND MAGNETISM. 15 electrified metal ball similarly insulated. Suspended from the under side of AB are five pairs of pith balls, one pair at each variation of end of the cylinder, one pair in the middle of the foregoing cylinder, and the other two pairs one on each side of experiment, the middle pair. When the electrified body c is brought near AB, all the pith balls diverge, with the exception of the central pair; but, as is shown in the diagram, the balls at the two ends of the cylinder are more powerfully acted upon than those between the ends and the central pair, while the latter remain entirely unaffected. If the electrified body be now removed, all the balls will, as in the first experiment, immediately collapse. Again, let us pause to ask ourselves the meaning of these two experiments ; and before going farther, let us attempt j^g^t^ to realise, if we cannot altogether explain, the condition explanations, of the pith balls during the experiment. Now, it must be evident that the electricity of c is not conducted through the air to AB, because if it were the whole of AB and there- fore all the pith balls would be similarly electrified, and their elec- tricity would moreover be of the same kind as that of the electrified conductor c. That the latter is not the case may be easily proved by bringing an electrified glass rod near them, when those near one end of the cylinder will collapse, while those near the other end will still farther diverge. What really happens, so far as our theory of elec- tricity enables us to explain it. seems to be this : the electrified body, when brought near to the unelectrified conductor, decomposes its electricity, attracting towards that end of the cylinder which is near itself the electricity which is of the opposite kind to that with which the ball is endowed, while the electricity which is of the same kind 16 ELECTRICITY AND MAGNETISM. as that upon the ball is driven to the opposite end of the cylinder ; this taking place by virtue of the law which we verified at the com- mencement of our studies, viz., that like electricities repel, xmlike electricities attract each other. This action of an electrified body upon a neutral body without actual contact is known as INDUCTION. If, before removing the (we will suppose) positively electrified body the cylinder AB be touched for an instant with the finger, and then the ball c be removed, the cylinder will be found to possess only nega- tive electricity, and in this case the positive electricity of AB, being repelled by that of o, escapes through the body to the earth, when it is enabled to do so by the hand being brought into contact with it ; and the ball c being then removed, the negative electricity is set free and diffuses itself over the whole of AB, which thus remains negatively electrified. Numerous other experiments, besides those already mentioned, may be devised to illustrate the facts and phenomena of induc- tion, and many will no doubt suggest themselves experiments to to the ingenious .student. We may, however, mention illustrate the following. Balance a lath upon an egg, or in some induction. otber w& ^ ^ that i( . ig free iQ rotate horizontally about its centre, and place under one end of it a gold-leaf electroscope. Now bring near the other end an excited stick of sealing-wax, and cause the lath to rotate as in our first experiment ; it will be found that every time the lath passes over the electroscope, the gold leaves of the latter will diverge with negative electricity. Another experiment is the following. To one end of your insulated cylindrical conductor fasten a short piece of stout wire, and so place the conductor that this wire is just immediately over a gas-burner which has the gas turned on but not lit. Now get a piece of thoroughly dry cardboard, and after rubbing it briskly with caout- chouc, bring it near the end of the conductor which is opposite to that which has the wire fixed to it : a spark will pass from the wire to the burner and ignite the gas. Turn out the gas and remove the card, a second spark will probably pass between the wire and the burner and re-light the gas. In this latter case the positively electrified card repels to the further extremity of the cylinder its positive electricity, while the Explanation of negative is attracted towards the end nearest the card. foregoing But the end of the cylinder at which the positive elec- expenment. tricity is accumulated is extended by the Avire until nearly in contact with the burner, and the burner is connected to the earth by the gas pipes. The positive electricity at the end of the wire causes by induction a similar accumulation of negative elcc- ELECTRICITY AND MAGNETISM. 17 tricity at the burner ; and the attraction of these quantities of elec- tricity for each other determines their union and mutual neutralization with the production of a spark between the two conductors. When the excited card is withdrawn, the negative electricity which it held at the end of the cylinder nearest to it distributes itself over the cylinder, and induces an accumulation of positive electricity at the burner. These two electricities then react as before if their tension is sufficiently great. Having now some notion of what induced electricity is, we may proceed to consider a very important though simple piece of apparatus which depends for its efficiency upon the principle of induction. Get a tin plate, such as can be bought for a penny at a toy shop. Procure also a stick of sealing-wax, and either a small sheet of vul- canite or a piece of brown paper. Warm the plate over a candle, and fix the sealing-wax vertically to its centre, so that it shall form an insulating handle. Next excite your vulcanite or brown paper in the usual manner, and place the tin plate upon it. Now touch the plate for an instant with your finger, and then raise it by the insulating handle. Upon bringing your knuckle near the plate a small spark will result, and several sparks in succession may be obtained by simply repeating the process without re-exciting the vulcanite or brown paper. This simple apparatus is a rough form of electrophoros (Gr. elektron, amber from whence electricity phareo, I bear). The principle of its action is as follows. The upper surface of the vul- pri^p^ canite or brown paper is negatively electrified, and when O f the the tin plate is placed upon it very little actual inter- electrophoros. change of electricity occurs, as the two surfaces only touch one another at a few points, and the vulcanite, being a bad conductor, does not permit these isolated points to become recharged. The plate, when placed upon the vulcanite, has its electricity decomposed by induction, its positive being fixed by the negative charge of the vulcanite, while its negative is repelled to its upper surface. This accumulation of negative induces a positive charge in one's finger as it approaches, and these two electricities neutralize each other with the production of a spark, leaving the plate with its surplus of positive electricity as fixed by the vulcanite. When the plate is withdrawn from the vulcanite its excess of positive electricity becomes free, and it is therefore charged positively. The plate may be charged in this way a great number of times without re-exciting the vulcanite, as the charge on the vulcanite is only lessened by accidental causes, which will be diminished as the instrument is more perfect. The electrophoros acts merely by induction. If the vulcanite is laid 18 ELECTRICITY AND MAGNETISM. upon a sheet of tin-foil, so that its under side is in gocd electric contact with the earth, it will be found that it may be more highly charged than when otherwise arranged. The metal receives by induction a positive charge, and this attracts the electricity of the vulcanite, enabling it to receive a greater quantity. Such is the principle of the electrophoros, and our readers will no doubt be anxious to make a more effective one than the le form which we have just described. Such a one is shown in our diagram (fig. 8). Into a shallow metal dish, or a wooden one lined with tin-foil, is poured a melted mixture of gum- mastic, shellac, marine glue, and Venice turpentine, in the proportion of five parts each of gum-mastic and shellac to two of Venice turpentine and one of marine glue. This mixture is allowed to cool and harden, and is the so-called "cake" of the electro- phoros, the dish into which it is poured being the " form." Air bubbles must be got rid of, and the best way to do this is to pass a gas-burner over the cake as it hardens, which will burst the bubbles. The next important part of the apparatus is the plate B, which should consist of a circular brass disc with an insulating handle, D, of varnished glass, which latter may be made to fit into a socket on the upper side of the plate. The electrophoros is now complete, and its principle and mode of action is precisely that given above in the description of the more simple and inexpensive form of apparatus. CHARTER IV. DISTRIBUTION OF FRICTIONAL ELECTRICITY. Experimental illustrations Meaning of foregoing experiments Use of points Discharging effect of flames and heated bodies Faraday's net Another method of illustrating distribution. THE next important feature connected with frictional electricity is the manner in which it disposes itself over the bodies of substances which are electrified. In order to illustrate this portion of our sub- ject, it will be necessary to construct a conical conductor ; it may be made either of metal, or of wood covered with tin-foil or gilt paper ; ELECTRICITY AND J/.-l BNETISM. 19 or it may be made simply of a piece of cardboard bent into the shape of a cone, and covered with metallic paper. Whatever may be the material of which it is constructed, it will be advisable to make the base of the cone of such a size that it will easily fit on to the end of the conducting cylinder already constructed so as to form a conductor haying one end rounded and the other conical. Having fitted up this irregularly-shaped body, insulate it, either by placing it upon an insulated stand, or by suspending it with silk dipped in melted paraffin. There should also be suspended from it two pairs of pith balls hung by cotton, one pair from the rounded extremity, and the other near the pointed end of the conductor. Now give to the conductor a charge of electricity by any convenient means, and it will be found that while the pith balls at the rounded end of the con- verge but slightly, those near the pointed end suffer a very considerable amount of divergence. Again take your metal ball, and sus- pend from it two pairs of pith balls in the manner shown in fig. 9. Now insulate it, and give it a charge of electricity ; it will be found that both pairs of balls diverge, and that the amount of their divergence will be equal. What then do these experiments tell us 1 Obviously, that in our irregularly shaped conductor the electricity is more evident at the pointed extremity than at the rounded one ; while in the M eanin _ O f second case our experiment shows us that the electricity foregoing of a sphere is equally spread over its whole surface, experiments. Careful investigation has shown this to be the case, and if we imagine an atmosphere or ocean of electricity to surround electrified bodies, the "tension" of that electricity on differently-shaped conductors may be represented as in the diagrams (tigs. 10, 11, 12), the distance between the dotted lines and the outline of the figures representing the depth of this hypothetical ocean or atmosphere, or in other words the " tension '' of the electricity it being of course thoroughly under- 20 ELECTRICITY AND MAGNETISM. stood that no such atmosphere or ocean is known to exist, but its existence is merely supposed for the purpose of assisting the graphic representation of the facts. By our diagrams it will be seen that on a cylinder the tension of the electricity is greatest at its two ends, and the same is seen to be the case with an oval body ; and if either of the ends of the latter be drawn out to a point, the tension or tendency to separate from itself becomes so great that the electricity no longer remains upon the conductor, but escapes into the atmosphere. When the tension of the electricity upon any body is sufficiently great to cause it to discharge into the air, the latter becomes electrified, and its particles being mutually repellent, give rise to what is known as an " electric wind." This electric wind may be excited at opposite points of a light body, and by this means it may be made to float in stable equilibrium in the air, to the great astonishment of the uninitiated. The experiment is also interesting, as it proves the possibility of the particles of the air assuming the electric state. This action of points in determining the concentration of a charge upon them, while they of all forms of conductors are the least able . to hold a charge, may be stated in a somewhat different eo poin . . tne for obtaining an electric connection between a solid conductor and the air. It thus becomes obvious that if we would retain a charge upon a solid, we should avoid the use of points, and that they are useful when electricity is to be caught from the air. as in the case of lightning-conductors. The effect of points in dissipating electric charges into the air may be demonstrated by bringing a needle near to, but not touching, the disc of a charged electrometer ; the gold leaves will gradually fall together Flames and heated bodies in general behave like points. If a ELECTRICITY AND MA GNETISM. 21 lighted match is used instead of the needle just referred to, the electroscope will be discharged, and the same effect may be obtained by holding a white-hot iron ball over the disc of the apparatus. It is worthy of note that as a heated iron ball cools, a point is arrived at when it will no longer and heated discharge an electroscope charged positively, but is bodies, still hot enough to discharge one whose leaves diverge with negative electricity. A heated iron ball also shows a difference in its behaviour to positive and negative electricity in becoming charged itself, for at a certain temperature it will be found possible to communicate to it a charge of negative while it is still too hot to accept a charge of positive electricity. In addition to our knowledge of the manner in which electricity is spread over the surfaces of electrified bodies, it is also easily shown that electricity has a tendency to seek those surfaces ,. , t rather than to occupy the interior. A very simple and * interesting way of showing this is by means of Faraday's net, which is shown in fig. 13. As will be seen, it consists simply of an ordinary butterfly net mounted upon an insulating stand, and provided with a string, by which it can be turned inside out. If to such a net a charge of electricity be given, it will be found, upon testing it with a proof plane, that the interior gives no indication of electricity, while the exterior is found to be electrified. Now turn the net inside out, and it will be found that the surface which is now inside, and was a moment ago outside, gives no indication of electricity, while the surface which is now exterior, and which was just now interior, is powerfully electrified. Another method of showing the same facts also due to Faraday 22 ELECTRICITY AND MAGNETISM. is shown in a diagram in fig. 14, which represents a roller mounted upon glass insulating supports, and having upon it a oTmuItTating long strip of tin-foil. To the tin-foil are attached by distribution. co tton threads two small pith balls. Upon giving to the tin-foil a charge of electricity, the pith balls will of course diverge ; but it will be found that if the tin-foil be unrolled they will gradually collapse, owing to the spreading out of the charge, whereas if the foil be rolled up they will again diverge. Here the electricity is concentrated as the conducting surface is reduced, and vice versa. Another easier method of performing the same experiment is to simply roll a long strip of tin-foil upon a glass rod, and connect the free TIG. li. end oE the foil with the electroscope by a piece of copper wire. By holding the glass rod in the hand, and giving, by means of the proof plane, small charges of electricity to the foil, the movements of the leaves of the electroscope, as the foil is rolled up and unrolled, will indicate that the electricity is spread only over its surface. The distribution of electricity upon the surfaces only of bodies is also well illustrated by the following experiment. Fig. 15 is a sphere of brass mounted upon a varnished glass pedestal. To this sphere a charge of electricity is given, and then two hollow hemispheres, held ELECTRICITY AND MAGNETISM. 23 by insulating handles, are placed over it ; upon being removed, it is found that while no electricity is present upon the sphere, the exterior surfaces of the hemispheres at once give indication of a charge. All these experiments tend to show that electricity confines itself to the surfaces of bodies and avoids the interior, and a number of further experiments illustrating the same fact will doubtless suggest themselves to the student. Any insulated hollow body, such as a hat or a glass covered with tin-foil, may be employed, with invariably the same result. This accumulation of electricity upon the surfaces rather thau the interior of conductors may be accounted for in two ways. An electric charge is self -repellent like electricities repel and therefore forces tself outwards away from its centre as far as possible. The other cause is merely a matter of accident, depending upon the presence of bodies outside the experimentally electrified substance. If the operator works in a room, then the walls become oppositely electrified by induction, and so draw the charge as near to them as possible, that is to the outside of the body electrified. It is easy to more than overcome these two causes, and so determine the accumulation of a charge on the interior surface of a hollow object. Insulate a metal saucepan by standing it on a dry glass, and charge it as usual ; the charge will be found on the outside. If now a metal ball connected to earth is carefully introduced into the saucepan so as not to touch it, the ball will become charged by induction, and will draw the elec- tricity of the saucepan to its interior surface so completely, that if the outside is tested with a proof -plane and electroscope it will probably give no indication of a charge. CHAPTER V. ELECTRICAL MACHINES. Earliest form of electrical machine The cylinder machine How to make ft cylinder machine The prime conductor The rubber How to make amal- gam Principle of the cylinder machine Chronology of electrical machines The plate machine The principle of the plate machine Winter's plate machine Holtz machine Principle of the Holtz machine How the machine is worked Inductive influence Action of the armatures Value of electrical machines The insulating stool. HAVIKG now made ourselves acquainted with some of the more important fundamental facts of electricity, we shall be enabled to understand the structure and principle of those larger pieces of apparatus for generating electricity which we term par excellence electrical machines. 24 ELECTRICITY AND MAGNETISM. The earliest electrical machine, devised in the year 1670 by Otto von Guericke, burgomaster of Magdeburg, consisted merely of a ball Earliest form of ^ su lph ur which was excited by friction with the dry electrical hand. Since this, however, numerous arrangements machine. ^ave ^ een d ev j se( j f or obtaining electricity upon a larger scale than can be done by any of the methods we have yet described ; and we will now endeavour to give such a description of the most common forms, that any one with a little ingenuity may at a small cost make them for himself. One of the oldest and most easily constructed of electrical machines The cylinder is tne so-called cylinder machine, the appearance of machine. which is familiar to most students (see fig. 16). It consists of a glass cylinder which is made to turn upon an axis supported on a wooden frame. What is known as the prime con- ductor of the machine consists of a brass cylinder with rounded ends, mounted upon varnished glass legs, and having upon its inner side a number of small spikes which extend from the prime conductor to the glass cylinder but do not quite touch the latter. On the opposite side of the cylinder is a pad or cushion, made of several layers of flannel and covered with a piece of black silk, which is so arranged as to extend in the form of a flap over nearly the whole of the upper surface of the cylinder. The cushion is sup- id on a varnished glass leg similar to those which support the prime conductor, and like the latter, it should have its ends well rounded. The silk flap and rubber being covered with a zinc and tin oadgam (which we will presently give instructions for making), the handle is turned, and the cylinder revolving, electricity is generated ELECTRICITY AND MAGNETISM. 25 which reveals itself by sparks when the knuckle is presented to the prime conductor. Before giving the theoretical explanation of the action of this machine, we will give a few hints to those who may desire to make one. The glass cylinder may be bought at a glass How to make a blower's, where they usually keep some on hand. In cylinder selecting the cylinder care should be taken to see that machine, it is, as far as possible, " true." The cylinders are usually sold by the pound, and one about ten or twelve inches long (which is a convenient size) should cost somewhere about three shillings. The stand of the machine should be of dry, well-seasoned wood (mahogany Is the best), and after being made as smooth as possible may be polished or var- nished to improve its appearance. For the axis of the cylinder an ordinary round ruler passed through from end to end answers very well, though it will be better if the ruler can be dispensed with in the interior of the cylinder, in which case turned caps to fit on the ends will be required. For cementing on these caps, or for fastening the axle in the cylinder, a mixture of resin and shellac in about equal parts, used hot, will be found to answer best. The uprights which support the axle may both be of well-seasoned wood. The prime conductor may be made from a piece of brass tubing about an inch and a half in diameter, and for the rounded ends the knobs of two door-handles may he used. Upon the side of the conductor which is to face the cylinder about a dozen small brass tacks may be fastened. The best way to do this is to get a narrow strip of thin brass and to put the tacks through it, soldering them so as to keep them firm ; then care- fully solder the strip of brass to your prime conductor, the glass sup- ports of which may be made to fit into sockets upon its under surface. The rubber may be made of several layers of perfectly dry flannel, put on as evenly as possible. The flap of silk (which should have its raw edges neatly hemmed), must be tacked on to the under surface of the rubber and then brought up in front of it and allowed to lie loosely upon the upper surface of the cylinder. Care should be taken to round off as much as possible every corner and edge of the machine, so as to prevent the loss of any electricity by discharge into the atmosphere. Upon the back of the wooden or brass rod to which the rubber is fastened should be placed a small hook, to which a chain may be attached in order to connect the rubber with the earth. If wood be used for this rod, it will be found advantageous to fasten a narrow strip of tin-foil upon its under surface, and to connect this by a small piece of wire with the hook at, the back. Wood may also be used for the support of O 26 ELECTRICITY AND MA GNETISM. the rubber, but in this case only one kind of electricity can be obtained from the machine. If this plan be adopted, a very good method is to hinge the support of the rubber on to the bottom of the stand, and attach to it a spring or other suitable appliance to obtain the necessary pressure. The amalgam for use upon the rubber is made as follows: melt in an iron ladle one ounce of tin; to this add gradually one ounce of zinc scraps ; thoroughly mix these with an ir n stirrer > and tten allow the mixture to COOl ^11 nearly solid ; re-melt, and add two ounces of hot mer- cury. Stir, throw into a warm iron mortar and stir up with an iron pestle till cold. Keep in a stoppered bottle. When used mix with the amalgam a very little lard which has been well washed and dried. Spread a little of this mixture upon the rubber of your machine, and also upon the silk with which you rub your glass rod ; it will be found to greatly add to their efficiency. With this description and a little patience we feel sure that any one may make a cylinder machine ; but in order to make the matter perfectly clear the student should endeavour not at all a difficult thing in these days to see one in actual operation. It remains now only for us to explain the principle upon which the machine works. The rubber being connected with the earth by Princi le of the means ^ a cna i n passing from the hook on the back to cylinder a gas pipe, fender, or other large metal object which is machine. a g OO d conductor, the handle is turned, and by the friction of the rubber upon the glass positive electricity is generated upon the latter and negative upon the silk. The glass is a non- conductor, and as it revolves carries the positive electricity round to the points of the prime conductor, and the points by facilitating the passage of electricity, determine an equalization of the electricity of the prime conductor and that part of the cylinder opposite its points. As the cylinder is positive and the prime conductor neutral, the equalization leaves them both positively electrified. The charge on the cylinder, which is thus impoverished, is reinforced when it passes again under the rubber, and the rubber by being kept in contact with the earth is maintained in a neutral condition in spite of the negative electricity it is continually receiving. The negative charge of the rubber, instead of the positive charge of the prime conductor, may be utilized, if the former is insulated and the latter connected to earth. The principle upon which the action of the plate machine depends is precisely the same as that which governs the action of the cylinder machine. In both rases electricity is developed hy the friclion of a ELECTRICITY AND MAGNETISM. 27 silk rubber upon glass, the two machines differing from each other mainly in the shape which the glass assumes. Although more easy to make, and therefore very fully described, the cylinder machine is not so good a form as the plate machine, which is an improve- ment made upon the former by Ramsden, in. the year 1760. As we have already stated, the first form of electrical machine consisted merely of a ball of sulphur which was mmauAagy of turned on an axis and rubbed electrical with the hand, and was made machines, about the year 1670 by Otto von Guericke, burgomaster of Magdeburg, to whom we also owe the air-pump. Subsequently resin was substituted for the sulphur ; and finally by Hawksbee a glass cylinder was used, the hand 11 being employed as the rubber. Cushions of horsehair covered with silk were first employed as rubbers by Winckler in 1740. and in the following year Boze used an insulated tin-plate cylinder as a 28 ELECTRICITY AND MAGNETISM. collector for the electricity produced by the machine. The next step in advance was the substitution by Ramsden of a glass plate for the cylinder, and the construction of the so-called plate machine, of which the diagram (fig. 17) represents a form which departs least from the original devised by its inventor. Here a large plate of glass or vulcanite, A, is suspended between two supports by an axis passing through the centre and turned by a glass handle, M. D, D, are two quadrant-shaped flaps of The plate O Q G( J s jjjj. w hich pass from the rubbers, which latter are cushions fixed upon the inner side of the supports, so as to clasp the plate as it revolves, one pair being placed above and another below the axis. These cushions consist of pads of flannel fixed upon wood and covered with leather. The object of the silk flaps is to prevent the escape of electricity into the air. C, C, are two horseshoe-shaped rods of brass provided on their inner surfaces with a number of spikes. These rods are attached to the two prime con- ductors, P, p, which are supported on insulated legs, and are united at their ends opposite to the plate by a small rod, Q. It will be easily seen that this form of machine differs from the The principle y linder machine merely in shape, its details being of the plate varied only so far as is necessaiy to accommodate them machine. to tne a ite re d form of the revolving glass. As in the case of the cylinder machine, the rubbers are connected with the earth by a chain, and when the plate is turned it becomes positively and the rubbers negatively electrified. By contact with the earth the tension of the negative electricity of the rubbers is reduced to zero. As the plate revolves the positive electricity upon it acts inductively upon the prime conductors, and decomposing their electricity, attracts their negative and repels their positive. By this attraction of the negative electricity to the points of the prime con- ductors, its tension there becomes so great that it can no longer remain on them, but passes across the air to the plate and tends to neutralize its positive electricity, while the positive of the prime conductors is left upon them, and (they being insulated) is unable to escape. In this manner a quantity of positive electricity accumulates upon the prime conductors, and they become electrified, not merely by the glass plate conferring positive electricity upon them, but also by that plate robbing them of their negative. If it is desired to obtain negative electricity from this machine instead of positive, it must be mounted upon insu- lating supports, and the prime conductors connected with the ground, when sparks of negative electricity may be obtained from the supports ; and it is noticed that these sparks are sharper than those of positive electricity obtained in the ordinary way from the conductors ELECTRICITY AND MAGNETISM. 29 The form of the plate machine just described is not the only one in use. A very good modification of Kamsden's machine, and one which is much more easily constructed by an Win ter^aplate amateur, is that known as Winter's, shown in fig. 18. Here, as will be seen, there is but one rubber, and the prime con- ductor consists of an insulated ball, to which are attached so as to almost touch each side of the plate a pair of rings furnished with spikes. The rest of the diagram will explain itself, and will serve as a useful guide to any one wishing to construct a machine of this kind. The chief advantage of the plate machine is that both sides of the glass are rubbed at once. In that form of it just described a large circular ring is frequently used, which very greatly increases the quantity of electricity accumulated. This ring, which is shown in fig. 19, fits into the ball of the prime conductor, and contains a metallic wire which communicates with the metal ball, as shown in the diagram. A form of machine now very largely in use, and which we must by no means omit to mention, is the one shown in fig. 20, and was invented in 1865, by Holtz, of Berlin. Here two thin discs of glass or vulcanite are placed near together. Of Holtz maohine ' these the larger, D, is fixed, and the smaller, A, can be made to rotate on an ebonite axle, its rate of rotation being considerably increased by means of the two multiplying wheels, E, E'. The axle upon which A revolves passes through a large hole in the centre of the fixed plate, 30 ELECTRICITY AND MA QNETISM. , In the fixed plate are also two other holes of somewhat triangular form and in the lower edge of one hole or window" and the upper eeteeof the other window are glued two pieces of varnished paper, P P' with pointed tongues which project into the openings. These are the so-called armatures. The fixed plate is supported on insu- lators, as shown at V, v. At the extremities of the horizontal diameter of the revolving plate and opposite to the windows of the fixed plate, are two rows of metallic points or rakes, c, c', which collect the electricity from the movable plate as it revolves between them and the fixed plate. Connected with these rakes are two rods, B, B', through the outer ends of which pass somewhat loosely the two smaller brass rods, termi- nated with knobs, B, B', and having wooden handles, H, H', by which the knobs may be moved so as to approach or recede from each other. The principle upon which the action of this machine depends is the continuous inductive action of an electrified body, as in the case of the electrophoros already described. Its theoretical e* cx P^ anat i n is not quite so easy of comprehension as ' that of the ordinary plate or cylinder machine, but a little patient attention on the part of our readers will enable them to master it. In order to set the machine to woik it requires a small capital of ELECTRICITY AND MAGNETISM. 31 electricity to work upon ; and supplied with this, it speedily accu- mulates a very large stock in hand, which may be em- ployed in any way desirable. The first thing to do is to connect the two knobs, B, B', and this having been done, a sheet of ebonite, which has been electrified by striking it with catskin, is brought near one of the armatures, P for instance, and a few turns are given to the movable plate, A A'. Now the sheet of ebonite is, of course, charged with negative electricity, and this negative electricity, acting by induction on P, decomposes its neutral electricity, repelling its negative and attracting its positive. The negative electricity of the armature is discharged by its tongue on to the movable plate, and the armature therefore remains positively electrified, while the plate becomes negatively electrified. After half a turn of the latter its negative electricity is brought opposite to the other armature, P 7 , and acting by induction upon it, takes away a portion of its positive electricity and leaves it negatively electrified. In this way, after a few turns of the movable plate, both armatures of the fixed plate become electrified, one positively, the other nega- tively. This having been done, the electrified plate of ebonite is removed and the knobs, B, B', separated. If the plate be now continuously turned, a perfect torrent of powerful sparks will pass between the knobs. Now, we have already seen that when a conductor is submitted to the inductive influence of an electrified body, it becomes charged with opposite electricities upon its opposite surfaces. The same is also the case with non-conductors, only in Inductive them the separation of the two electricities is a gradual, not an instantaneous one. If, however, a bad conductor, such as a plate of glass or ebonite, be interposed between the source of electri- city and a good conductor, the inductive action of the electrified body is somewhat modified. If, for instance, the source of electricity be positive, it is found that, after a time, the good and bad conductors are toth charged with negative electricity on the side turned towards the source of electricity, and with positive upon their opposite sides. If, on the contrary, the inductive action is of only short duration, the influence is weak, and the glass plate then becomes charged with negative electricity on both faces. Now, it will be remembered that the armature P is after a few turns of the plate positively electrified. When, there- fore, the plate is made to revolve, the portion of it opposite to P will become negatively electrified, while at the same time the conductor BB, which has imparted its electricity to the opposite side of the plate A, has thereby become positively elec- 32 ELECTRICITY AND MAGNETISM. trifled. The glass plate continuing to rotate, that portion of it which just now was opposite P will soon be opposite the other armature, P*, which, it will be remembered, is negatively electrified. There will then be acting upon the other conductor, E' B', both the negative electricity of the armature and the negative electricity of the plate, and these acting together will withdraw from R' B' its positive, and leave it charged with negative. Now, therefore, we shall get both conductors charged, one with negative, the other with positive elec- tricity. The portion of the plate opposite to the armature P' will tend to keep the latter negative by withdrawing its positive, and will itself return to the neutral condition. By continued rotation of the plate A the same phenomena are reproduced, and an accumulation of opposite electricities upon the opposite conductors takes place, and these unite to produce a spark which, with a good machine and under favourable circumstances, may reach a length of six or seven inches. In order to increase its power a pair of small Leyden jars are sometimes suspended to the conductors E, E', the external coatings being connected by a rod with each other, and the internal coatings with the conductors of the machine. The effect of this machine is also much increased by doubling the number of plates. With either of the electrical machines which we have described, a number of very interesting and important experiments may be per- formed. Many of these we shall mention in the course of this treatise on Electricity, and many others will doubtless su Sp st themselves to our readers ; but in all cases it is desirable that experiments should be looked upon not merely as amusing, but also as instructive. Every one of these ex- periments has a meaning, which with the aid of the information given none of our readers need despair of comprehending, and we earnestly trust that they will endeayour to grasp to its fullest extent the mean- ing of the reply which Nature gives to every question with which, in the language of experiment, they attempt to ply her. A very useful adjunct to the electrical machine is an "insulating stool." This consists merely of a piece of hard, well-seasoned wood. * ith four varnished g la s legs. If a person stands upon such an insulating stool, and places one hand on the prime conductor of the machine while the latter is in action, sparks may be drawn from any part of his body by his com- panions. Here, of course, the body of the individual upon the stool becomes temporarily a portion of the prime conductor of the machine. When the spark is taken from the person on the insulating stool, he ex- penences the same pricking sensation as when he himself takes a spark from the prime conductor of the machine by placing his knuckle near it. ELECTRICITY AND MA GNETISM. CHAPTER VI. CONDENSATION OF FRICTIONAL ELECTRICITY. The condensing pane Principle of condenser Use of the condenser The dis- charging tongs The Leyden jar How to use the Leyden jar Theory of the Leyden jar Residual charge Experiment with Leyden jar Leyden jar batteries Cascade arrangement Quantity and density -Capacity of Leyden jars Potential Electric discharges Action of points and brush dis- chargeDuration of electric spark Piercing action of spark Heat of spark Appearance of spark Magnetizing action of spark The condensing electroscope Theoretical explanation The unit jar Use of the unit jar. PERHAPS the most useful kinds of apparatus for use with the elec- trical machine are those employed for the purpose of condensing the electricity obtained from it. As we have already seen, there is a limit to the tension of electricity accumulated upon a surface, and when that limit is reached the electricity passes off, or, as we say, is " discharged " into the air. In order to increase the tension of the electricity which we obtain upon a surface, without causing it to discharge, we employ several pieces of apparatus which condense it. Of these the first which we shall describe is what is known as the " condensing pane," which was invented by Franklin. This, in its simplest form, consists of a sheet of varnished glass, which has attached to it, near each end, a loop of tape The condensing by which it may be easily lifted. Beneath it is laid a sheet of tin-foil, and upon it another sheet somewhat smaller than the plate of glass. (See fig. 21.) If now the upper sheet of tin- foil, A, be connected with a gold-leaf electroscope, P, by means of a piece of thin copper wire, a number of charges of electricity may be given to A by means of a proof- plane, without in the least affecting the leaves of the electroscope 34 ELECTRICITY AND MAGNETISM. If however, the upper sheet of foil, A, be raised by lifting the glass plkte by the tape loops D and E, the leaves of the electroscope will this fact is as follows. Suppose the electricity given to A to te positive. This positive electricity will act inductively through the glass plate upon the lower sheet of foil, c Principle of ( w hich is connected with the earth by means of the wire condenser. and ^ decompose its neutral electricity, repelling its positive and attracting its negative. The positive electricity of A, therefore, will have opposed to it the negative of C, and will attracted to it, bound captive by the very power which it has its evoked. It is therefore powerless to affect the leaves of the electro- scope, until it is removed from the influence of c by the raising of the glass plate B. With the electrical machine the condensation of condenser 6 electricit y bv Franklin's pane may be shown in a very striking manner. Let the upper sheet of foil be connected with the prime conductor of the machine by means of a piece of thin copper wire. Now turn the machine several times, so as to charge the pane, and, having done so, touch the lower sheet of foil with one hand, and bring the other hand near the upper sheet : a smart shock will be felt, and a bright spark of electricity be seen. Here the opposite electricities of the two sheets of foil unite through the body of the experimenter ELECTRICITY AND MAGNETISM. 35 As it is not always pleasant or convenient to discharge the con- densing pane and other similar electrified bodies by the hands, a pair of "discharging tongs" will be found useful. These are represented in fig. 22, and consist merely 6 d jjj h r ' in!r of two metal rods having brass balls at their ends and furnished with a glass handle or handles. In the figure 1 is for use with one hand, 2 requires two, and is used when the distance between the two points to be connected is considerable. In its permanent form, for use with the electrical machine, Franklin's condensing pane has the two sheets of foil the same size but somewhat smaller than the pane of varnished glass. They are then pasted on to the glass, and the lower sheet has a small tongue of tin-foil passing from it over the edge of the glass so as just to reach the upper side. It is also convenient to make a long tongue to the upper sheet, and then to roll it round a varnished glass rod. By this arrangement the end of the upper sheet of foil may be brought any required distance from the tongue of the lower sheet of foil. This arrangement is shown in fig. 23, where V is the sheet of glass, c the tongue of the lower foil, B the upper sheet of foil, p the glass rod round which the tongue of B is rolled, E the wire connecting the lower foil with the earth, and P the wire by which B is connected with the prime conductor of the machine. Another very important piece of apparatus for condensing elec- tricity is the Leyden jar, fig. 24. It will, of course, be easily seen that if we could take our condensing pane and roll it up so as to form a cylinder, we could, by putting a bottom to the cylinder, readily convert it TheLe y den J ar ' into a jar coated on its inside and outside with tin-foil ; and this is really what the Leyden jar consists of. A wide-mouthed glass jar is carefully coated both inside and out- side with tin-foil, with the exception of an inch or so from its neck. Into the top is fitted a disc of well-baked wood, and through this passes a stout wire which terminates externally in a brass knob, and internally in a hook to which a small piece of brass chain is attached, which rests upon the bottom of the inside of the jar. The part of 36 ELECTRICITY AND MAGNETISM. the outside of the jar which is not covered with tin-foil is coated with shellac varnish. It must be understood that both the outside and inside of the jar must be covered with tin-foil, including the bottom. The rod should be fastened to the wooden disc through which it passes by shellac. To use the Leyden jar, hold it in one hand, and place its knob in contact with the prime conductor of the machine. After it has re- mained there a second or so, remove it, and touch the 8 ' he knob with the otber hand; a smart shock and spark will be the result. If preferred, the jar can of course be discharged with the discharging tongs, (in which case be careful to touch the outside coating first) ; or if it is desired to give a shock to several persons at once, let them join hands ; then let the person at one end hold the jar by its outer coating, while the person at the other end touches the knob. It will be readily seen that the action of the Leyden jar is precisely the same M that f the coudensin ? P ane > tne inner coating of the jar corresponding to the upper foil of the pane, and the outer coating of the jar answering to the lower foil of the pane ; consequently the inner coating of the jar is posi- tively, the outer negatively electrified, and the union of the two kinds of electricity is rendered evident, when the two coatings are connected, by a spark. A curious phenomenon of the Leyden jar is the pis. 24. Residual so-called " residual charge." If a jar charge. be charged and then allowed to stand for a short time before being discharged, it will be found that a second, although very much smaller spark, may be obtained a short time after the first, and sometimes at short intervals a third and even a fourth may be seen. The production of these sparks is usually explained by saying that the two electricities slightly, as it were, soak into the glass, being impelled thereto by the attractive force which they exert upon each other. When the jar is first discharged, the surface electricities, which form by far the larger portion, unite, and the jar being sub- sequently allowed to stand, the residual electricity oozes out again to the surfaces, whence it may be discharged. That this explanation is probably correct, is seen from the fact that a spark will sometimes pass through the glass of a jar if left standing for some time after being charged. The two electricities of a charged jar are generally supposed to accumulate upon the outer and inner surfaces of the glass, rather ELECTRICITY AND MAGNETISM. 37 than upon the tin-foil coatings ; and that this is the case, at any rate when the coatings are removed from a charged jar, may be shown by constructing a Leyden jar with movable coatings. This may be done in the following manner. Take a clear open glass, such as an uncut tumbler, and make a wooden cone so as to fit easily the interior of the glass. (See fig. 26.) Cover this carefully mth tin-foil, and fix to it a brass rod and ball, placing round the rod a small piece of varnished glass tube, so that the cone may be raised without touching either the tin-foil or the brass rod. This cone placed inside the jar will constitute its inner coating. The outer coating may be made of thin cardboard covered with tin-foil on both sides, or, better still, may be made of tinned sheet-iron or zinc, and should be just large enough to allow the glass jar to slip easily (but not too easily), in and out. The sur- face of the jar above the coatings should be coated with shellac varnish. When these parts are placed together, the jar can be used in precisely the same manner as an ordinary Leyden jar. Charge the jar in the ordinary way, and then stand it upon the table and remove the inner coating by catching hold of the E rfment varnished glass tube, and test it by with Leyden means of the electroscope ; it will Jar ' exhibit only a very small charge of electricity, if any. Now take hold of the outer coating and shake out the glass jar, which proceeding will of course neutralize any electricity which the outer coating may possess, as by laying hold of thei outer coating you connect it at once with the earth. Having done this, proceed to rebuild your jar by placing the outer coating over the glass, and, after inverting the latter, place in it the inner coating, lifting this of course by the varnished tube. If you now join the inner and outer coatings, a spark will be obtained which will be almost if not quite as bright as if the jar had not been taken to pieces. This experiment without doubt clearly shows that when the metallic coatings of a Leyden jar are removed the electricity stays upon the glass. When large quantities of electricity are required, several jars are employed, having their internal and external coatings respectively joined together. These are placed in a box lined with tin-foil, which lining is connected with two metal handles in the sides of the 38 ELECTRICITY AND MAGNETISM. box. The inner coatings are all connected together by metallic rods, and the battery is charged by placing the internal Leyden jar coa ti ngs i n connection with the prime conductor of attenes. ^ electrical machine, and connecting the external coatings with the earth by means of a chain which is attached to the handles of the box. Such an arrangement of Leyden jars is known as a Leyden jar battery, and is shown in fig. 26. From these batteries extremely powerful discharges may be obtained, and they should invariably be discharged by means of a discharging rod, the outside coating being first touched. Beside the arrangement described above, a series of jars may be arranged in what is termed a " cascade." In this case Cascade the j ars are p i ace d each upon a separate insulating 1 ' support, and the knob of the first is placed in connec- tion with the prime conductor of the machine, while its outer coating is connected with the knob of the. second, the outer coating of the second with the knob of the third, and so on ; the outer coating of the last being connected with the earth. The first inner coating becomes positively charged, and acts inductively upon the first outer coating, which latter discharges its free positive electricity, not, as in the ordinary way, to the earth, but to the inner coating of the second jar, which thus becomes positively electrified, and in like manner passes on a charge to the third jar, and so on. The jars may be discharged singly, by connecting the inner and outer coating of each jar, or simultaneously, by connecting the inner coating of the first with the outer coating of the last. A little consideration on the part of the reader will enable him to see tnat by this arrangement the same quantity of electricity which would, *n the ordinary way, charge only one jar, is made available to charge several. ELECTRICITY AND MAGNETISM. 39 There are a few matters with regard to electric charges that will be conveniently considered here in connection with Leyden jars, though they are of far more extended application. If we take two Leyden jars, alike in all respects except that one is twice as large as the other, that is, has twice the extent of coated surface, and then put into both an equal quantity of elec- tricity, which may be measured by counting sparks of a definite length ; we shall have in each the same "quantity" of electricity, but its "density" will be twice as great in the smaller jar, because there it is crowded into half the space. The greater density of the charge of the smaller jar may be proved by observing the length and nature of the two sparks obtained by discharging the jars, the denser charge giving the longer and more powerful spark. The apparatus shown in fig. 14 affords another means of demonstrating the effect of a change in density of any given quantity of electricity. The pith balls diverge to a greater extent as the available surface is diminished, and collapse as the surface is increased, while the quantity of the charge remains the same because the tin-foil is insulated. A charge of electricity may thus become so thinned out by spreading it over a large surface that its presence cannot be recognized, although the same quantity in a more concentrated, that is more dense, con- dition may be dangerous to manipulate. The " capacity " of a Leyden jar, that is with reference to the quantity of electricity that it can receive, does not depend merely upon the extent of the tin-foil surface, but also upon the ease with which induction can take place between Capacity of the inner and outer coatings. This will depend first upon their nearness, that is the thinness of the glass which separates them, and also upon the quality of the glass in reference to the ease with which induction takes place through it (its specific inductive capacity, see p. 46). Thus a good jar is more likely to be shattered or pierced by discharging itself through the glass than a bad one, not merely because the thinner glass is a weaker barrier to the electricity, but also because being thinner it allows a greater and therefore denser charge to accumulate in the jar. " Potential" is yet another quality of electric charges. It may be defined as capability of doing work, and is a term applied not merely in connection with electrical movements and pheno- mena, but to nature in general Potential is a relative 3 and not an absolute term, and in order to give it any definite mean- ing we must assume a level or a condition at which potential becomes zero. If we assume a stone at the sea level to have no potential, then by throwing the stone thirty feet above that level at tho 40 ELECTRICITY AND MAGNETISM. moment that it begins to fall it is invested with a certain definite potential or capability of doing work. If the stone weighed exactly one pound, and we threw it thirty feet high, its potential at that moment would be equal to thirty foot-pounds ; that is, in falling back to its original position it would do work, or expend energy, equal to the raising of a weight of one pound through a distance of thirty feet. The potential of electric charges is referred to that part of the earth where the experimenter works, the potential of the earth being considered as zero. It follows, therefore, that by connecting any body with the earth by a good conductor the potential of that body is reduced to zero, because the fact of its connection with the earth makes it electrically one therewith. If the potential of the body was previously different from that of the earth, then electricity FIG. 27. would pass along the conductor ; and if none does pass, when a good way is open for it, we have proof that none can pass. The potential at all parts of a conductor is the same, however the density of the charge vanes ; and if two electrically charged bodies having different potentials are connected by a conductor, then we have a movement electncity until the potentials are equalized. If a charged body connected with the earth by a good conductor, its potentfal is re" I to zero, and the force or energy with which the electricity moves is the measure of its previous potential The nature and effects of the electric dischargeare subjectsthat call Electric dis- , a8 ? ch attention as s P a <* will allow. There are charges. two or three ways by which a charged body may be effected is if ^ff dectricall y w ^ the air ; its discharge is thus ed as if lt had been connected to earth, though with greater ELECTRICITY AND MAGNETISM. 41 difficulty and slowness. The most important of these methods is by the use of points at which the tension of a charge Action ofint]| becomes so augmented that it breaks away into the air and brush dis- in spite of the fact that the air is so poor a conductor charge, that it is classed with the dielectrics. The air being a gas and Its particles free to move, it is set in vigorous motion by such a discharge because of the repelling action of similarly electrified bodies. The particles of the air are thus repelled by the point and by each other, aud produce so perceptible a wind that a candle may be blown out by it, or a light evenly balanced suitable arrangement may be caused to rotate. Fig. 27 shows such an apparatus. The discharge from points is called the " brush" discharge, because of its appearance in a darkened room ; and it becomes more vigorous if an oppositely electrified or earth-connected body is brought in front of the point. If instead of a point we employ two balls or rounded surfaces, no appreciable amount of discharge will take place until they are brought within " striking distance," when a spark passes. The electric spark appears to last for a very appreciable time, but in reality it exists for only about jf^nr P ar t f a second, so that if a spinning top in a dark room is illuminated by it, the fraction of a second during which the top is made visible is so small that it does not revolve perceptibly in the time, and therefore appears to be standing still ; and any pattern that may be painted upon it is as clearly seen as if it were still. The spark may be caused to strike through solid dielectrics ; even a feeble spark will pierce cardboard. If the holes so made are examined they will show a burr on both sides of the card as if produced by an explosion within the thick- ness of the card, and this appears to show that the spark does not pass through the card in one direction as we might push a pin through. However, so far as one can see, the spark appears to pass from the positive to the negative substance ; and we know that minute particles of the positively electrified body are torn off and volatilized by the great heat of the spark by examining the light it gives spectroscopically. Indeed this method is constantly used for vaporizing metals and alloys that they may be identified by their spectra. The heat of the electric spark may also be utilized for firing gun- powder or other explosives at a distance from the operator, but there are certain precautions that have to be observed. To illustrate this, secure the ends of two pieces of wire Heat of sparkl within a short distance of each other to a piece of board, and over the break in the conductor heap a small quantity of gunpowder. a 42 ELECTRICITY AND MAGNETISM. Now hold the loose end of one of the wires against the outer coating of a charged Leyden jar, and then bring the loose end of the other wire to the knob of the jar. A spark is produced in the midst of the powder, but it is not inflamed by the spark it is merely scattered. If, however, the experiment is repeated, and instead of providing so good a conductor for the electricity a few inches of wet string are introduced into the circuit, the powder will be ignited. *IG. 28. The general appearance of the spark in air when taken between rounded surfaces, such as brass balls, varies chiefly according to the distance across which it has to strike. When this is ApP g e paA. Ce f sma11 the s P ark is Poetically straight, but a slight in- crease will cause its path to be very perceptibly irregular, zigzag, similar to forked lightning. These deviations from a straight course are probably caused by the motes of the air ever varying their positions, so that the path of least resistance between the two con- no. 29. ductors is never twice the same. By still further increasing the iistance over winch the spark has to travel it becomes branched as resented in fig. 28 ; and occasionally, if the distance is almost too reat for the charge to pass across, the spark will appear to dissipate rSed b l, T^ \ Wlth Ut & PP arentl y rea <*^ the negatively elec- trified ball. These branches all grow, as it were, from the positive The intimate connection between electricity and magnetism is a ELECTRICITY AND MAGNETISM. 43 subject belonging rather to the study of voltaic than of frictional electricity ; but the disturbing effect of thunderstorms on telegraphic instruments and ships' compasses is so well known that the subject demands a passing reference here. The electric discharge acts in these cases partly by affecting the magnetic state of the needles used. A needle may be magnetized or demagnetized by the discharge, by laying it across a piece of tin- foil, through which the prime conductor of a machine discharges itself to the earth. Fig. 29 shows the arrangement, with the polarity of the needle marked that the discharge would bring about. If a magnetized needle is placed with its poles in a reverse position to that shown, it will first be de- magnetized by the discharge, and then magnetized, as shown in the engraving. Before leaving the subject of condensation, we must not omit to describe an adaptation of t condenser to the ordinary gold-leaf electroscope, so as to convert it into a " con- densing electroscope." This form of the instrument is shown in fig. 30. It consists, as will be seen, of the gold-leaf electroscope, with a much larger circular brass disc than usual attached to the top of the wire, from the other end of which the gold leaves are suspended. This brass plate has its upper surface well coated with shellac varnish, its lower surface being left bare. A precisely similar brass plate has its under surface var- nished, and to its upper surface an insulating glass handle is attached, by which the plate may be lifted. The action of this instrument is as follows: The varnish upon the opposed surfaces of the two discs prevents their actual metallic contact, but they are sufficiently near to act powerfully upon each other by induction. Accordingly, if repeated small charges of electricity be given to the upper disc, B, and if at the same time as these charges are given the under surface of A be touched with the finger, so as to connect it momentarily with the earth, a quantity of electricity will accumulate upon both 44 ELECTRICITY AND MAGNETISM. plates, and by mutual attraction will be there held captive, to that no effect will be had upon the gold leaves, which, so long as the two discs are in contact, will remain undisturbed. So soon, however, as the upper disc is removed, the electricity upon the lower disc, being liberated, spreads itself over the wire and gold leaves, and the latter diverge with electricity of the opposite kind to that with which the upper plate, and therefore the body under examination, was charged. The theoretical explanation of this will be readily understood by those who have mastered what has been previously said about induc- tion. Suppose the charge given to the upper disc B to be positive, this will act inductively upon the neutral electricity of A, attracting its negative and repelling its positive. The repelled positive escaping to the earth, A is left charged with negative electricity, which by a repetition of the process gradually accumulates, but being held bound by the positive of B, cannot escape from the plate until, B being re- moved, it is set free. Instead of employing the proof-plane, the upper disc may be directly connected with the body under examination by a wire or other con- ductor, the under surface of the lower disc being at the same time similarly connected with the earth. It is under these circumstances that the special use of this apparatus becomes evident in showing the presence of electricity when the charge is too minute for detection in the ordinary way. Let us suppose it desired to search for electricity on a conductor of large surface which gives no indication of a charge when connected with the common electroscope. The method of pro- cedure would be to connect one disc of the condensing electroscope to the earth and the other to the object to be tested. The feeble charge would then spread itself over the disc, and induce opposite electricity in the earth-connected disc, which would react upon the other, attracting the electricity of the object according to the principles of condensation already explained. It only remains now to break the earth-connection, then the connection with the object under examination, and lastly to remove the upper disc, when the diverg- 2nce of the gold leaves will indicate the charge that was tested for. Another very useful piece of apparatus which we must briefly The unit jar describe is the "unit jar," for measuring the charge of electricity given to an ordinary Leyden jar. As will be seen by fig. 31, it consists of a small Leyden jar, A, supported in a horizontal position by a varnished glass stem, B, its outer coating being in connection with a metal knob, c. A horizontal rod, E, which is in connection with a machine, is placed also in connection with the inner coating of A. To this horizontal rod is attached a movable arm by which the knob B may be brought to any ELECTRICITY AND MAGNETISM. 45 required distance from c. Finally, from the rod c to the inner coating of the jar to be charged passes a conducting wire, while the outer coating of this latter jar is connected with the earth. If a charge of positive electricity from the conductor of the machine be given to the inner coating of A, the outer coating will become negatively charged. In the same way the inner coating of the large jar becomes charged with positive TTse of .^ e ""* (repelled from the outer coating of the unit jar), and the outer coating with negative electricity. This goes on until A FIG. 31. becomes sufficiently charged for a spark to pass between c and B, and the negative of the outer coating of A becomes nearly neutralised. This leaves the jar D positively charged, the amount of the charge depending upon the previous accumulation of electricity in its in- terior, which, in its turn, depends upon the distance between B and c. The use of the unit jar is to charge a jar equally at different times, by allowing a certain number of sparks to pass between the knobs B and c. 46 ELECTRICITY AND MAGNETISM. CHAPTER VII. SPECIFIC INDUCTIVE CAPACITY. Dielectrics Faraday's researches Faraday's theory of induction Objections to Faraday's theory Dielectric phenomena- Results of Faraday'sresearches Dielectric value of certain substances. IN treating of the various phenomena of induction we have hitherto considered the action of an electrified body upon a non-electrified Dielectri ^^J w ith which it is not absolutely in contact purely as an action at a distance, and have neglected altogether the condition of the medium (such as the air), existing between the two bodies. It is, however, necessary that we should, before quitting this branch of our subject, consider what happens to the intervening medium, and endeavour to understand the theory of induction, pro- pounded by the great electrician, Faraday. First let us approach the subject from the practical side. Faraday in his researches performed the following simple but instructive ( experiment. He took a vessel containing turpentine. researches. and sus P end ed in it a number of small silk threads! He then placed in the liquid two conductors, one of them charged with positive electricity and the other connected with the earth. Upon completing this arrangement all the filaments of silk were attracted towards each other, so that they joined end to end, and formed a continuous chain between the two sides of the vessel. This of course tended to show that the alternate filaments were oppositely charged, and formed a chain of elements having positive at one end and negative at the other. A more conclusive experiment even than this of Faraday was one subsequently performed by Matteucci, an Italian physicist. In this case a number of thin plates of mica were taken and placed quite close together, while the two end plates were on their outside provided with a sheet of tin-foil, in the same way as the fulminating pane of Franklin before described, with the important difference that only the outside of the terminal- plates were so provided. This arrangement was then electrified, and the tm.foil removed by insulating handles. Upon examining electro- ecopically the condition of the plates of mica, it was found that each was charged with positive electricity on one side and with negative on the other. Now, it is evident, in this case, that however thin the layers into which a body may be divided, each layer is both positively and nega- ively electrified, and not half the layers of the body positively and the other half negatively endowed. By continuing in imagination ELECTRICITY AND MAGNETISM. 47 the breaking up of a body into finer and finer particles until the ultimate particles or " molecules " are reached, it is quite legitimate for us to suppose that each one of them, is also positive at one end and negative at the other. If this view of the matter be correct, we might consider an insulator, across which induction is taking place, to be represented graphically as in the annexed diagram (fig. 32). Faraday supposed that electrification by induction was not really an action at a distance, but merely a continuous polari- ^ da y' f 8 sation through the air particles connecting the two induction, bodies. According to this (Faraday's) theory of induction, the action of an electrified body upon another, with which it is not directly connected by a conductor, may be graphically represented as in fig. 33, where A represents the inducing electrified body, B the body upon which A is acting, and c to H a row of air particles connecting A and B. *IG. 33. Ingenious, however, as is Faraday's theory, it has not wanted for assailants, and some of the objections urged against it obj ect um g to are entitled to considerable weight, the most important Faraday's being that induction takes place in a vacuum. This theory, objection would at first sight seem to be fatal to the theory ; but we must remember that a perfect vacuum has never yet been obtained, and it may be that in the best vacuum we can at present command there still remains a sufficient number of air particles to propagate the polarisation. Whatever may be the value of Faraday's theory of induction, it is perfectly certain that it is necessary to consider the action of inter- vening media in inductive phenomena, it being clearly proved that different insulators have different capacities for transmitting electrical influence. To show this, the apparatus represented in figs. 34 and 35 was employed by Faraday. Here fig. 34 represents a brass sphere made up of two halves, which are made to fit very accurately into each other. In the interior of this sphere is a smaller one, also made of brass ; and attached to this is a metallic rod, which terminates outside the larger sphere in a 48 ELECTRICITY AND MAGNETISM. knoh. B. This rod is insulated from the envelope PQ by a thick mass of shellac, A. Into the space m n, between the two spheres, the substance whose inductive capacity is to be tested is placed. The foot of the apparatus is hollow, and is provided with a screw and tap, so that it may be connected with an air-pump, and the air within P Q, if necessary, rarefied or exhausted. It will be sufficient for our purpose if we describe one of the experiments performed by Faraday with this apparatus. Two such Results of double spheres precisely similar to each other were used, IWM? End in the fil ' St instance the spa 06 m in each contained only air. Both the envelopes PQ were connected with the ground, and to the knob of one of them a charge of positive electricity was given. The inner sphere, therefore, in this case appa- ELECTRICITY AND MAGNETISM. 49 a torsion of 114, while that of the second sphere containing shellac was 113, the two amounts together only being equal to 227 instead of 290. Of the total quantity of electricity, therefore, the apparatus charged with shellac had taken 176, and that containing air 114. Faraday therefore came to the conclusion that the inductive power, or. as he termed it, the specific inductive capacity of air, is to that of shellac as the proportion of 114 : 176 or as 1 : 1'55. By a series of similar experiments with other insulators, the specific inductive capacities of these substances, or Dielectrio value llelectrics as they are termed, (conductors being called of certain anelectrics,') have been ascertained. In the subjoined ttbrtancei. table the most important of these results are given : Air. . . . 1-00 Glass . . .1-90 Spermaceti . . 1-45 Shellac . . . 2-00 Resin . . . 1-76 Sulphur . . . 2'24 Pitch . . . 1-80 India rubber . . 2'80 Wax . . . 1-86 Guttapercha. . 4-00 By the following simple experiment we may illustrate the effect of a dielectric in induction. Let an electrified body be placed at a Certain distance above a gold-leaf electroscope, by which, of course, a certain divergence of the leaves is produced. If now a disc of sulphur or shellac be interposed between the electrified body and the disc of the electroscope, the divergence of the leaves will be sensibly increased. CHAPTER VIII. ATMOSPHERIC ELECTBICITT. Franklin's experiments Phenomena of thunderstorm Varieties of lightning The return shock Action of lightning conductors Normal atmospheric electricity. THE facts connected with this branch of our subject are tolerably familiar to us all, and we shall therefore only very briefly allude to them. The well-known philosopher, Benjamin Franklin, was the first to prove conclusively that the electricity which produces Franklin'* the lightning and thunder is precisely identical with experiment*, the electricity which we obtain upon a smaller scale by artificial means. For this purpose Franklin constructed a kite armed with a number of sharp points, and held by a damp string, the lower end of which was connected with a dry silk cord to serve as an insulator, which 60 ELECTRICITY AND MAGNETISM. was attached to a tree. To the lower end of the string a small key was also attached. This kite being sent up on a stormy day, Franklin found that he could obtain sparks of electricity from the key, just in the same manner as from any ordinary electrical appa- ratus, and by various trials he was enabled to thoroughly satisfy himself that the "fire" which he thus veritably "brought down from heaven," was of the same nature as the electricity with which he was familiar. The electricity thus obtained was not, as Franklin thought, absolutely abstracted from the thunder-clouds, but was the result of the decomposition of the electricity in the kite and string, by the inductive action of the electrically charged atmosphere. In an ordinary thunderstorm we have clouds which are oppositely electrified discharging towards each other, or we may have a direct discharge between the earth and the oppositely elec- trified clouds. In either case the lightning-flash is, of course, the electric spark, while the thunder is the noise accompanying that spark, which is usually greatly intensified by echoes. There are usually said to be three or four kinds of lightning. First there is what is generally known as " sheet lightning," which seems t0 CUr in the clouds themselves ; then there is the %htnteg. common " zigzag " lightning, in which we see the exact counterpart of the discharge from an electrical machine ; then again we have " summer lightning," after which no thunder is usually heard ; again we have the much more dangerous, and happily also more unusual, form of lightning known as " globe lightning," in which the flashes appear as globes of fire, frequently descending in the form of "thunderbolts" to the earth, and exploding with great noise; while, lastly, we have the kind of brush discharge which takes place from pointed bodies, such as the mast of a ship or a soldier's bayonet, towards the clouds, and is popularly known as "St. Elmo's fire." A peculiarity of these atmospheric distarbances is what is known as the "return shock," which is often experienced, and which, though The return "^ fteQ atal> is sometimes very disastrous in its shock. eff ects. This shock, which is frequently experienced at a distance from the place where the lightning dis- charge passes, is due to the inductive action of the thundercloud upon bodies on the earth. By this inductive action the neutral elec- tricity of a body is decomposed, and it eventually becomes charged with electricity of the opposite kind to that of the cloud. When, LtloTcel Cl U ^ eCOmes neutralized by its discharge, its inductive action ceases, and there is at once a return to the neutral condition ELECTRICITY AND MAGNETISM. 61 on the part of all bodies which were previously influenced by the cloud, and the sudden movement of electricity in order to effect their neutralization produces what is known as the return shock. One of the most important fruits of the discovery of the identity of atmospheric electricity with that artificially obtained was the invention of the lightning conductor. As ia well known, this consists usually of a rod tipped with gilt copper or platinum, and projecting some eight or ten feet above the roof of the edifice which is to be protected. This is connected with a conductor, consisting of an iron rod (or better, a copper band), passing down the wall of the building and penetrating some few yards into the earth, where it is either led into a well or imbedded in a hole filled with wood-ashes, which latter serve not only as good conductors, but also to prevent the rusting of the metal. A lightning conductor protects a building from injury when a storm is passing over it in one of two ways. Either a lightning flash may pass, and, instead of striking the building, descend into the earth by the rod, which is a much better conductor Action of than the building to which it is attached; or the lightning cloud may be neutralized without a discharge by the conductors. easy .passage of electricity along the conductor. This latter way of preventing a flash of lightning by neutralizing the thundercloud is probably the most usual way in which the conductor acts. Although the electricity in the atmosphere is most apparent during a thunderstorm, it must be borne carefully in mind that there is always some amount of free electricity in the air. Normal When the sky is cloudless this electricity is always atmospheric positive varying, however, in intensity with the height electricity, of the locality and with the hour of the day. When the sky is obscured by clouds, the electric condition of the air is sometimes negative and sometimes positive, while in foggy weather it appears to be strongly positive. A great many attempts have been made to account for the exist- ence of free electricity in the atmosphere, but no thoroughly good hypothesis has yet been propounded. Amongst other theories it has been supposed to result from the friction of the air against the ground, from the phenomena of vegetation, and from evaporation; while by others it has been supposed that the earth is a huge voltaic pile, and by still others that it acts as an immense thermopile. No one of these hypotheses is thoroughly satisfactory, and it is probable that the true explanation will be found in the combination of several of these supposed causes. ELECTRICITY AND MAGNETISM. CHAPTER IX. ELECTROMETERS. Electrical measurement The unit of electrical measurement The pith-ball electrometer The torsion electrometer Employment of the torsion electro- meter Explanation of foregoing experiments Thompson's quadrantelectro- meter. WE have more than once in the course of these chapters had to speak of the quantity of free electricity which a body possesses ; and before closing these chapters upon f rictional electricity, it is necessary that we should understand the structure and mode of action of the principal instruments used in electrical measurements. First, however, it will be necessary for us to adopt some The unit of unit o measure- electrical ment in which we measurement. ma y ex p ress an y results which, by means of such instruments, we may be able to obtain. The unit usually adopted is one which considers the velocity conferred upon an ^HBBH9B@ electrified body when repelled by a similarly electrified body, and is expressed as " that quan- tity of electricity which, wJien concentrated at a point, will repel a like quantity at a dis- tance of one centimeter with a force that, after acting for one second, will impart to it a velo- city equal to one centimeter per second for each gramme weight of matter.^ To those who are not familiar with the metric system this definition may seem somewhat puzzling, but a glance at a table of the metric system with which, by the way, every student should familiarize himself will suffice to make it perfectly clear. One of the simplest pieces of apparatus used in electrical measure- The pith-ball m6Ut iS the s - called Quadrant, or, better, Pith-ball electrometer. e ketrometer. This, as its names imply, consists of a small quadrant scale, D (fig. 36), which is attached to a brass rod, B c, which fits into a hole in the prime conductor of an ELECTRICITY AND MAGNETISM. 53 electrical machine. To B c, at B, is hinged a light wooden rod, on the further extremity of which a pith-ball, A, is attached. When the machine is worked the ball A is repelled until a maximum height, indicating a maximum potential of the electricity of the machine, is reached. Beyond this point all electricity produced by the machine escapes into the air. If a Leyden jar be attached to the prime con- ductor, it will take a longer time to reach this maximum, but as before, when it is reached, no amount of working the machine will increase the charge of the jar. A more useful electrometer with which one may demonstrate the laws of electrical attraction and repulsion is the torsion electrometer, already referred to in our description of Faraday's experiments upon dielectrics, and which is represented in fig. 37. As will be seen, it consists of a cylindrical glass vessel, A A A A, about half way from the bottom of which is a circular band of paper, a a, upon which a scale, divided into 360 degrees, is marked. Upon the bottom of the glass case is placed a small dish containing chloride of calcium, which, by absorbing moisture readily, keeps the air in the interior of the vessel perfectly dry. Upon the top of the 54 ELECTRICITY AND MAGNETISM. glass vessel a glass plate, B B, is placed, and is secured by three wooden buttons, c c c. Upon the top of this glass lid, at its centre, is placed a tall cylinder, D, also of glass, but much smaller in diameter, and at the top of which is a micrometer, shown in section at the side. This latter consists of two cylindrical drums, the upper moving freely upon the lower. The lower one, A, is fastened with gum mastic upon the top of the glass cylinder, D D, while the upper movable one B bears a plate upon which a scale of 360 is marked. Through the centre of this plate passes a small rod, D D, which can be made to turn by means of the nut at its top, while a pointer, c, serves to mark upon the micrometer scale, in degrees, the rotation of the movable drum with respect to the fixed drum. This micrometer being fixed upon D, a fine wire passes from the bottom of Dinto the larger cylinder, where it has attached to its other extremity a horizontally disposed rod of shellac,/, at one end of which is a gilt pith-ball,^. Finally, through the lid B B of the larger cylinder passes a stout wire, h, to the lower extremity of which another gilt ball s attached; the two balls, when the instrument is at rest, remaining in contact, and being opposite to the zero of the paper scale, a a using this apparatus, a charge from the body under examination IB gl ven to h, and this charge will of course be immediately conferred Employment of t t ^ * 6 tW P ith balls ' * 9- These becoming P eb y simil ^ly electrified, will repel each other and shellac rod will ca its ith-bal away from ELECTRICITY AND MAGNETISM. 55 force of torsion is employed to overcome the force of repulsion, which is of course still acting, sufficiently to reduce the distance between the balls to 18 that is, by one half and the degree of torsion required to do this is 114. In the experiments, third instance the distance between the two balls is reduced to one- fourth the original distance, and the degree of torsion required to effect this is 576. Now, this evidently shows us that the nearer the 3. QUADRANT ULECTBOMETBB. cwo balls approach each other the more powerful is their mutual repulsion, and it remains only for us to see the proportion in which this repulsion varies with the distance. Tabulating the results of the experiment detailed above, we have the following : Distance. Force of repulsion. 36 36 18 144 = t x 36, 9 576 ~ 16 x 36; ELECTRICITY AND MAGNETISM. that is to say, if the distance between the two balls be halved, the repulsion is not doubled but quadrupled while, if the distance be reduced to one-fourth, the repulsion is sixteen times as great. In other words, the force of repulsion (and therefore also of attraction), varies in- versely as the square of t/te distance. In this way the application of the law of squares to electrical attraction and repul- sion is, by means of Coulomb's torsion balance, satisfactorily proved. One of the most important and inge- Thompson'! n ^ ous electrometers is that quadrant of Sir William Thompson ter. _ hig so . called Quadrant Electrometer." In fig. 38 a view of this instrument is given, while in fig. 39 the instrument is seen dissected. As will be seen, it consists of four brass quadrants, A, B, c, D, insulated by glass supports, in the same horizontal plane, form- ing a circular disc with strips cut out across the diameters. For those who care to make one of these instruments for them- selves, we may say that these brass quadrants may be replaced by a ground-glass disc with a hole in the centre, upon which similar quadrants of tin-foil may be pasted in such a manner as to leave the diameters of the glass disc bare. Each quadrant is then connected below with the opposite one that is, A with c and B with D. Two brass knobs with wires E, p, arc attached to two neighbouring quadrants, and form the poles of the instrument. A needle, o, formed of a thin piece of aluminium, and shaped as hown in the diagram, is suspended above the quadrants by a verv thm silver or platinum wire, which is attached to the inside of a ELECTRICITY AND MAGNETISM. 57 small inverted Leyden jar, H. This Leyden jar is supported by a rod, I, and its inner coating is connected with the rod and knob K, while the whole apparatus is covered by a glass shade, the air inside which is kept dry by means of a dish containing sulphuric acid. Finally, above the needle G a small vertical mirror is fastened, which will thus of course move with the needle. The manner in which the instrument is worked is as follows : A small lamp (not shown in the figure, but any lamp will do), casts a beam of light upon the mirror above G, and this is reflected by the mirror so as to throw a spot of light upon a screen suitably arranged to receive it, and upon which a scale is marked. Of course, as long as the needle remains still the spot of light will do the same ; while, if the needle moves the spot of light will also move along the scale in one direction or the other, according to -the movement of the needle. If now a charge of positive electricity is given to the jar H, and one of the knobs, say E, is connected with the earth so that the quadrants D and B are earth-connected ; and if at the same time F receive a charge of positive electricity, the quadrants c and A must share this charge, and they will repel the needle, which will then move in the direction of the hands of a watch, the direction and amount of the movement being registered by the movement of the spot of light upon the screen. If the charge given to F be one of negative electricity, the needle will of course move in the opposite direction to the hands of a watch, and the spot of light will therefore move in the opposite direction along the scale. This instrument can also be employed with voltaic electricity by connecting E and F with the opposite poles of a voltaic cell. CHAPTER X. ELECTRICITY OBTAINED BY CHEMICAL ACTION. Galvani's discovery Galvani's conclusions disputed Further discoveries Modern theory of voltaic electricity Volte's pile Effect* of the pile Volta's contact theory Volta's error The crown of cups How to make a simple cell Action of the voltaic cell Parallel effects with frictional elec- tricity Differences in quality Differences in quantity Observations of Faraday Changes in the voltaic cell Proofs of contact theory Explana- tion of last experiment- Electricity from a single metal Explanation of changes in the cell The molecular theory Definition of a molecule The atomic theory Chemical affinity Application of theory Electrical con- ductivity of water Negative and positive metals Electro-motive series. ABOUT a century ago (namely, in the year 1780), an Italian named Galvani, professor of anatomy at the university of Bologna, was busily B 58 ELECTRICITY AND MAGNETISM. engaged in making researches into the effects of electricity upon animals, and in the course of his experiments noticed G-alvani's that the u m b s o f d ea d frogs, which had been recently Very ' skinned, were convulsed by frictional electricity. One day, however, he found that similar convulsions were produced in the legs of a dead frog without the aid of an electrical machine at all. A frog had been skinned and the legs suspended from the horizontal iron bar of a balcony, to which they were attached by a brass or copper wire which passed through the frog's backbone. Every time the wind blew, the frog's legs were blown against the vertical bars of the balcony ; and to Galvani's surprise he found that when this contact of the frog's feet with the metal took place the legs were violently convulsed, in the same way as they would have been had an electric shock been given to them by an electrical machine or a Leyden jar. This discovery caused a considerable commotion in the scientific world, and a discussion fierce and long soon arose over its theoretical Galvani's interpretation ; Galvani and his supporters maintaining conclusions that the convulsive movements were due to the flow of disputed. a v ital electric fluid" from the spinal cord to the muscles, while others (the chief of whom was Volta, Professor of Natural Philosophy at Pavia), maintained that electricity was pro- duced by the mere contact of the different metals, the frog's muscles only serving as a highly sensitive electroscope. In support of his theory Galvani adduced the further experiment of connecting the frog's spinal cord with the muscles of the legs by a tingle piece of wire, when the same results, though discoveries U0t f S violent a character, were produced. Further, he showed that similar effects might be obtained with- out any metal at all, by taking the sciatic nerve of a recently killed frog and letting it come in contact with the muscles of the frog's thigh. In this way the contest went on for a considerable time, and from it have arisen results far exceeding in magnitude anything that its first observers in their wildest dreams could possibly have imagined. From the simple and apparently trivial discovery that a dead frog's legs may be made to jump, have sprung all those wonderful inventions which culminate in the electric telegraph and the telephone. As it will be our duty hereafter to show, both these philosophers Modern theory were P ai % right in their surmises as to the reason of of voltaic the phenomena they observed. The existence of animal nwty. electricity has been, since Galvani's day, abundantly proved, while the passage of electricity from one metal to another ELECTRICITY AND MAGNETISM. 59 through a conducting medium forms the basis of that science to which we have now to direct our attention. Volta, in the prosecution of his researches for facts to support his theory, constructed what has since been known as Vol , g Ue Volta's pile, a representation of which is given in our next diagram, fig. 40. It consisted of pairs of zinc and copper discs, each pair separated from the next by a piece of cloth soaked in water acidulated with sulphuric acid, the base of the pile being formed by a single zinc plate, its summit by one of copper. The pile was in its finished form mounted in a frame, as represented in our diagram. The discs of copper and zinc, with the exception of the terminal ones, were soldered together, and from each end of the pile a wire passed in such a manner that the two wires might be joined together, or might be attached to any body to which a charge of electricity was required to be given. When the construction of his pile was completed, Volta found that the effect upon a dead frog was the same as in his first experiments, but Effectsofthe much more powerful, while if the two wires were held in the hand a shock was experienced similar in character to that obtained from a very feebly charged Leyden jar. Further, he found that when the wires were united no such effect could be obtained, but that the temperature of the wires was con- siderably increased. These facts ascertained, Volta fancied he saw a complete verifica- tion of his contact theory of electricity, and it remained only to con- nect these facts with those relating to the electricity of an ordinary machine. For this purpose he invented Vol t'a contact and employed the condensing gold-leaf electroscope which we have already described in connection with Frictional Elec- tricity. Upon connecting first the zinc and then the copper plate with the collecting plate of the electroscope, the other plate being in 60 ELECTRICITY AND MAGNETISM. each case connected with the earth, he found that while the copper plate exhibited positive electricity, that upon the zinc was negative. In other words, the copper formed the positive, the zinc the negative pole of the pile. . ., This experiment appeared to Volta as a certain and unmistakable Volte's error. pr f f the correctness of his theory that the metals were of themselves endowed with opposite electricities by the mere fact of their coming in contact with each other, he regarding the moistened pieces of cloth which alternated between them as furnishing merely a good conducting medium. It was, however, in this latter supposition that Volta's error lay It was soon noticed that the zinc plates of the pile became corroded' and this fact at once drew attention to the acidulated water with which the cloth discs were moistened. Opposition to Volta's theory soon sprang up, and the importance of the acidulated water was at FIG. 41. first more than fully recognised, the whole credit being given to it r the production of the facts observed. As we shall, however see the truth lies between these two extremes ; the mere contact of two ifferent metals being sufficient to produce in them opposite electrical condttion,, but the presence of a body capable of setting up chemical action being also necessary for the production of what we shall here- after know as an electric current. The next step in advance was to construct a more convenient The Crown source of electricity than the pile, and this took the ' A modified form of this is shown in fig. 41. As will be seen it r acidulated water or brine. A number of strips of zinc r'^ stri r of , - opper ' c > *> - to 2 barS being reversed > one le g of the U was in one jar, and the other in the next jar, in such a manner ELECTRICITY AND MAGNETISM. 61 that each jar contained a strip of zinc and a strip of copper at- tached respectively to the copper and zinc of the jar on either side of it. The terminal strips of zinc and copper were, however, unattached, but instead had copper wires soldered to them. The crown of cups and the pile are exactly similar in construction, the discs being re- presented by the united strips of zinc and copper, and the moistened cloth by the acidulated water or brine. Here we have the simplest form of voltaic battery, and from it we shall be enabled to trace the various modifications which have from time to time been made in its construction; but it will first be neces- sary for us to show the nature of the electricity obtained by chemical FIG. 42. action, and its connection with that obtained by means with which we are already acquainted. For this purpose take a simple glass or earthenware jar, as shown in fig. 42. In this jar place a strip of zinc, z, and a strip of copper, c. As it is impossible to obtain pure zinc, the strip used must be covered with quicksilver, or the acid will act upon it in the ordinary way and produce chemical decomposition without available electricity, and thus vitiate the experiment. As we shall frequently have to speak of amalgamated zinc, it may be as well to give at once the necessary instructions for coating the metal with mercury. The process is an extremely simple one. First thoroughly clean 62 ELECTRICITY AND MAGNETISM. the zinc by immersion in dilute sulphuric acid and rubbing it if necessary, then pour a little mercury upon it and spread it by rul bing it with a rag until the zin/; is uniformly bright. Take then such a strip of amalgamated zinc and a strip of copper, and place them in the jar, and add some sulphuric acid diluted with about twelve times its bulk of water. It will be found that, as long as tlie metals do not touch each other, no change will take place. If, however, the metals be allowed to touch either inside or outside the liquid, a sensible com- motion is immediately set up, and bubbles of gas (which may be shown to be hydrogen) collect upon and rise from the copper plate. The same effect may be also produced by connecting the zinc and copper plates outside the liquid by a wire. Let the wire be broken, and immediately the action ceases, while if the two ends be again united the action as immediately recommences. By means of a delicate condensing electroscope it may be shown that the two plates of metal, as soon as they are immersed in the liquid, and before they are connected with each other, assume opposite electric conditions, the zinc exhibiting a feeble charge of negative electricity, the copper a similarly feeble charge of pwitive. Here, then, we have one proof of the identity of the two electricities, inasmuch as both affect the gold-leaf electroscope in the same way. Another proof may be obtained as follows. Take your cell and stand it so that the wire connecting the plates shall be in a north and south direction. Now bring near the wire a magnetic needle. It will be found that the needle is deflected from its normal position Again, if an ordinary sewing needle or any similar piece of steel be laid above and across the connecting wire, it will be found after a short time that the needle has become magnetized. It will be found, moreover, that, as in fig. 43, the end A of the needle will point to the north, but if the wire o be connected with the copper plate instead of the zinc, so that the current flows in the opposite direction, as in fig. 44, the end B will point to the north. To both these facts parallels may be obtained by means of frictional Parallel effect, J^S*' F r j nStailCe > if ^ke a long coil of silk th frictional or cotton-covered wire, and place in it an "astatic" electricity, needle, see p. 112, the needle will be deflected, when through the con' "" C ndUCt r * * maCMne ^ & ^ itself earth, the needle will after a time become magnetized; and ELECTRICITY AND MAQNETISM. 63 moreover, if the direction of the current be reversed, the polarity of the needle will also be reversed. (See page 42). Here then we have evident traces of agreement between the elec- tricity of the machine and the electricity obtained from a galvanic batteiy, yet at the same time we have evidences that there are some differences in the nature of their manifestation. In point of fact, the two electricities differ in quality, though they are the same in kind. As we have before seen, the electricity obtained by friction is pro- duced in a very concentrated form. It can leap across very considerable intervals, and a brilliant spark may Diffe 2| l t l es in be obtained from the most simple apparatus. As we say, its tension is very high, and hence this electricity is times spoken of as high-tension electricity. On the other hand, the electricity obtained even from a considerable galvanic battery cannot pass across the most trifling interval, its passage being stopped at once if the wires from the poles are separated the most minute frac- tion of an inch. Its tension we say is very low, and consequently it is often spoken of as low-tension electricity. When, however, we come to take into account the quantity of electricity produced by the two methods in question, we find that that produced by chemical action vastly exceeds in quantity that produced by friction. It is only with Differences in extreme difficulty and with powerful apparatus that frictional electricity can be made to decompose water into its two gaseous elements, hydrogen and oxygen ; while, as we shall presently e _ 45 a consideration of the rationale of the method by which the latter is produced. We have seen that when a plate of zinc and a plate of copper are placed in dilute sulphuric acid or brine they assume opposite elec- trical conditions > and we nav e seen further that when voltaic cell. * the two P lates are brought into contact with each other, or united by a wire, certain changes take place in the liquid and in the wire which indicate the presence of electricity We see the bubbles of gas on the copper plate ; we find that the zinc plate is gradually being eaten away ; we find that a magnetic needle placed over or under the wire is deflected from its normal position we find that the wire is slightly heated, and that it is capable of attracting iron filings ; and we find/ moreover, that all these effects instantly cease when either the wire is broken or one of the plates lifted from th<* liquid. Now, we are unfortunately totally ignorant at present as to the ultimate cause of all these phenomena, but they almost irresistibly suggest to us that something is passing along the ELECTRICITY AND MAGNETISM. wire and through the liquid ; and that something, because it appears as it were to flow through the system, we, for want of a better name, term an electric current. Moreover, for purposes of convenience, we determine to consider the course of this current as being from the copper to the zinc in the wire, and from the zinc to the copper in the liquid. It must, however, be perfectly understood that the actual flow of electricity is not a proven fact, but, like the direction in which it is supposed to flow, only a very convenient theoretical convention. A change in the condition of the connecting wire the facts observed abundantly testify, but as to what is the exact nature of that change of con- dition nothing has as yet been satisfactorily ascertained. Before leaving this part of ; of our subject, it will be well to mention one or two^^JJj"*^ experiments which tend to confirm the truth of the so-called " con- tact theory " of the origin of the electric current. As we have already mentioned, it is assumed by the defenders of this theory that the mere contact of two dissimilar metals causes them to as- sume opposite electrical conditions, and that the metals thus touching are maintained in these oppo- FIO. 46. site electrical states. In proof of these suppositions the following experiments may be adduced. If, as in fig. 45, a positively charged needle be suspended over the line of junction of two semicircular plates of zinc and copper, the needle will turn towards the copper, as shown in the diagram by the arrow proving that the copper must be negatively and the zinc positively electrified. Again, if. as in fig. 46, a small bar of copper, b, be soldered to a similar bar of zinc, a, and the latter being held in the hand, the compound bar be brought into contact with the lower unvarnished surface of the plate of a condensing electroscope, while the finger is placed upon the upper unvarnished surface of the plate of the con- denser, it will be found, upon removing the bar and the finger, and 66 ELECTRICITY AND MAGNETISM. subsequently raising the plate of the condenser, the leaves diverge with (as may be proved in the usual way), negative electricity. Here positive electricity must accumulate on the upper and negative upon the lower plate of the electroscope. But if the compound bar is reversed, so that the copper end is held in the hand, it will be found upon proceeding as before that the leaves of the electroscope will not become charged with positive electricity. And if the zinc end of the bar is held in the hand and protected by flannel, or similar sub- stance, no electricity is produced, even though the rod is connected with the earth or with the upper plate of the condenser by a wire. Now, there are two ways of looking at this latter experiment. The opponents of the contact theory assert (and it would seem that they are correct in doing so) that the development of electricity here ia due to the chemical action nt^ f the 8aline Perspiration of the hand upon the zinc. On the other hand, the supporters of the contact theory, while they admit the chemical action between the moisture of the hand and the metal, regard the per- spiration merely as a conductor which undergoes decomposition, and in the second case explain the non-accumula- tion of electricity as due to the equal and opposite current which is, they say. set up between the condenser and the zinc. With regard to the first experiment, which seems at first sight most conclusively to support the contact theory, it is argued by the opponents of that theory that the oxygen and moisture of the air so act upon the discs of zinc and copper as to corrode the zinc and form what is really a very feeble galvanic cell , ; \ WC haVe re P resented "* experiment designed to illustrate the production of electricity by the action of a single liquid upon a 'Electricity from Smgle "^^ T an ordinar y gold-leaf electroscope a a single metal. zlnc ^ late > Zn - is attached, and to this a narrow strip of zinc, zn, is soldered at one end, the other being left free. Upon the zinc plate is placed a very thin, dry, varnished glass plate, G, and upon L this again a little water, A w, acidulated with sulphuric S J f ^ f the Zinc StTi P zn is then brou ght into contact iTat f T f * ^^ ^ UP n liftin S the gl^s plate, G, the leaves o the electroscope diverge with negative electricity. Positive electricity must therefore have accumulated in the acidulated water ELECTRICITY AND MAGNETISM. 67 If copper be substituted for the zinc, both in the piate and in the strip zn, a negative charge is still given to the leaves of the electro- scope ; but it is of a feebler character than the charge obtained when zinc is employed in the experiment. Let us now return to our jar of acidulated water and metal plates or, as we must call it, our " voltaic cell." We see that when the two plates are unconnected no chemical action takes place, but that immediately they are united, either inside or outside of the liquid, the zinc is violently attacked. Exp^a^,, of Let us see if we can, in the light of the principles we changes in the have just established, give some explanation of this ceU> phenomenon, and ot the production of electricity by its means. ' For the sake of simplicity, we will first suppose the liquid in the cell to be pure water, neglecting for the present the sulphuric acid which it contains. Water is composed of two gases, oxygen and hydrogen, which having a certain strong liking, or, as we say, chemical affinity for each other, remain, so long as they are not too power- fully affected by external circumstances, firmly united Thejnolecular together to form this well-known liquid. Now, just as with every other kind of matter, whether solid, liquid, or gaseous, the physicist believes water to be made up of a vast number of extremely minute particles which he calls " molecules," or little masses, each of these molecules being so extremely small, that by no mere physical process can it be subdivided ; but nevertheless, small as it is, being to all intents and purposes water just as much as the largest body of that fluid which it is possible to imagine. Here the physicist stops. This molecule, which he defines as " the smallest particle of a substance which can exist in a Definition of* separate state," he takes as the unit which he employs molecule, in all his theories and calculations. But where the physicist stops the chemist comes in and takes up the tale. He takes the molecule of the physicist and by chemical decomposition splits it up into its component atoms, defining an atom as the smallest particle of an ele- mentary substance that can be moved in the molecule or from one molecule to another. This atom is the unit of the chemist, in the same way as the molecule is the unit of the physicist. If we have made ourselves perfectly clear, it will be understood by our readers that the molecule is the smallest particle of a substance, whether simple or compound, which can be obtained by mere physical division, when carried to the farthest extent which the imagination can conceive ; while the atom is the smallest particle of a simple ELECTRICITY AND MAGNETISM. substance or clement which can be obtained from the chemical dp- composition of a molecule. Employing, then, in this sense the words atom and molecule, we say that each molecule of water is made up of three atoms, two atoms of hydrogen and one atom of oxygen. This fact the chemist signifies by writing the symbol for water, in a kind of chemical shorthand, OH 3 or HjO. Now, when in our cell the zinc and copper plates are united, a disturbance of the chemical equilibrium of the water takes place. Strong as is the affinity of oxygen for hydrogen, it has a much greater liking for zinc. The molecules of water are decomposed the oxygen uniting with the zinc to form oxide of zinc, and the hydrogen bein? given off at the copper plate. to this fa f , m lecules of watcr -^etching zinc to the copper plate. The zinc plate, having a greater Chemical \ ^ ^^ tbm the CO PP er > attracts towards affinity. f the oxygen of the first molecule of water in our hydrogen ThThT 7 Chain> thereby weakcai e j ts hold upon its bu t inS S 10Dg M the P lates are -connected, the connection is made the positive electricity at the ELECTRICITY AND MAGNETISM. 69 end of the wire from the copper unites with the negative electricity at the end of the wire from the zinc, and an electrical discharge takes place through the whole system or circuit. The oxygen of the first molecule of water in our chain is torn away from its hydrogen, and unites with the zinc to form oxide of zinc ; while the released hydro- gen unites with the oxygen of the second molecule, which is in its turn decomposed,"and this decomposition and rearrangement passing along the whole line of molecules, the hydrogen of the last having no oxygen with which to unite, and having no affinity for copper, is there given off in the state of gas. This discharge of course produces electrical equilibrium, but the same phenomena being continuously repeated result in that apparent flow of electricity which we denominate an electric current. We are now in a position to learn that an electric current is pro- duced whenever two joined conductors are dipped into a liquid that acts more vigorously upon one than upon the other. It is usual to apply the name of electro-motive force to the force thus produced. If our principle be true we can at once prophesy the result of employing two plates of the same metal instead of plates of different metals. Suppose, for instance, we take two App ^* tion of plates of zinc. Here obviously the action will be the same on each, and the electro-motive force, which is due to the differ' cnce between these actions, is of course nil a result which is amply verified by experiment. From the same principle we may naturally be led to expect that the greater the difference between any two metals in their liability to be acted upon by the liquid employed, the greater will be the electro-motive force produced ; and this is found to be correct. In our explanation of the production of an electric current in a cell we assumed the liquid employed to be water, although, as we have stated, dilute sulphuric acid is generally used. H .. . The reason for this is that the oxide of zinc formed d-ictivity of does not readily dissolve in water, but tends to cling to water. the zinc plate, and, being a bad conductor of electricity, it impedes the action of the cell. The explanation of the chemical action which takes place when sulphuric acid is employed is precisely similar to that just given for water. The molecule of sulphuric acid consists of one atom of sulphur, two atoms of hydrogen, and four atoms of oxygen a fact which the chemist expresses in his symbolical notation by HjS0 4 . Fromthis compound molecule the hydrogen is displaced, and the S0 4 , or sul- pliion as it is sometimes called, unites with the zinc to form sulphate of zinc (ZnSOJ. 70 ELECTRICITY AND MAGNETISM. ' By a knowledge of the relative facilities with which the metals unite with oxygen, we may easily predict, not only the proportional Negative and ma g ni t the dilute sul P huric acid unites ^th the oxide of zinc (formed by the union of the oxygen evolved at the zinc pole with the zinc), and forms sulphate of zinc. The disengaged hydrogen of the molecule of sulphuric acid imme- diately adjacent to the zinc plate reacts upon the next neighbouring molecule and displaces its hydrogen, hydrogen gas at the immediate moment at which it is disengaged from a compound, or as it is usually expressed when in its " nascent " condition, having a greater power of exciting chemical action than at any other time. The hydrogen displaced from the second molecule of sulphuric acid reacts in like manner upon the third, and so the process goes on from molecule to molecule until the porous cell is reached. Here the nascent hydrogen meets with the sulphate of copper, and displaces from its nearest molecule the copper, forming with its other atoms sulphuric acid ; and this copper, reacting upon the copper of the next molecule, dis- places its copper and so the process is carried on and the molecule adjacent to the negative plate deposits its copper upon the copper plate. By this it will be seen that in the outer cell sulphuric acid is formed, which of course tends to replace the sulphuric acid used up in the porous cell, while the copper plate is increased by the de- posihon upon it of the copper resulting from the decomposition of the sulphate of copper. To replace this used-up salt, crystals of suphateof copper are placed upon the shelf attached to the copper pole. The amount of sulphuric acid liberated in the outer cell is ELECTRICITY AND MAGNETISM. regular, and is proportional to the acid used up in the porous cell, which causes the action of the latter upon the zinc to proceed regularly and to keep the current constant. The porous cell is employed to prevent the liquids from mixing together thoroughly, but at the same time to allow the current to pass. Another well-known form of constant battery is that invented by Grove, of which a representation is given in fig. 52. Here A is a glass or earthenware vessel containing dilute sul- phuric acid of the same strength as that employed in Grove's Daniell's battery; B is a thick sheet of amalgamated zinc bent into the form of the letter U, with a piece projecting from the cell so as to afford a means of junction with the neighbouring element ; C is a porous cell containing strong nitric acid ; and D is a B ' "^ C Xfc b A i - - eM --~-- - h - -- --\ 1 m plate of platinum, which when the cell forms part of a battery is clamped to the zinc of the next element. Here, as in the Daniell's battery, the zinc is in contact wilh dilute sulphuric acid, and therefore, as before, we shall have sulphate of zinc formed in this portion of the cell, and hydrogen set free. At the porous cell, however, this hydrogen meets, not with sulphate of copper, but with nitric acid. From this nitric acid the hydrogen takes part of its oxygen, with which it forms water, while the acid is by this process reduced to a compound containing less oxygen. Here, then, as before, the hydrogen is got rid of by compelling it to form a compound with some other element present in the battery. Grove's form of battery is one of the most convenient, as also one of the most powerful forms of a constant battery, but is expensive, owing to the high price of platinum. The fumes which it gives off are also objectionable. 76 ELECTRICITY AND MAGNETISM. A form of battery frequently employed instead of that of Grove is one invented, in 1843, by Bunsen, and known by his name. It is represented in fig. 53. Here V is the glass or Bunsen'. porcelain cell containing dilute sulphuric acid ; Z is a battery. ^^ cylinder of zinc . p is a por ous cell containing strong nitric acid, in which is inserted a cylinder of gas coke, c. The connection of the cells with each other is made in the same way as with a Daniell's cell by strips of thin copper. The theory of the mode of action of this cell is the same as that in Grove's battery described above, than which it is much cheaper. It is used almost universally in France and Germany, but is more expensive to work and more clumsy to manipulate. Beside these more common forms of constant battery, there are two or three other forms of more recent invention which deserve a passing notice. Sulphate f One of these is represented in fig. 54, and is the mercury battery, sulphate of mercury battery devised by M. Mari6 Davy. Like the Bunsen battery, each cell contains a hollow zinc cylinder, Z, a porous cell, P, and a carbon cylinder, c. The outer vessel contains ordinary water or brine, and the porous cell sulphate of mercury. The sulphate of mercury is well shaken up with about three times its volume of water, in which it is with difficulty soluble. The liquid is then poured off from the sulphate, which is mainly left at the bottom of the vessel in the form of a thick paste. This thick paste is placed round the carbon cylinder so as to fill up the space between it and the porous cell, and then the solution of sulphate of mercury which was poured off is poured into the porous cell. When this battery is set to work, the zinc decomposes the water or brine, setting free hydrogen in the usual manner. This hydrogen acts upon the sulphate of mercury and reduces it, forming sulphuric acid ELECTRICITY AND MAGNETISM. 77 and metallic mercury. The latter is deposited at the bottom of the cell, and the former, passing through the porous cell, attacks the zinc, and thus increases the action. From the mercury which collects at the bottom of the porous cells, a quantity of sulphate of mercury may be formed, equal to that decomposed during the action of the battery. This form of battery soon fails when used continuously, but answers very well with dis- continuous work, such as that of the telegraph. In all the two-fluid batteries hitherto described, a porous cell is employed to separate the two liquids; others, however, Gravitation have been devised in which, owing to the difference batteries. between the specific gravities of the substances employed, no porous cell is necessary. 54. V BATTJ One of these so-called " gravity " batteries, devised by M. Callaud, of Nantes, is represented in fig. 55. Here an earthenware vessel, v, contains a copper plate, to which a wire A, insulated by gutta percha I, is soldered. Upon c a number of crystals of sulphate of copper are placed, and the cell being filled up with water, a zinc cylinder, Z, is placed in it. When the battery commences to act, the lower part of the liquid becomes saturated with sulphate of copper, and, the action of the cell being similar to that of a Daniell's cell, sulphate of zinc is formed, which, owing to its less specific gravity, floats upon the top of the solution of sulphate of copper. A variation of Daniell's cell, known by the name of Kenotti' its inventor, Menotti, is represented in fig. 56. battery. Here the porous cell is replaced by a layer of sawdust or sand, B C. At the bottom of the containing cell v a layer of coarsely powdered 78 ELECTRICITY AND MAGNETISM. sulphate of copper, A, is placed, and upon this is laid a copper plate, provided, as in Callaud's battery, with an insulated wire, I. Upon the top of this is placed the sand or sawdust, and, the whole being filled up with water, a zinc cylinder, z, completes the arrange- ment. The action is precisely the same as in a Danielle cell. A great objection to the sand or sawdust is the considerable resistance it offers to the current, A form of one-fluid battery which is coming somewhat generally into use for telegraphic purposes is that known as Leclanche's battery. In it a rod of carbon in a porous cell is tightly packed Leclanche's j n a Yes sel containing peroxide of manganese and coke battery. mixed together. In the outer cell a zinc cylinder is placed. The liquid employed is a solution of ammonic chloride (sal- ammoniac). When set to work chloride of zinc is formed and hydro- gen and ammonia set free. The latter escapes as a gas, while the former is oxidized by the manganese, and forms water. A form of battery recently devised by Mr. Fleming has both poles Fleming'* ^ tne same metal. Fig. 57 represents this form of battery. battery. As will be seen from the letters under the cells, two liquids are employed in alternate elements, nitric acid in one cell, and penta. sulphide of sodium in the next, and so on throughout the battery. Each cell contains a plate of copper, Cu, and a plate of lead, Pb but it will be noticed that the copper of one cell is connected with the copper of the next, and not, as would be the case in another battery, with the lead. By using an even number of elements the terminals of such a battery will both be of copper. This arrangement is a very good example of the fact previously mentioned that the ELECTRICITY AND MAGNETISM. 79 polarity of metals to each other depends upon the liquid in which they are immersed; Thus, as in the battery now under consideration, lead is positive and copper negative when the liquid employed is nitric &cid, but in penta-sulphide of sodium the copper plate becomes positive and the lead negative. Consequently, in each element the current through the wire will be in the direction of the upper arrow, and through the liquid in the direction represented by the lower arrows. In speaking of constant batteries it should be understood that the term is used relatively, no battery being absolutely constant. We have already seen, in our inquiries into the origin of the voltaic current, that it results from certain chemical decompositions con- stantly going on in the cells of a bat- tery, and we have also seen that the attraction of the poles of the battery is sufficiently strong to tear apart the atoms of which compound bodies are composed, and so to maintain that constant condition of chemical activity upon which the existence and main- tenance of the voltaic current entirely depend. We have now to turn our attention to n consideration of some important and interesting facts, which tend to show us that, not only within the generating cell itself, but also at a distance from it, the electric current may be made of service in the decom- position of various compounds into their respective elements. At the commencement of the present century Nicholson and Carlisle simultaneously discovered that water could be decomposed by the voltaic current into its con- stituent elements oxygen and hydrogen, and thatdecom- Decomposition position is now exhibited as one of the stock experiments of the physical lecture table. Various forms of " voltameters " (as the instruments for decom- posing water are termed) have been devised ; but by far the simplest form, and one which we should strongly recommend our readers to make for themselves, is the one shown The voltameter - in fig. 58. It consists essentially, as will be seen, of a glass basin, into the wooden bottom of which two small strips of platinum have been fastened, and to these pieces of copper wire, which ate also in con- 80 ELECTRICITY AND MAGNETISM. uecbion with the binding screws a, J, are attached, while' from the latter wires proceed to the battery. Over the two strips of platinum two test-tubes are placed, and in these the gases which result from the decomposition of the water are collected. Before proceeding to describe the action of this apparatus, it will be well to define certain. terms which are employed in this depart- ment of electrical science. First, then, the decomposition of any compound by the electric Explanation of current is termed electrolysis, from the two Greek technical tenn. words elelttron, amber, and luv, I loose or set free. The liquids which undergo electrolysis are termed electrolytes. The platinum or other terminals jvhich are placed in the electrolyte are termed electrodes, from the Greek words elelitron, amber, and odox, which signifies a way. The electrode which constitutes the positive pole of the battery is called the anode, from the Greek word anodos, an ascent, because it ia by the anode that the current enters the electrolyte. The electrode which constitutes the negative pole of the battery is termed the katlwde, from the Greek word kathodos, a return or descent, because at the kathode the cm-rent leaves the electrolyte. The products of electrolysis which appear at the kathode are called Itations ; the elements appearing at the anode are termed anions. Having made ourselves acquainted with the technology of electric decomposition at a distance from the battery, we may now proceed to consider what takes place when water is decomposed in the appa- ratus just described. A battery of two or more cells is connected by means of its terminal wires with the binding screws of the voltameter, and the glass cell and test-tubes are filled with acidulated water, the acid aeC water 1S * being added to increase the conductivity of the liquid. As soon as the connection is complete, bubbles of gas appear at the electrodes, and the water in the test-tubes begins to be gradually displaced by these gases, which occupy the upper portion of the test-tubes. It will be noticed very early in the experiment that the liquid in the test-tube over the anode does not sink so rapidly as that in the other tube, in fact, the quantity of gas in the latter throughout the experiment will be found to be about twice that in the former. These two gases are the results of the decomposition of the water, which has by the force of the electric current been decomposition. *P* U P iuto its elementary constituents, oxygen and ' hydrogen, the former being evolved at the arode, the latter at the kathode. From the fact that the hvdroscn is twice as ELECTRICITY AND MAGNETISM. 81 great in quantity as the oxygen, we are justified in concluding that two volumes of hydrogen united with one volume of oxygen form that familiar fluil water. But our readers are probably tempted to ask, " How do we know that these gases are oxygen and hydrogen ? " And we reply, From the following facts : If we prepare oxygen by some other method than by the decom- position of water with the electric current, we find that it is an invisible, colourless gas, without taste or smell, which p ro ertieg ef has the property of supporting combustion very power- oxygen and fully. If, for instance, a smouldering match be plunged hydrogen, into a jar of it, it is re-kindled ; if any body which burns in ordinary air very feebly (such, for instance, as charcoal) be ignited and plunged into oxygen it burns very rapidly and vividly ; while some substances, such as iron and steel, which will not burn at all in air, may be made to burn rapidly and brilliantly in oxygen. If the test-tube be removed from the anode of the voltameter, and a half -extinguished match be plunged in it, it will be re-kindled; and if the gas be collected in sufficient quantity the other tests for oxygen may be successfully applied. In this way the gas given off at the anode may be shown to behave precisely as oxygen does, and it can easily be seen that it is, like it, an invisible gas. We are, therefore, justified in concluding that it is oxygen. In the same way we know hydrogen to be an invisible gas, which will not in the same way as oxygen support combustion, but which will itself burn with a feeble flame. If the test-tube, placed over the kathode, be removed when full of gas and a light be applied to it, it will burn with a pale flame, giving but little light. Finally, if through a mixture of oxygen and hydrogen (in the proportion of one volume of the former to two of the latter) an electric spark be passed, the two gases will combine and form water, no residue of gas being left uncombined. In making a voltameter we should advise our readers to get a large glass funnel and cut off the stem. Then fill the bottom up with plaster of Paris, into which, while moist, place the platinum strips, having first soldered to each a piece of stout copper wire. The glass vessel thus formed can then be easily fastened into a wooden stand, and the wires from the electrodes soldered to two binding screws on the stand, and to these binding screws the wires fiom the battery can be attached when the voltameter is used. 42 ELECTRICITY AND MAGNETISM. Another plan is to stop the bottom of the funnel-top with a la compounds may be readily divided into two other classes those which are and those which arc not compounds, susceptible of decomposition by the electric current. With regard to gases and vapours, we find that all of these without exception refuse to conduct the current, and therefore cannot be decomposed by it, as it is an indispensable and indeed obvious con- dition that an electrolysis must be a conductor. Solids may be divided into conductors and non-conductors: but those which conduct the current undergo no chemical change, and current ^ ^ C ndUCt Cami0t f ^^ ^ &ffectcd by thc Some liquids, such as molten metals and mercury, conduct freely without bemg decomposed by the current ; others refuse altogether by. athird ciass Some of the earliest, and at the same time most valuable results Sir Humphrey of electrolysis were obtained by Sir Humphrey 'Davy diSi. e year 1870 ' ^ which he succeeded in showing di. ' ucceee n sowng w P r P i r f SeV6ral substances formerly thought to be elements were in reality compounds. bv hH ? perated upon by Davy was OKlinar y p tash > , by a battery of about 250 elements, he succeeded in splitting Potassium. U P nd showing that its base consisted of an extremely Soda n electroljtioaHy e^tacd by Iv y , M d Iron, it he Wtan. 3 8ucceed J " bt totag a metal greatly resembling potassium, which he named sodium. ELECTRICITY AND MAGNETISM. 83 In these experiments it was found necessary to fuse the potash and soda, as in the solid state they, in accordance with what we have stated above, refused to conduct the current, and could not, there- fore, be affected by it. In the decomposition of potash and soda we have a process precisely analogous to that which we have when water is decomposed. One clement (oxygen) goes to the positive electrode, and the other (potassium or sodium, as the case may be) is liberated at the negative electrode. Until the molecules of the potash or the soda are to a certain extent separated by heat, the current is powerless to effect their decomposition. The substances separated at the positive electrode are, in accordance with our frequently enunciated law of electrical attractions, termed electro-negative elements, while those liberated at the negative electrode are termed electro-positive elements; and it is possible to arrange the elements in such a way that any one is electro-negative to any below it, but electro- positive to any one above it. Such a list is termed an electro-motive series. In such a series oxygen would be placed at the top and potas- sium at the bottom. It must be clearly understood that any element which is electro- positive in one combination may be electro-negative in another, and vice versa. A very simple electrolytic experiment, and one which may be performed with a single cell, is the decomposition of iodide of potas- sium. This substance consists of iodine and potassium, Electrolysis O f and in its electrolysis the iodine is set free at the posi- iodide of tive electrode and the potassium at the negative potassium, electrode. In performing the experiment with a single cell, a piece of bibulous paper (such as blotting-paper) is soaked with a solution of starch and iodide of potassium. This paper is placed between the electrodes, and at the place where the positive pole touches it a blue spot is pro- duced, due to the action of the liberated iodine upon the starch, the production of this blue colour with starch being an infallible test for free iodine. The electrolysis of iodide of potassium, as just described, brings us to a consideration of the electrolysis of those compounds which the chemist terms " salts." Without going into the exact details of the meaning which the chemist Elec g ^J ais of attaches to this term, we may say that a salt is a com- pound in which a metal is always present, usually in combination with an acid. 84 ELECTRICITY AND MAGNETISM. These salts may be decomposed by the electric current when brought to a state of fusion by heat, or when existing in a state of solution. For their electrolysis in the latter condition, which is by far the more convenient one, the Toltameter mentioned on p. 79 may be employed, or instead a piece of strong glass tubing bent into the form of the letter V, and supplied with platinum electrodes, may be used. This latter simple piece of apparatus is represented in fig 59. The decomposition of a solution of common salt, which consists of the metal sodium and the gas chlorine combined to- Decomposition g e th er i n equivalent proportions, forms a very interesting ' experiment, as not only the salt, but the water also, is decomposed by the passage of the current. To make the experiment effective, the solution should be coloured blue with indigo or litmus. If this be done, chlorine being a very powerful bleaching agent, its liberation will be marked by the disappearance of the colour of the solution. As soon as a current is sent through the solution, the oxygen of the water and the chlorine of the salt appear at the positive electrode, while the hy- drogen of the water in company with the sodium of the salt are liberated at the negative electrode. Another experiment in electrolysis, which is rendered more effective by colouring the water with litmus or some other vegetable blue, is Electrolysis of ^ decom P osition of sulphate of soda. This salt is sulphate of soda. formed b J the action of sulphuric acid upon soda, and when an electric current is made to pass through it in solution, the connection between the soda and the acid is severed, the former being attracted to the kathode, tlie latter to the anode. It being one of the special properties of an acid to turn blue vegetable colouring-matters red, the presence of the sulphuric acid will be marked by the reddening of the solution. Some of the most beautiful experiments which can possibly be performed are those connected with the electrolysis of the salts of Electrolysis of silver and lead > of which the nitrate of silver and the 8UVe salS l6ad a etate f lead aff rd the best exam P les - In thcse experiments crystals of silver or lead appear upon the negative electrode ; and if the decomposition be effected in a glass cell, tfic image of which is, in a darkened room, focussed upon a ELECTRICITY AND MAGNETISM, 85 screen, the experiment forms a most effective and beautiful lecture illustration. By reversing the direction of the current through the cell, the crystals may be seen at once to appear at the opposite electrode, and to dissolve off from the one at which they were first seen. The discovery of the electrolysis of metallic salts soon developed into commercial importance, and led to the now well-known process of electro-plating which simply consists of the decomposition by the electric current of some compound of the metals, gold, silver, copper, etc., and the consequent deposition upon materials placed in the solution of the particular metal with which the articles are required to be coated. It would be, of course, beyond the purpose of these papers on electricity to go into the details of such a special subject as that of electro-plating, which has long since been raised to the condition of a fine art ; it must therefore suffice to give Hectr -P latin S- a general idea of the manner in which the process is carried out in the coating of the baser metals with silver. Fig. 60 represents a simple form of apparatus by which this process may be effected. A is a jar containing a solution composed of 100 parts by weight of water, 10 parts of cyanide of potassium, and 1 part of cyanide of silver. Across the top of this jar is placed a metal rod, B. and from this rod the article (we will suppose a spoon) to be coated is suspended by a metal wire, coated with wax except where it is attached to the spoon, c is another metal rod, from which is suspended by a silver wire a silver plate, D. The rod 86 ELECTRICITY AND MAGNETISM. B is in connection with the zinc plate and the rod C with the copper plate of a Danicll's cell. The article to be plated must, before its immersion in the solution, be very carefully cleaned, in order that any grease spots, etc., may be removed. When the current passes, silver is deposited upon the spoon, which is, as will be seen, the negative electrode, and the released cyanogen dissolves from the silver plate a quantity of that metal equal to the quantity deposited upon the spoon, and by this means the solution is kept at its original strength. In the course of from one to two days the process is complete. Before concluding our notice of the interesting subject of electro- Faraday'g lysis, we must not omit to notice the laws by which electrolytic laws, the process is governed, as discovered and formulated by Faraday himself. It was early noticed by this distinguished investigator, that if the same current were sent through a succession of cells, containing different substances, the same weight of substance was not decom- posed. If, for instance, the current is sent successively through water oxule of lead, chloride of lead, iodide of lead, and chloride of silver, the weights of these substances decomposed by the current are re- spectively 9, 111-5, 139, 230-5, U3'5. It will be found further, that m the first case the weight of oxygen will be to the weight of hydro- gen produced in the proportion of 8 to 1 ; in the second case the proportion of oxygen released to that of the lead deposited will be as 8 to 103-5 ; in the third case the proportions between the chlorine and lead resulting from the decomposition will be 35'5 to 103-5 in the case of iodide of lead the proportions between the iodine and the mete will be as 127 to 103-5 ; while in the last case the proportion of chlorine liberated to the silver deposited will be as 35-5 to 108 g Now, these figures must evidently have a meaning, and the meaning is at once evident when we turn to a consideration of what are known Atomio f th6 at mic wei g hts of ^e elements. The term weights. atomic weights" requires some explanation. As our readers probably know, or as they may easily learn from our handbook of Chemistry in this series, there are known to :hem ls ts about sixty-five simple substances, which are termed Ele- Tta / r T f thes * elements thing simpler than themselves can be wkh ea rt u if 16SS WayS ln which these elements combine with each other result all the compounds known bu?kIheVl f T d * h l 0f . aU ^ese elements hydrogen is, bulk for ; S-- ELECTRICITY AND MAGNETISM. 87 weights, be compared ; and its weight is therefore called 1. With this standard the other elements are compared, and the weight of a given volume of the vapour of each, as com- pared with the weight of an equal volume of hydrogen under the same conditions, is taken as their "atomic" weight. Com- pared in this way, oxygen is found to have an atomic weight of 16; chlorine of 3 5- 5 ; and iodine, 127. Now, it is found that not only have these bodies the specific weights set down to them, but also they always combine with each other in these same definite propor- tions, or in multiples of these numbers. For instance, in a compound containing chlorine you may have 35-5 parts by weight, or 71 parts, or 106-5 parts of that element, but you will never get any proportion between these numbers existing. These numbers are consequently frequently spoken of as the combining numbers of the elements ; but from considerations into which it is not our province here to enter, it is regarded as certain that they do actually represent the atom, weights of the respective elements concerned. To return to the consideration of the electrolysis of the compounds mentioned above, we find that the weights of the elements resulting from their decomposition are in the exact proportion of p ropor tioate their combining or atomic weights, or else in some pro. weights of portion that is very simply related to these, as in the electrolyses case of water, in which we have one atom of oxygen united to two atoms of hydrogen, the weights of these elements present in water being therefore respectively 16 and 2, which are of course in the proportion of 8 to 1, given above. The first and fundamental law of electrolysis, as i, awg O f enunciated by Faraday, is therefore as follows : electrolysis. The same quantity of electricity that is, the same electric current decomposes chemically equivalent quantities of all the "bodies which it traverses; consequently,*^ weights of the elements separated in these electrolyses are to each other as their chemical equivalents. To this Faraday appended many other laws, of which we have already sufficiently dwelt upon the two following : Electrolysis cannot take place unless the electrolyte is a conductor. The energy of the electrolytic action of tfie current is the same in all its parts, We have seen that when water is decomposed by the electric current oxygen is evolved at the positive and hydrogen at the nega- tive electrode, and we know that therefore these gases are respectively electro-negative and electro-positive, From this knowledge it will easily be understood that when a current is sent through an electrolysis cell, the films of gas 83 ELECTRICITY AND MAGNETISM. which cover the two electrodes are capable of acting towards each other as plates of different metals would ; and that, in fact, a current may be set up in the cell, the oxygen playing the part of a copper plate and the hydrogen that of a zinc plate. This condition is known as the polarization of the electrodes; and when this polarization has been effected, upon disconnecting the battery from the electrolysis apparatus, a current passes through the water in the latter from the hydrogen to the oxygen. Such a current is termed a polarization current. As we have already seen, a similar current may exist in the cells of the battery itself, tending to destroy the original current. If instead of employing a simple electrolysis cell several be used, joined together so as to form a battery, upon sending a current through this battery all the cells become charged with oxygen and hydrogen films, and upon uniting the terminals of this so-called secondary battery, a current of considerable intensity may be obtained. A modification of the voltaic pile has been constructed upon this Hitter's principle, and is known from the name of its inventor, secondary pile, as Rater's secondary pile. This consists merely of a series of plates of the same metal sepa- rated from each other by pieces of bibulous paper or cloth soaked in acidulated water. A current is then sent through the pile, and, like the battery just mentioned, it becomes polarized, each plate having a film of oxygen on one side and a film of hydrogen on the other. Having now obtained some knowledge of the production and nature of electricity in its two forms of high-tension or frictional and low- tension or voltaic electricity, we have now to turn our Connection between attention to the connection between electricity and electricity and magnetism. The interesting and intimate relations that magnetism. ex j g ^ between these two forces were not brought to light in a complete way until Faraday worked at the subject his discovery of magneto-electricity being the grandest labour that adorns his memory. Of the two forms of electricity mentioned above, it is the low-tension that yields itself the more readily in tracing the connec- tion between electricity and magnetism, but we shall find in our subsequent study that this subject will lead us to the interesting topic of the relations existing between the two widely differing forms of electricity itself. But it will be first necessary to make ourselves acquainted with the most important phenomena of natural magnetism, and this subject will therefore occupy our attention for the next few chapters. ELECTRICITY AND MAGNETISM, 89 CHAPTER XII. MAGNETISM. Natural magnets Multiplication of magnets How to magnetize steel bars Simple experiments Results of the previous experiments Attraction re- ciprocal Law of attraction and repulsion The magnetic meridian Further experiments Molecular magnetism Illustrative experiment Condition of neutral bodies Further experiments Magnetic induction Coercive force Difference between iron and steel Magnetic curves Magnetic poles- Magnetic meridian Variation of declination Secular variations Agonic lines Isagonic lines Annual variations Diurnal variations Inclination or dip Variations in dip Magnetic measurements The declination com- pass How the compass is used The inclination compass Use of the inclination compass Possible errors The mariners' compass Intensity of earth's magnetism Magnetic attraction and repulsion Coulomb's torsion balance How the torsion balance is employed Methods of magnetiza- tionSingle touch Double touch Saturation of magnets Magnetization without magnets Effect of heat on magnets Electro-magnetism Mag- netism by electricity Electro-magnets Law of electro-magnetic attraction Difference between iron and steel Reason of the difference Diamagnetism Law of diamagnetic repulsion Comparison of intensities of magnetism and diamagnetisin. EVEN so far back as the days of Athenian greatness the Greeks were acquainted with the fact that a certain mineral, now known as the magnetic oxide of iron, had the property of attracting iron and steel, and had also the peculiarity of setting itself in a nearly north and south position when freely suspended. It has been suggested that the word magnet was derived from the fact that this iron ore was found in great abundance near Magnesia, a city of Lydia, in Asia Minor. This magnetic iron ore, the molecule of which has the composition of three atoms of iron to four of oxygen, and is written symbolically by chemists as Fe 3 O 4 , forms what is termed a natural magnet. The magnets most commonly in use are, however, pieces of steel, either in the shape of a bar or a horseshoe, to which magnetic properties have been artificially imparted, and which are therefore termed arti' ficial magnets. It being a special property of both artificial and natural magneta to impart to steel properly prepared magnetic properties, it will easily be understood that the number of magnets can be in- definitely multiplied ; and an artificial magnet being more convenient than a piece of loadstone, we will commence the consideration of the subject of the properties of mag- nets by examining the behaviour of an ordinary steel bar magnet. G 00 ELECTRICITY AND MAGNETISM. Such a magnet can be purchased for a shilling at a philosophical instrument maker's, and a horseshoe magnet can be bought for a few H . pence at the nearest toyshop. In point of fact, provided roagnetbe steel with the latter and one or two sewing or knitting bars. needles, the bar magnet may be dispensed with ; and although it is somewhat anticipating matters, we will, in order that our readers may be enabled to perform the experiments we are about to describe, tell them how they may convert two or throe of their needles into bar magnets. Take then one of the steel needles and lay it across the ends of the horseshoe magnet, and let it remain there for some time ; on removing it you will find that it has become a magnet. Do the same with, say two more of your steel needles (knitting needles are the most con- venient), and you will be fully well equipped for the following series of experiments. In each case make a mark with a file on that end of the needle which was in contact with the end of the magnet marked S. Simple * Bring one f y ur magnetic needles near any small experiments, iron or steel objects, and attraction will result. . 2. Sprinkle some iron filings (to be had for the asking at the lock- emith's) over one of your bar magnets. The filings will adhere very firmly, and in considerable quantity, to the ends of the needle, but at and near the centre of the needle no filings will be found attached. 3. Suspend by a piece of thread a small piece of iron wire. Bring either end of a bar magnet near the iron ; the latter will be attracted. 4. Now reverse the conditions of the experiment : let the magnet be suspended, and bring the iron wire near to either end ; the magnet will now be attracted by the iron. 5. Bring near to the marked end of a suspended bar magnet the unmarked end of another magnet ; they will attract each other. Now present to the same end of the suspended needle the marked end of the needle held in the hand ; the suspended needle will be repelled. 6. Suspend a magnetic needle and allow it to come to rest ; it will set in a direction which is nearly north and south ; and if it be caused to turn from that position by any external influence, it will return to its original position when free to do so. Results of the Befor e proceeding further with our experiments, let previous us pause to sum up the results of the practical inquiries experiments. we h ave j ug<; b een ma kj ng> From these, then, we see that a magnet has the property of attract- ing iron and steel, and that this attractive power resides principally in the ends of the magnet, which are therefore termed poles, and this two-endedness of a magnet is termed magnetic polarity. The central ELECTRICITY AND MAGNETISM. 91 part of the magnet, where no attraction exists, is sometimes termed the magnetic equator. The third and fourth experiments show us that the attraction be- tween a magnet and iron or steel is reciprocal ; that Attraction not only does the magnet attract iron, but iron attracts reciprocal, the magnet. The fifth experiment proves to us that in magnetism, as in elec- tricity, there is a law of attraction and repulsion, L awo f attraction which may be thus formulated : Like poles repel, un- and repulsion, like attract each other. The sixth experiment shows us that a freely suspended magnetic needle sots itself in a nearly north and south position. The position in which it thus remains at rest is known as the mag- netic meridian. Further, the pole of the magnet which turns towards the north pole of the earth is termed the north pole of the magnet, while the end of the needle which points towards the south pole of the earth is denominated the south pole of the magnet. As it is believed that the reason of this "direc- tive" property of the magnetic needle is that the earth itself is a great magnet, it will be obvious, from a consideration of the law of magnetic attraction and repulsion, that this nomenclature is incor- rect; and it has been suggested to substitute for the terms " north " and" south," the words " north-seeking " and " south- seeking." The former terms have, however, become so wedded to the science, that it is almost hopeless to attempt to replace them, and it must therefore be borne in mind that the north pole of the magnet is endowed with the opposite kind of magnetism to that of the north terrestrial mag- netic pole, and so with the south. Having siimmed up the conclusions to which our experiments have so far led us, let us proceed to question nature still Further further, and endeavour to ascertain the manner in experiments, which the special properties of a magnet are distributed throughout its mass. Take one of your magnetized knitting needles, and marking its north pole so as to be able to tell which end is the north and which south, break it in two. It will be found that each half has become a magnet, with its poles in the same direction as those of the original magnet. Repeat the operation upon each piece of the original magnet, and again it will be found that the fragments are perfect magnets. In this way you may break your needle into the smallest possible portions, and each piece will be in every respect a perfect magnet. Now, if we follow this subdivision of a magnet in imagination to 92 ELECTRICITY AND MAGNETISM. its logical conclusion, we must be impressed with the fact that the ultimate particles or " molecules " of the needle are also Molecular perfect magnets, and that the magnetic properties dis- *' played by the needle, as a whole, are merely the sum of the magnetisms of its molecules. In fig. 61 an attempt is made to represent this idea in a graphic form. A simple experiment by which this imaginary disposition of the Illustrative magnetic fluids in a magnet may be to certain extent experiment, verified, is the following : Take a small piece of glass tube, and after jtartly filling it with steel filings, pass the pole of a strong magnet several times along the outside, being careful not to shake the tube. This will of course magnetize the steel particles, and so long as the tube is not shaken it will act as an ordinary magnet, and will have, like it, a south and a north pole. The moment, however, the tube is shaken, so as to disturb the relative position of the filings, all trace of polarity disappears. It is presumed that before magnetization the two magnetic fluids are mingled together in the molecules of a magnetic substance, and therefore neutralize each other ; and that magnetization neutral bodies. consists in the separation of these two fluids, developing an equal but opposite magnetic force, at the two poles or thereabout^; an idea which is comparable with that which regards static electric induction as a separation of negative and positive elec- tricity from their combined neutral condition. It must, however, be clearly understood that, as in the case of the two- fluid theory of electricity, this idea as to the existence of two magnetic fluids is purely hypo- letical, and that magnetization consists in the separation of these two fluids, developing an equal but opposite magnetic force at the ELECTRICITY AND MAGNETISM. 93 two poles or thereabouts : au. idea that is comparable with that which regards static electric induction as a separation of negative and posi- tive electricity from their combined neutral condition. The student must beware of accepting it as a proved fact. Later on in the course of these iu-westigations of ours, we shall have to consider the origin of magnetism in another light. A striking experiment, which illustrates the neutralization of one pole of a magnet by the other, is the following, shown Further in fig. 62 on the preceding page. experiments. Take two bar magnets, and to the north pole of one of them attach any small object, such as a key. Now bring near the south pole of the other magnet ; the key will fall. On the other hand, if you place the two north poles of the magnets together it will be found that the attractive power will be much increased. Take a bar magnet as represented in fig. 63, and suspend from one of its poles, say the north, as many pieces of soft iron. wire as it will hold. What is the condition of each of these small pieces of wire during the experiment? Obviously each one of them must act as a little magnet. For instance, disturbed owing to the influence of the magnet which sepa- rates the two kinds of magnetism, and at- tracts the south while it repels the north ; B will accordingly be a north pole, and FIG - 63 ' A a south pole. This first piece of wire acts in the same way upon the second, the second upon the third, and so on. Here we have what, by analogy with electrical action, we may term magnetic induction. Inasmuch as the magnetism of the pieces of wire disappears the moment they are removed from the magnet, we say that they form temporary magnets. But not only may we have the magnetic influence exerted by contact of the magnet with a magnetic body ; we may also have this influence exerted at a distance. Take the north pole of a bar magnet and bring near to it a bar of soft iren. Near to, but not touching; the other end, place a freely- suspended magnetic needle. The needle will be affected in just the same way as it would have been had the needle been directly acted upon by the magnet that is to say, its north pole will be repelled, 94 EtECTtilClTY AND MAGNETISM. its south pole attracted. Here we have another instance of magnetic induction. Our readers will have already seen that although both steel and iron Coer ivef r h&Ve the power of acce P tin g tne magnetic condition, yet oe ' they differ from each other in the readiness with which they accept it, and with respect to the length of time during which they retain it. In point of fact, it is found that whereas soft iron very speedily, Difference indeed immediately, accepts the magnetic condition, it between iron as immediately loses it upon the mnupncinf magnet and steel, being withdrawn. Steel, on the other hand, can only be magnetized with some diffi- culty, but having once accepted the magnetic condition it retains it for a prolonged period. This resistance on the part of steel to mag- netism and its retention of it when once acquired is termed, somewhat unhappily, coercive force, and this is said to be very great in steel but almost absent in soft iron. A very beautiful experiment either with a horseshoe magnet or with one or two bar magnets is the following : Take a sheet of glass or of writing-paper, and place beneath it first an ordinary horse- ioe magnet. Then sprinkle lightly upon the glass or paper a few iron filings, gently tapping the glass every now and then. The filings will arrange themselves round the poles of the magnet in the most beautiful curves, as represented in fig. 64. These lines are known as the magnetic curves or lines of magnetic Magnetic curves. /( ' ?W - Now substitut e for the horseshoe a bar magnet, and, proceeding as before, the arrangement of the magnetic curves will be as in fig. 65. Finally, place two bar magnets parallel to each other with their reversed-that is to say, the north pole of one opposite to the south pole of the other. We have already said that the earth is considered to be a great magnet, having its north and south poles, and have in this way Magnetic poles. attem P ted to account for the tendency of rf freely suspended magnetic needle to set itself in a direction nearly corresponding to the geographical north and south. As we ELECTRICITY AND MAGNETISM. 95 have also intimated, the north and south magnetic poles are not precisely coincident with the north and south geographical pole's. Both the magnetic poles of the earth were discovered through the investigations of Sir James Ross : the north magnetic pole in 96 43' west longitude, and 70 north latitude ; and the south was calculated to be at that time (1830) in about 154 east longitude, and 75i south latitude. The magnetic poles of the earth are unfortunately named in an opposite sense to the ordinary nomenclature of mag- netic poles, for the north pole of a magnet is attracted by the north pole of the earth and repelled by its south pole. This difficulty may be to some extent removed by calling the poles of a magnet north- seeking and south-seeking respectively, though no change of nomen- clature will obviate the confusion consequent upon the fact that what is called the earth's north pole is really a magnetic south pole. An imaginary line joining the two poles of a magnetic needle ia termed the magnetic meridian, and it is therefore in this direction that a magnetic needle settles to rest. The line joining the north and south geographical poles is termed the geographical meridian, and the angle which at any place is made between these two meridians, or, what is the same thing, the angle which the needle makes with the geographical meridian, is termed its declination or variation. This declination, which is expressed in degrees () and minutes ('), is said to be east or west, according to whether the north pole of the needle is east of west of the geographical meridian. The declination of the needle varies not only at different places, but also at different times at the same place. Some of these varia- tions take place daily, some annually, some only after long intervals of time. All of them are comparatively regular, and are so denominated ; but, in addition, irregular disturbances of the needle take place, which are known as mngnctic storms. 00 ELECTRICITY AND MAGNETISM. The variations in the declination of the magnetic needle which are only apparent after long intervals of time are Secular termed secular variations ; and the following facts with regard to these changes in London and Paris will suffi- ciently illustrate their character. At London, in 1580, the declination of the needle was 11 36' east ; in 1663, it was zero that is, the magnetic and geographical meridians coincided. From 1663 to 1818 the needle gradually turned towards the west, and in the latter year reached its maximum western de- clination, which was 24 41'. Since 1818 it has gradually diminished, and is now (1882) about 18 46'. At Paris the needle has varied in declination according to the following table : Year. Declination. 1580 ...... 1030'B. 1663 1700 810'W. 1780 19 55' W. 1785 22 W. 1805 22 5' W. 1814 2231'W. Year. Declination. 1825 2222'W. 1830 223 12 ' w. 1835 22 4' W. 1850 20 30' W. 1855 1957'W. 1860 19 32' W. 1865 . . 1S41'W. At some places on the earth's surface there is no declination of the magnetic needle. These places are connected by a Agomc lines. curved line on tne mapj w hi c h is termed a line of no variation, or agonic line. To those imaginary lines which connect places having the same declination, the term isagonic lines is applied, and the mes ' maps on which such lines are depicted are termed declination maps. Beside the secular variations in declination, the magnetic needle undergoes minute annual variations. These were first discovered by Cassini in 1780. At Paris and London this variation is greatest about the spring equinox (March Annual 21st), and diminishes from that time to the summer variations solstice (June 21st), and then during the following nine months slowly increases. The amount of this variation is from 15' to 18'. The diurnal variations in declination were first discovered in 1722 Diurnal by Graham, and require the employment of very deli- variations, cate instruments, for their detection. From observations which have been made, it is found that in England the north pole moves from east to west from sunrise until about one or two o'clock. It then begins to return, and at about ten o'clock at night regains its original position, and during the night remains almost stationary. ELECTRICITY ASJD MAGNETISM. 1:7 Another important point to be noticed in connection with the directive tendency of the magnetic needle is the angle inclination which it makes with the horizon. or dip. If a magnet be so suspended that it is free to move in a vertical plane, it will be found that it does not come to rest in a horizontal position, but that (in this country at least) it rests with its north pole pointing downwards. This is termed the inclination or dip of the needle, and the angle which the dipping needle makes with the horizon is termed the angle of dip. This angle of course varies with the position of the needle upon the earth's surface. If the needle were taken to the north magnetic pole it would hang vertically, its north pole pointing downwards. If it w r ere then carried southwards the north pole would gradually rise, until, when the magnetic equator was reached, the needle would be perfectly horizontal, that is, there would be no dip. Continuing to travel southwards, the south pole would gradually be depressed, until, when the south magnetic pole was reached, the needle would again be vertical, but this time with its south pole downwards. This change in the position of the needle may be experimentally illustrated by passing it along a long bar magnet, when its position would vary, as shown in fig. 66. At London the angle of dip is at the present time (1882)'abotit 67 4(X. Like the declination, the inclination of the needle is subject to variations secular, annual, and dmt'nal. Vanation8m * The secular variations may be illustrated by the following particu- lars respecting the alterations in the dip at London. In 1821 the dip was 70 3' ; in 1838, 69 17' ; in 1854 it was 68 31' ; and in 1859, 68 21'. It will thus be seen that the dip in London is slowly decreasing. The variations of the dipping needle which occur annually and diurnally are very slight. 98 ELECTRICITY AND MAGNETISM. The dip is about 15' greater in summer than in winter, and 4' or 5' greater before noon than after. For the measurement of magnetic declination and inclination Magnetic two veI T ingenious instruments have been devised, a meaurements. description of which will now be given. The first of these, the so-called declination compass, is shown in fig. 67. It consists of a brass or copper box, M N, in the bottom of ' which is a divided circle. In the centre of this circle is sus P encied U P U a very delicate pivot, by an agate cap, a small lozenge-shaped magnetic needle. The box MN is itself mounted upon a copper circle, p Q, divided into degrees, 14 ELECTRICITY AND MAGNETISM. In working the instrument, it is first placed in a perfectly hori- zontal position by means of the spirit level and the levelling screws attached to the feet of the tripod stand. The next thing is to find the astronomical 'meridian, by methods which are explained in the manual on Astronomy. The box M N is next turned until the telescope is in this astronomical meridian. The angle made by the magnetic needle with the diameter, n s. of the box M N, and which corresponds with the zero of the scale, and is exactly in the plane of the telescope, is read off on the graduated circle. If the north end of the needle stops east of n s, the declination is so many degrees east ; if it stops on the west side of n g, the declination is so many degrees west. These observations are, however, only correct when the magnetic axis (that is, the line joining the two mag- netic poles) of the needle coincides with the line connecting its two ends, or its axis of figure. This is very rarely the case, and it is therefore usual to reverse the needle in such a way that its under surface is placed uppermost, and to proceed as before. The mean of the two observations is then taken as the true declination. The second important instrument is one for measuring the inclination, or dip, of the magnetic needle, and is known as the inclination com- pass. It is represented in fig. 68. Here A B is a graduated circular stand, resting upon a tripod stand, supported by levelling screws, and pro- vided with a spirit level. Moving freely upon this stand is a plate, c r>, which replaces the metal box of the declination compass. This plate supports, by means of two uprights, a graduated circular hoopj E F. Between the uprights, and supported by them, is a frame, on which a needle is pivoted so that it can move only in a vertical plane. The axis upon which the needle swings is of very fine steel, and is supported by two horizontal triangles attached to the frame. In using the instrument the magnetic meridian is first ascertained. This is done by turning the plate, c D, npon A B until the needle is vertical -which will of course be when it is in a plane at right angles loo ELECTRICITY AA T D MAGNETISM, to the magnetic meridian. The plate is then turned 90, which will Use of the bring the needle into the magnetic meridian. The inclination angle which the needle now makes with the hori- compasa. zontal diameter of the vertical graduated ring is the inclination. Several errors may creep into observations made by this instru- ment, and of these the three following are the most important. Possible errors. First, an error may arise owing to the magnetic axis not coinciding with the axis of figure of the needle ; and this may be remedied, as in the declination compass, by re- versing the needle and taking the mean of two observations. The second error arises if the centre of gravity of the needle does not coincide with the point through which the axis of suspension passes. This error is guarded against by demagnetizing the needle and then remagnetizing it in such a way that its poles are reversed. The mean of two observations, one made before and the other after the change of polarity, is then taken as the true inclination The third error exists when the plane of the vertical ring does not coincide with -he true magnetic meridian. It should be in that plane when the M has its minimum deviation ; an observation of this kind is usually taken along with that described above, by which the needle is moved 00 from its maximum deviation. ELECTRICITY AND MAGNETISM. 101 An instrument which we must by no means omit to describe is the well-known and invaluable mariners' compass. This is represented in fig. 69. It consists of a cylindrical case, which, in order that it may be kept horizontal during the rolling The mariner8 ' of the vessel, is supported by two concentric rings, called gimlals. Of these, the inner is attached to the case itself, and moves about an axis which plays in the outer ring, which again moves about an axis at right angles to the first. The needle is mounted on a pivot in the centre of the box, and rests on an agate cap. Above this is fixed a very thin disc of mica, the diameter of which is slightly greater than the length of the needle. This disc has traced upon it a star or rose having thirty-two branches show- ing the eight points of the wind, the half points, and the quarters. The branch ending with a small star corresponds to the magnetic needle beneath. The intensity of the earth's magnetism may be measured by the number of oscillations performed by a magnetic needle after it has been removed from its position of equilibrium. If the j ntengity of same magnetic needle be taken to two or more places earth's upon the earth's surface, and be in each place moved ma K netism - from the magnetic meridian, it will, after a certain number of oscilla- tions, return to its original position ; the number of oscillations thus made will, however, be different in each place. These oscillations are similar in character to the movements of the pendulum, and are governed by analogous laws. By these laws we know that the intensity of the earth's magnetism at any two places is proportional to the square of the number of oscillations performed in a given time. Suppose the number of oscillations at two different places to be respectively 28 and 30, then the relative intensity of the earth's magnetism at these places will be in the proportion of 784 to 900, or the magnetic intensity at the latter place to that at the former will be as 1-148 to 1. When the force of magnetic attraction and repulsion is quantita- tively examined, it is found to vary in the same way as the force of electrical attraction and repulsion. That is to say, the jraietio attraction or repulsion varies inversely a* the square attraction and of the distance a law the meaning of which has already plon. been sufficiently explained. This law was first conclusively proved and formulated by Coulomb, who proved it both by the method of oscillations and CoulomVs by his torsion balance, an instrument which we will torsion balance, now describe. This ingenious apparatus is represented in fig. 70. It consists of a 102 ELECTRICITY AND MAGNETISM. glass case, covered with a glass lid, in which latter is a small aperture through which a magnet may be introduced. Upon the top of the glass hd is a glass cylinder, provided at its upper extremity with a micrometer. This micrometer is shown on a somewhat larger scilo on the left hand of the diagram, and will be seen to conslfof wo circular pieces : B, which is divided on the edge into 360 degrel while E which is movable, carries a mark to indicate the amount of' its rotation. Upon E are two uprights, through which passes a screw which supports by a fine silver wire a magnetic needle a I EonZ the sides of the glass case, about half-way up and on a levef wfth the suspended needle, a paper scale of degrees is placed The zero of this scale and the suspended magnetic needle are'so arranged t t " * * is turned so that case. ** are in the of ~, , niLU 1C t^ micrometer. at the zero of the paper scale on ..-...s-ir-r''"-""" "'""'" ssrsSsSSSSvw: turned iTnf-ji * * _ * * ^*^ do this. tVi^ mir*! ELECTRICITY AND MAGNETISM. 103 one of his experiments, found that the piece E had to be turned 35 in order to move the needle through 1 that is, the earth's mag- netism was equal to a torsion of 35. The action of the earth's magnetism having thus been determined, the magnet A is placed in the case through the aperture in its lid, so that the similar poles are opposed to each other, when repulsion of course results. In one experiment Coulomb found that this repulsion drove the suspended needle through 24. The force which tended to bring the needle into the magnetic meridian was therefore reprssented by 24 + 24 X 35 = 864, of which the part 24 was due to the torsion of the wire, and 24 X 35 was the equivalent in torsion of the direc- tive force of the earth's magnetism. The needle being in equilibrium, the repulsive force which counterbalanced those forces must be equal to 864. The micrometer disc was then turned until a b made an angle of 12. In order to do this eight complete revolutions of the disc, or 2880, were found to be necessary. The total force now tending to bring the needle into the magnetic meridian was composed of the 12 of torsion by which the needle was removed from its start- ing point, of the 2880 torsion of the wire, and of the force of the earth's magnetism represented by a torsion of 12 x 35 = 420, making in all 3312. But the torsion at 24 was only 864, which is about one-fourth the torsion at 12. In this way Coulomb demon- strated that the forces of attraction and repulsion vary inversely as the square of the distance. Before leaving the subject of magnetism we must not omit to notice the manner in which magnetic properties may be imparted to steel bars, so as to convert them into artificial magnets. And this we shall now proceed to do. Amongst the most simple methods by which magnetic properties may be imparted to steel bars, so as to convert them Methods of into artificial magnets, will be found the following : magnetization. 1. Let one extremity of the steel bar be brought into contact with one pole, say the north, of a permanent magnet, and let it remain there for some little time. Upon removing it, it will be found to possess magnetic properties, the end which was in contact with the north pole of the magnet being a south pole. 2. Draw repeatedly one end of the steel bar in the same direction across the north pole of a permanent magnet, and then draw the other extremity of the bar across the south pole of the magnet. The bar will be magnetized, its former end being a south, its latter a north pole. 3. Lay the bar across the poles of a horseshoe magnet and let it remain for a few moments. Upon removing it, it will be found to 104 ELECTRICITY AND MAGNETISM. be magnetized, each end having an opposite polarity to that of tLe magnetic pole with which it was in contact. 4. The most common and most ancient method of magnetizing a steel bar is by the method known as that of single Single touch. touch This is represented in fig. 71. Here one pole of the magnet is placed in the middle of the bar to be magnetized, and is then drawn to the end of the bar. The magnet is then returned through the air to the middle of the bar and the same process is repeated. After this has been done several times, the other pole of the magnet is taken and placed on the middle of the bar as before, and is then drawn to the other end of the bar. This is done several times, and at length the bar becomes thoroughly magnetized, its ends having the opposite polarity to that of the pole of the magnet wjth which they were respectively magnetized. 5. A more efficient method of magnetizing steel bars is that known as double touch. Here, as shown in fig. 72, the bar to be magnetized, a b, is laid flat ui>on a table, and two magnets, A B, with Double touch theh tw dissimilar Pp les opposite, but not touching each other, are placed in the middle of the bar. They are then drawn in opposite directions to the ends of the bar ; are then returned through the air to the middle of the bar, and the process being several times repeated the bar becomes permanently magnetized. A modification of this method is shown in fig. 73. Here the two dissimilar poles of the two magnets are tied together with a small piece of card or wood between them, and starting as before at the middle of the bar, the magnets are first drawn along the tar to one of its ends. They are next carried through the air to the ELECTRICITY AM) 105 FIG. 73. other end of the bar, and then drawn along its whole length. This process is repeated several times on both sides of the magnet, care being taken to leave off in the middle. In all these processes of magnetization the same principle is employed : the poles of the magnet employed separate the magnetic fluids of the neutral bar, attract the opposite kind of jiolarity to themselves, and repel that of the same kind as their own. With all methods of magnetiza- tion, there is found to be a limit to the intensity of the magnetism which a given bar of steel will accept, and when this limit is reached the bar is said to be saturated. Although, as we have previously observed, steel retains its magnetism for a very lengthened period, that period is not unlimited, and there is notice- able a distinct deterioration in the mag- netic properties of a steel bar which has been magnetized, if it is kept for a con- siderable length of time. The enfeeble- ment of permanent magnets may to a great extent be prevented by attaching to their poles a piece of soft iron, which is commonly known as the armature or keeper of the magnet. It is not always necessary to employ a magnet to induce magnetic properties in soft iron or even steel. A Magnetization simple experiment by whichwithout magnets, this fact may be exemplified is shown in fig. 74. Let a common sewing needle be fixed in the magnetic meridian against a table. Then take two pokers and place one above and the other below the middle of the needle, as represented in the figure. Then move the pokers in dif- ferent directions towards the ends of the needle, and repeat the operation several times ; it will be found that the needle has become magnetized. The explanation of this curious result is that the mere fact of placing the pokers in the direction of the magnetic dip con- verts them into temporary magnets. For the same reason pokcrsrmd other metal bars which have been by accident allowed to stand vcrtt- H FIG. 74. 106 EKE'CTSICITY AND MAGNETISM. cally so as to be in the direction of the magnetic dip, frequently exhibit feeble magnetic properties. Magnets are very sensitive to changes of temperature, a fact that must be borne in mind in making comparative experiments with the same needle, but at different times. A magnet partially saturated at 60 C. will lose magnetism by cooling, but the most ordinary effect is the loss of magnetism by a rise of temperature. By Effect of heat heating a magnet only a few degrees centigrade it will become sensibly weaker, but will almost regain its original condition on cooling. Coulomb found that a magnet at 15 0. had its intensity reduced to less than one-half by heating it to 50 C., and to about two-fifths by heating it to 100 C. The greater part of the effect of heat on a magnet takes place directly it reaches the given temperature. A steel bar entirely loses its magnetism at a red heat, and at such a temperature iron is not attracted by a magnet. The intensity of a magnet may be seriously impared by other means than heat ; in general, any action that is likely to cause molecular disturbance or motion will lessen the coercive force of steel. Friction, percussion, etc., have this tendency. But whatever temporarily lessens coercive force may be made to facilitate magnetization as well as demagnetization under suitable circumstances. A bar of steel may be powerfully magnetized by suddenly cooling it from a red heat between the poles of an electro magnet ; it is easy to see that in this case the coercive force is really developed in an intense magnetic field, which, therefore, has full opportunity to affect the steel. For the same reason a steel bar that is hardened by cooling it in a vertical position, will give evidences of being a magnet, because of the action of the earth's magnetism upon it. Having made ourselves acquainted with the phenomena of ordinary magnetism, we are now in a position to take up that magnetism Branch of electrical science which treats of the con- nection between electricity and magnetism, and which is usually known as electro-magnetism. We have already seen that when the metallic plates of an ordinary Voltaic cell are connected by a wire, the latter, during the passage of a current, has the power of attracting iron filings, and so resembles a magnet. Moreover, we have seen further that if a current be passed through a coil of wire in which a steel bar is lying the latter becomes magnetized ; and we have also seen that when a freely suspended magnetic needle is brought near a wire through which a current is passing, the needle is deflected from its normal position. The magnetic action of a wire through which an electric current passes may be shown by scattering iron filings upon such a wire. The ELECTRICITY AND MAGNETISM. ior filings will become magnets, and will cling to each other and to the wire. If a piece of paper is laid over the wire, and the filings scat- tered upon it, they will be seen to arrange themselves in lines at right angles to the current, and these magnetic lines of force may be shown to encircle the wire in concentric rings, by causing the wire to pierce a card vertically. The filings will then arrange themselves as shown in fig. 75, on gently tapping the card. These facts all tend to show that there is a direct connection between the electrical and magnetic conditions, and this is more fully shown when we introduce an iron Magnetism by bar into the voltaic circuit. If we take a battery wire, which is uncovered, and wind it round a bar of soft iron, the latter becomes a part of the circuit. If, however, the wire is insulated, as is ordinarily done by covering it with cotton or silk, the wire may be wound round the bar of soft iron without coming into contact with it. Never- Af^ theless, although not in the circuit, the bar / will exhibit very powerful magnetic pro- L, perties, and the strength of its magnetism will be increased with the increase of the number of coils of the wire which sur- FIG ' rounds it. By making these coils very numerous a temporary magnet of enormous strength may be obtained, vastly exceeding in power any of the steel permanent magnets of which we have recently been speaking. Such a magnet is termed an electro-magnet, and is usually made in the form of a horseshoe and provided with a strong soft iron. armature. The covered wire is generally coiled round a hollow reel, and forms an electro-magnetic helix. In this the soft iron bar, which is termed the core, is placed. Immediately a current passes through the wire surrounding the core, the latter becomes a powerful magnet. When the Electro- current is broken the magnetism of the core almost magnets, instantly disappears. Although a core greatly intensifies the power of an electro-magnet, it is not indispensable. A helix without a core will, when a current is passing through it. exhibit magnetic properties ; it has two poles, with a magnetic equator between ; it attracts iron and other magnetic substances, and will, if free to move, set itself in the magnetic meridian. When, however, a helix and core are employed together, the strength of the combination is much greater than that of the helix alone. In fig. 76 an electro-magnet of the ordinary form is represented. 108 ELECTRICITY A.VD M is the horseshoe-shaped soft iron bar or core, P N are the ends of the helix of covered copper wire, K is thc soft iron armature, the ring at the bottom of which is employed to attach any weights or other bodies which it is desired to suspend from it. The law which governs the attraction of electro-magnets is one which requires careful attention in order that its import may be thoroughly understood. If the attraction of a certain electro-magnet law of u{K>n a piece of soft iron electro-magnetic be taken as 1, upon attraction. tlou > jling tnc strength of the magnet thc attraction between it and thc iron will not be 2, but 4. In thc same way, if the strength of the magnet be trebled the attraction will not be 3, but 9. That is to say, the attraction of the magnet upon soft iron is proportioned not simply to the strength, but to the square of the _ strength of the magnet. If, however, the substance acted upon by the electro-magnet is hard Difference steel, and not soft iron, between iron the attraction is pro- and steel. p 0rtiona i s i mp l y to the strength of the magnet. That is to say, double the strength, and the attractive force is doubled ; quadruple the strength of the magnet, and the attractive force is quadrupled. To make the reason of this difference clear, we must remind our Reason of the readers of what has been already said in the section on difference, magnetism with regard to the difference between soft iron and hard steel in the reception of magnetic properties. It will be remembered that whereas soft iron accepts magnetization with extreme readiness, hard steel is only magnetized with difficulty ; this resistance which steel offers to the reception of the magnetic condition being termed coercive force. It is this coercive force which is at the bottom of the difference just referred to. Now, when a piece of soft iron is placed near an electro-magnet, it is immediately converted into a temporary magnet, so that we have not merely a magnet attracting a magnetic substance, but two mag- nets attracting each other, and every increase in the strength of one is followed by an increase in the strength of thc other. Suppose, for instance, an electro-magnet has one imaginary unit of magnetism ; that one unit will evoke in the soft iron one unit of magnetism, and HG> 7fl< .ELECTRICITY AND MAGNETISM. 109 the attraction of the magnet and the iron will therefore be 1. If, however, the strength of the magnet is so increased that it possesses two units of magnetism, each of those units will evoke a unit of magnetism in the soft iron, and the pull between the two bodies will now be 2 x 2 = 4. In the same way, if three units of magnetism are given to the electro-magnet, three units will be induced in the soft iron, and the total attraction will be 3 x 3 = 9, and so tji. In this way it is easy to see how the attraction between soft iron and an electro-magnet is proportional to the square of the magnet's strength. On the other hand, when an electro-magnet attracts hard steel, no magnetism is induced in the latter, and consequently any increase in the power of the magnet produces no corresponding increase in the attraction of the steel upon it ; and therefore when an electro-magnet acts upon hard steel the attractive force increases simply as the strength, and not as the square of the strength, of the magnet. A very curious property possessed by certain substances, called diamagnetism, may be more clearly shown by an electro- Diamagnetism. magnet than by an ordinary steel magnet. The property in question is that of repelling the poles of a magnet, and being repelled by them, and is possessed by the metals antimony and bismuth in the most marked degree. The repulsion due to dia- magnetism is, however, vastly feebler than the attraction due to the force of magnetism. So far back as 1778 the repulsion of bismuth by a magnet was known. The repulsion of antimony was first observed in 1827. In 1845 Faraday showed that all matter could be divided into two kinds that which was attracted and that which was repelled by the poles of a magnet, and to the force of repulsion he gave the name of diamagnetism. Upon examination it is found that the repulsion of a diamagnetic substance by a magnet follows the same law as that law of which governs the attraction of soft iron by an electro- diamagnetio magnet ; that is to say, it is proportional to the square "P" 1 *""*- of the strength of the magnet. It is, therefore, obvious that dia- magnetism is induced in the substance by the influence of the magnet. Of all diamagnetic substances bismuth is most strongly repelled by the poles of a magnet ; but it has been calculated that comparison of the magnetism of a thin bar of iron exceeds the dia- intensities of magnetism of a similar mass of bismuth about two and a half million times. 110 jSZBCmiGlTY AND CHAPTER XIII. ESTIMATION OF ELECTRIC CUBBENTS AND RESISTANCES. Deflection of magnets by currents-A simple piece of appar atus-Measuremen of current strength-The galvanometer-Preliminary explanation-The astatic combination-How to make an astatic galvanometer-How to use the astatic galvanometer-The differential galvanometer-The tangent compass-Explanation of trigonometrical terms-The tangent galvanometer -The sine galvanometer-The voltameter-Ohm's laws-Definition tcrms- Keduced length of the circuit-Electric fall-Summary of Ohm's laws- Comments upon Ohm's laws-Application of Ohm's laws-To determine tho best arrangementforabattery-Arithmetical calculations-Measurements of resistances-The rheostat-Principle of the rheostat-The rheocord-Resist- P^, atl n f CUrrent to Distance- Wheatstone's Bridge- UnTts S reLTanTe < Bridge - Detei io a 0* Eternal resistant ONE of the most important facts bearing upon the connection between Deflection of electricity and magnetism is that an electric current This fact, which forms the starting-point in the development of the electric telegraph and many other equally useful though perhaps less generally known appliances, was first discovered in 1819 We have already alluded to this so-called directive influence possessed currents over magnets, for ' , ,. ELECTRICITY AND MAGNETIC!. Ill wire be above the needle. Now join the terminal wires to the poles of the battery, so that the current passes, as in the figure, from south to north. The needle will be deflected, so that ks north pole turns to the west. Now reverse the current by reversing the wires, and let it pass from north to south. The needle will be deflected, but its north pole will turn towards the east. If the needle be next sus- pended so that the wire is beneath it, and the same experiments repeated, it will be found that when the current passes from north to south the north pole of the needle turns to the west ; while if the current be allowed to pass in the opposite direction, the needle will be deflected with its north pole to the east. By this it will be at once seen that we have in this directive action of currents upon magnets a ready means of detecting not only the existence, but the direction of an electric current. Measurement Moreover, as may be proved by experiment, the deflec- of current tion of the needle being greater as the strength of the strength, current increases, we have also a ready means of measuring current strength. It is somewhat puzzling at first to accurately remember the way in which the north pole of the needle is deflected in obedience to the direction of the current and the position of the needle. To obviate this difficulty, the following memoria technica, due to Ampere, may be found useful by our readers. Conceive a man to be lying on the connecting wire, so that, whatever the direction of the current, it always enters at his feet and leaves by his head ; conceive, moreover, his face always to be turned towards the needle, the north pole of the needle will always le deflected towards this hypothetical man's left hand. A further examination of this directive action of electrical currents speedily led to the discovery of the fact, that by making a number of coils of wire -round the needle, the action The salvano- of the current was greatly intensified, so that a very small current might be made to influence the needle ; and this ELECTRICITY AND MA6NETI8M. soon resulted in the invention of the so-called galvanometer, by which the direction and comparative electrical amounts of small currents may be measured. In order to make the principle upon which the action of this in- strument depends perfectly plain to our readers, it will Preliminary ^ e ne cessary to enter upon a few preliminary considera- tions, before proceeding to a description of the manner in which the instrument is constructed and employed. Suppose we have, as represented in fig. 78, a magnetic needle sus- pended by a thread of unspun silk in the magnetic meridian. If we have a copper wire wound longitudinally once round the needle in the same vertical plane, and if we have a current passing through it in the direction indicated by the arrow in the diagram, it will be found, by making use of the memoria tcchnica given above, that the left hand of the man will always point towards the same point of the horizon ; that is to say, the north pole of the needle will be induced to turn in the same direction by all parts of the current. The effect of the current, therefore, has been multiplied by the doubling of the wire. In the same way, if several coils of wire, coiled in the same direction, and insulated from each other, be wound round the needle, the effect of the current will be increased still further. In employing, however, a tingle needle, we have the directive action of the earth upon it to take into account^ and therefore must, to get any effect, employ a current v.;at is strong enough combtoaton. * over come this directive action. This inconvenience is to some extent obviated by employing tn-o magnetic needles in the form of an astatic combination, as represented in fig. 79. Here a b, a' V, are two magnetic needles parallel to each other, and connected, as shown, by copper wire. The needles are so arranged that the north pole of one is opposite the south pole of the other, and ELECTRICITY AXD MAGNETISM. 113 vice versa. By this device the directive action of the earth is to a very great extent neutralized ; if the needles were absolutely of the same strength it would be quite neutralized, but it is hardly possible to perfectly secure this. But in addition to this neutralization of the earth's directive action, the influence of the current is increased by the employment of two needles in the place of one. The needles are arranged as shown in the diagram, the upper one being altogether above the wire, the lower one being between the two folds of the wire. Supposing the current to pass in the direction shown by the arrow, the upper needle will be influenced by two currents flowing in opposite directions ; of these, however, the upper and nearer one will have the greater effect, and will therefore tend to turn Val so that a! is deflected towards the east. The current flowing both above and below the other needle, will tend to turn it in the same direction. In this way the current tends to turn both needles in the same direction, this tendency being only slightly counteracted by the current in the lower part of the wire acting on the upper needle ; but this action being at a great distance, is corre- spondingly enfeebled. Having, we hope, now made clear to our readers the principle upon which the action of the galvanometer depends, we hope they will be 114 ELECTRICITY AND MAGNETISM. encouraged to make one, which will be found with ordinary care and skill by no means a formidable task. In' fig. 80 will be seen a view of the perfect instrument as ordi- _ k narily made for the lecture table ; one or two modifi- aTastatio cations may, however, be made in its construction by galvanometer. ^ Q ama t e ur electrician. The first thing will be to get about a hundred feet of fine copper wire covered with cotton \ No. 20 will be found a good size. This you must divide into halves, and of each half you must make a com- plete coil, leaving the two ends free. The best way to coil the wire is to get a small wooden block and carefully Mind the wire upon it, removing the block after securing the coil by tying it with thread. Those who have the time and skill may make a permanent wooden frame, as shown in the diagram, upon which to coil the wire, which will greatly add to the neatness and finish of the instrument. Having made the two coils, fix them on the frame side by side, leaving a space of say a quarter of an inch between them, and carefully solder two of the free ends of the coils together so as to convert them into one continuous coil. The next thing will be to get a wooden stand, which should consist of a disc of wood about three quarters of an inch in thickness. This may easily be cut from any ordinary piece of deal and the corners rounded off with a rasp. If then coated with shellac varnish, it will constitute a neat and useful stand. Make two small holes under the coil, and bring the two ends of the coil through these holes to the under surface of the stand. In what is to be the front part of the stand fix two binding-screws, so that their lower ends project slightly on the under surface of the stand. Join one wire of the coil to one binding-screw and the other wire to the other screw. To suspend the needle it will be best to get a stout piece of brass wire and bend it into the form of a flat-topped arch, as shown in the margin, fixing the ends firmly into the stand by the side of the coils. The next step will be to make an astatic needle. Magnetize two small pieces of steel wire (sewing needles will do well, pieces of knitting needle better), and join them by a piece of twisted copper wire, so that their opposite poles face each other. On the same axis above the upper needle fix a slender glass thread, about four inches long, to answer the purpose of an index. Suspend this com- bination by a thread of unspun silk. The glass index may be made by taking a piece of fine soft glass tubing, heating it in an ordinary gas burner, and drawing it out to the required fineness It will be found best to attach the upper end of the silk to a small cork roller, which can easily be fitted on to the top of the wire ELECTRICITY AND MAGNETISM. 115 arch before fixing the latter to the stand. The next, and perhaps most difficult task, is to get a piece of cardboard and cut it into a circle, and then graduate half the circumference of the circle into 180 degrees that is, 90 degrees on each side of the zero point. A little patience and a good protractor will soon, however, enable the amateur electrician to finish this part of his task to his satisfaction. Having done so, it only remains for him to put the card and astatic combina- tion in their respective positions as shown in our diagram, and to cover the whole with an ordinary glass shade such as is used to cover ornaments, and which may be purchased for a few pence. If a frame has been made, the card may be allowed to rest upon it, but if (as must be done when the frame is dispensed with) the coils have been fastened directly to the stand, the card must be supported by two pieces of wood or cork glued on to the stand. The card should not be fixed to these or to the wooden frame. In using this instrument allow the needle to come to rest, and then move the graduated card so that the end of the glass index points to the zero of the scale. Then connect the two ends of How to uge ^ the cell or other circuit to be tested with the binding- astatic galvano- screws of the galvanometer. The direction of the meter, deflection of the needle will indicate the direction of the current, and the amount of the deflection will vary with the intensity of the current. The galvanometer is, however, not well adapted to give exact quantitative measurements of the strength of currents, and is used only for the detection of currents, for determining their direc- tion, and for roughly comparing their intensities. For instance, if two currents be compared with regard to their intensities, and one gives a deflection of 50 and the other one of 25, it would not neces- sarily follow that the first current was of double the intensity of the second, but all that could be safely asserted would be thatjthe first was of greater intensity than the second, without giving any quan- titative estimate of the amount of their difference. Moreover, the graduation of every galvanometer has a special value of its own, a current producing a certain deflection with one galvanometer pro- ducing a different deflection with another instrument. It will also be found best to employ at the commencement of an experiment a current of knorvn direction, when the direction of the deflection by this current can be noticed, and this will, of course, serve as a guide for determining the direction of other currents which it may be re- quired to test. A modification of the ordinary galvanometer is sometimes used for determining the difference in intensity between any two fte differential currents, and is known as the differential galvanometer. galvanometer, 116 ELECTRICITY AND In this case two precisely similar wires are coiled round the wooden frame, and arc carefully insulated from each other, and their ends connected with binding-screws, so that separate currents can be simultaneously passed through both coils, but in opposite directions. If the currents are of the same intensity no deflection of the needle takes place, but if the needle be deflected the direction of its deflection indicates the stronger current. Although the ordinary astatic galvanometer is not available for measuring the absolute or relative intensities of cur- The tangent rcntS) t wo instruments, known respectively as the tangent compass and the sine compass may be success- fully employed for that purpose. To those of our readers who may not have studied trigonometry f the terms sine and tangent will prove unintelligible ; trigonometrical we shall therefore make no apology for giving a very terms. brief explanation of them. If, as in our diagram, fig. 81, we have an angle A formed by the M two lines A P, A M, and if we draw a line PM anywhere on A M and perpendicular to it, so long as the angle A remains unchanged there will always be a definite, unchanging ratio or proixniion between the three lines A P, A M, PM. To these ratios between the lines the term Trigonometrical Ratios has been applied, and to each ratio a special name has been given. For instance, the fraction produced by divid- ing the perpendicular p M by the hypothenuse A P is termed the sine of the angle A, and is usually expressed thus : = sin A. In the same manner the perpendicular PM divided by the base AM is known as the tangent of the angle A, and is usually expressed by the following formula: =tan A. In the same manner each of the other ratios between the three lines of the triangle hasja name, but as we are only concerned to know the meaning of the terms nine and tangent we will not pursue the siibject further. ELECTRICITY AND MAGNETISM. 11? All these ratios have been calculated for every possible angle, and a table of their values will be found in every book of logarithms. Having now some idea of the meaning of the term " tangent of an angle," we will proceed to describe the tangent compass, or as it is more frequently termed, the tangent galcano- ^, he tenffe t "* meter. It is found that when a magnetic needle is suspended in the plane of the magnetic meridian and a current caused to flow round it, the intensity of the current is directly proportional to the tangent of the angle of deflection. This fact is, however, strictly true only when the needle is very small as compared with the diameter of the circuit. Accordingly the tangent galvanometer consists, as shown in fig. 82, of a ring of copper, usually some twelve inches in diameter, which is fixed in a vertical position upon a stand, the lower half of the ring being generally fixed, to secure its steadiness, in a wooden frame In the centre a small magnetic needle, not more than a tenth of the diameter of he copper ring in length, is suspended. Beneath this needle is a graduated circle. To the ends of the copper ring wires- are attached, which are led into the mercury cups. a, b, and a battery can be easily connected with the galvanometer by plunging its wires into the mercury. When used the circle is placed in the magnetic meridian and the angle to which the needle is deflected is read off. It must, however, be remembered that it is not this angle, bitt its tangent, which indicates the intensity of the current; the value of the lattter must therefore be obtained from a table of tangents. 118 ELECTRICITY AND MAGNETISM. The sine compagg, or sine galvanometer, represented in fig. 83. is used for measuring strong voltaic currents. It consists of a circular frame, M, round which several turns of stout covered c PP er w * rc are coiled, their ends terminating in bind- ing-screws at E. In the centre of the ring is fixed a circular platform, and in the centre of this platform a magnetic needle, m, is suspended. Round the edge of the platform is a graduated circle, N ; and a light pointer, n, is attached to the mag- netic needle, so that it can move along the circle N when the needle is deflected. The circle M, and with it the circular platform N, is supported on a foot, o. which is movable about a vertical axis passing through the centre of the fixed horizontal circle H. In using the instrument, M is placed in the magnetic meri- dian, and wires, a, b, from the battery being attached to the binding- screws at E, the cur- rent is allowed to pass. The needle being deflected, the ring M is turned until it coincides with the vertical plane passing through the magnetic needle m. The directive action of the current is now exerted perpen- dicularly to the direction of the magnetic needle, and it can be proved that the intensity of the current is proportional to the sine of the angle of deflection. The angle itself is measured on he graduated circle H, by means of a vernier attached to the piece c, and the value of its sine may be obtained, as in the case of the tangent, from the tables. Besides these instruments for measuring the intensity of a voltaic ent, we must not omit to mention the voltameter, due to Faraday, The voltameter, which depends for ' its action upon the electrolytic action of the current upon water. This consists of a glass vessel in which two platinum electrodes arc ml. In the neck of this vessel a bent tube is placed, and this i fl mnected also with a graduated cylinder in which the gases result- ing from the electrolysis of the water are collected and measured. The amount of chemical decomposition is directly proportional to the strength of the current. In accordance with this principle it has ELECTRICITY AND MAGNETISM. 119 been proposed to adopt, as a unit of the intensity of the current, that intensity which in. one minute yields a cubic centimeter of mixed gas reduced to the temperature of C. and the pressure of 760 millimetres of mercury. To render this mode of measure- ment more exact it would be better to measure the volume of the hydrogen alone, for oxygen is rather more soluble in water than hydrogen, and moreover it has a tendency to form ozone. Both these facts of course tend to reduce slightly the bulk of oxygen evolved in electrolysis. In connection with the measurement of the electric current, certain very important laws have been enunciated which call next for our attention. The thorough investigation and m s .* wa formulation of these laws was effected by Professor Ohm, and they are accordingly known as Ohm's laws. Before giving these laws it will be necessary to explain certain of the terms employed in them. First, then, it is agreed to call the force which in a Definition of cell or battery urges the current forward the electro- terms. motive force. Secondly, the term resistance is employed to denote anything which opposes itself to this electro-motive force ; so that the strength of the current will be the electro-motive force as diminished by the resistance. Thirdly, when a current is passing through a closed circuit, we are dealing with dynamic electricity (Greek dunamos = force). Fourthly, when the circuit is broken the current ceases to flow, but the ends of the wires remain charged with static electricity (Latin sto = I stand). Now, when a wire connecting the poles of a battery is broken, the static electricity at the ends of the wire can. as we have before explained, be measured, and it is found to be proportional to the strength of the current which passes through the wire when the wires are united. The statical electricity may in this way be taken as a measure of dynamical electricity. In measuring the tension of the electricity in different parts of a wire connecting the poles of a battery, it is found that when the middle point of such wire is connected with the earth, the tension of that point is nil, while right and left of this point the tension gradually rises up to the poles of the battery, where it attains its maximum. If any point of the wire other than the central point be connected with the earth, it becomes the zero point of tension, right and left of which the electricities increase in tension towards the poles of the battery. In both cases the electricity will, of course 120 ELECTRICITY AND MAGNETISM. be positive on one side of the zero point, and negative on the other. If the negative pole 6f the battery be connected with the earth, the whole of the wire shows positive electricity, and vice versa. There will, as before remarked, in every circuit be a resistance offered to the passage of the current, an opposition to the electro- motive force tending to reduce it. This resistance sting throu g hout the circuit will be partly in the conducting wire and partly in the battery itself. The latter resistance may be represented by a certain length of the con- ducting wire, and when this is done the sum of the lengths of both wires is called the reduced length of the circuit. If the reduced length of the circuit and the electro-motive force are known, the tension of every point of the circuit may be easily calculated. It is^ usual to represent the distribution of tension throughout a A CDEFCHIKLB FIG. 84. circuit in a graphic form by means of lines. A horizontal line called Electric fall an af>scissa is usecl to represent the circuit; vertical lines drawn perpendicular to certain points in the abscissa, and called ordinatcs, represent the tension at corresponding points in the circuit. If a line is drawn joining the upper ends of all the ordinates, that line will indicate the distribution of the tension in the circuit. The inclination or steepness of this line represents what Ohm termed the electric fall. The electric fall is, however, more correctly denned as the decrease in the length of the ordinate for the unit of length of the abscissa. ,A11 this may at first sight seem rather puzzling to an amateur, but perhaps the difficulty may be done away with if we give a rough diagram such as that in fig. 84. It will, of course, be understood that in this and all similar diagrams the lines must be drawn to scale that is to say, a given length of the horizontal line must represent a given (but not necessarily equal) length of the circuit, and a given length of the vertical line must represent a certain amount of tension. Let AB then, be the abscissa representing the length of a certain. ircmt. And let ce, Drf, EC, etc., be vertical lines (ardinatcs), reprc- ELECTRICITY AND MAGNETI&M. 121 senting the tensions at certain points in the circuit AB. The line cJ indicates the distribution of tension in the circuit, and it was to its inclination that Ohm applied the term electric fall. In strictness, however, the electric fall is proportional to the difference in length between any two neighbouring ordinates. Finally, the area of the triangle C&B represents the total charge of the wire. Ohm, in his researches, assumed that the passage of electricity from one section of the wire to the other was due to the difference in tension of the two portions. The laws which Ohm deduced from his experiments may be summarised as follows, and readily remembered by the Summary of student :- Ohm ' s laws " 1. The intensity of the current is equal to tlw electro-motive force divided by the resistance. 2. The intensity of the current it inversely proportional to the length of the conductor, and directly proportional to its cross- sectional area as tveU as to its conductivity. The first of these laws needs no comment ; the second requires perhaps a little explanation. In the first place it means that, other things being equal, the sJwrter and thicker the wire the less the resistance, and therefore the greater the inten- sity of the current. The conductivity of a wire or other conductor is of course different in each case, and must be specially determined ; but being known, the greater this conductivity the greater the intensity of the current. The facts embodied in the second law may be formulated in a somewhat different mannar. thus : (1) If the wire uniting the poles of the battery is of the same material, and of the same thickness throughout, the "electric fall" will be the same throughout the wire. (2) If the wire is of the same material, but not of a uniform thick- ness, the "fall" is steeper on the thin wire than on the thick ; it is, in fact, inversely proportional to the cross-sectional area of the wire. (3) If two conducting wires are employed, of the same thickness but of different resistances, the electric fall is steepest on the more resisting wire. The "fall" is directly proportional to the specific resistances of the wires. These laws of Ohm, which have been since verified by Kohlrausch, are very useful in enabling us to determine how best Application of to adapt a battery to the special work it has to perform. Ohm's laws. The first law may be stated as a formula, thus : I = ^ T 122 ELECTRICITY AND MAGNETISM. I representing the intensity, E the electro-motive force, and K the resistance. Now, in applying this to the battery, we must remember that R is in this case made up of two elements, the resistance in the fluid between the plates, and the resistance outside the battery in the wire, etc. The former is called internal resistance, and is usually represented by R ; the latter is termed external resistance, and is represented by r. Our formula therefore may be written : I -- ^ Now, in a case where a short thick copper wire is employed to connect the poles of the battery, r is so small in comparison with R that it may be neglected ; and inasmuch as K will be correspondingly increased the greater the number of cells, we should get little or no more effect from a large battery than from a single cell. If, however, the external resistance is very great, as in the case of working the electric light or the electric telegraph, then R is so insignificant in comparison with r that we may neglect it. Now, r remains constant, and therefore the more cells we employ the better ; and within certain limits the intensity is proportional to the number of elements employed in the battery. If the plates of a battery are enlarged, there is no increase in the electro-motive force ; but the cross-section of the conducting medium being increased, the resistance will be correspondingly less, and there- fore the intensity greater. Increasing the size of the plates will not, however, indefinitely increase the intensity. By those laws of Ohm which we have just investigated, we may To determine the eas ^y determine, when the resistances of a battery are best arrangement known, the most efficient way in which the cells of ltter y- the battery may be coupled together. If, for instance, we have a battery of six cells, we may couple the cells up in four different ways. First we may unite the cells in the ordinary " tandem " fashion, by joining the copper of one to the zinc of the next, as shown in fig. 85. Again we may unite them, as in fig 86, in three pairs, the two coppers and the two zincs of each pair emg joined together. A third way would be to unite them in two threes, as in fig. 87. Lastly, all the coppers and all the zincs may be respectively joined together so as to form one large cell, as shown in fig. 88. ELECTRICITY AND MAGNETISM. 123 Now, if we suppose that the internal resistance R of each cell is say 4, and the external resistance r = 12, we can easily Arithmetical calculate which of these four methods of coupling the calculatisns. battery will be most advantageous. -I 2 FIG. 86. To take the first case, the following formula will represent the conditions which will obtain 6E 6E 6E 6x4 + 12 36 In the second case, by uniting the cells into pairs, we have virtually doubled the size of the plates, each pair of cells thus coupled being I equal to a single cell with plates of double size. This doubling the size of the plates will, in accordance with the law that "the resistance is inversely proportional to the sectional area of the conductor," halve the resistance ; and the electromotive force remaining in each of the three large cells being the same as in the small ones, our formula will become ' I = 3E _ 3E _ 3B _ 6E ~ 18 = 36' 124 ELECTRICITY AND MAGNETISM. In this instance, therefore, no benefit would arise from doubling the size of the plates. Let us see how the third arrangement comes out when tested arithmetically. The formula will now be _ 2E 2E _ 2E 6E = 2*+r ^TFTl^ = | + 12 = 44- Th is method, then, would be disadvantageous. In the last case the formula becomes j_ E _E 6E 6" ' This would be a still worse arrangement, with the supposed values of R and r. It must, however, be distinctly understood that the question as to which of these methods of coupling up a battery is the most economical depends entirely upon the relative values of R and r, and will differ in almost every case. If, for instance, the values of R and r were respectively 5 and 9, the foregoing formula; would be as follows : (1) I -= 6E 6E 6E (2) j 3E 3E 3E 6E 3f + ' rn T 2E 2E 2E 6E ~ ^JTr~ 2 x 4 + 9 V- + 9 37 m T E * E _6E =R Tr =. 9-5T Here the most advantageous method ot arranging the battery would be in three pairs, and the most disadvantageous would be that of uniting the cells so as to form a single element. In like manner, by taking different values for the internal and external resistances of any battery, the best method of coupling the elements may easily be ascertained. Tp If, however, we consider the equation I- -^ - in its application to an unlimited number of any certain cells and a given external circuit, and bear in mind that by increasing E (that is, adding more cells, as in fig. 84), we increase R proportionally, we shall find it most economical to make R and r equal to one another. So that we can easily tell Low many cells to employ for any given circuit i.e. to overcome any given resistance if the resistance of the circuit and of one cell are known by dividing the resistance per cell into the resist- ance of what is to form the external circuit. In order that we may be able to obtain a correct knowledge of the ELECTRICITY AND MAGNETISM. 125 relation to each other of the resistances of different conductors, it is necessary that we should have instruments by which resistances may be accurately measured ; and we will now proceed to a description of some of the most useful instruments employed in these measurements. The first of these which we shall describe is the "rheostat," which is employed to alter the resistance in a circuit without breaking the connections. It is shown in fig. 89 and consists of ^ rheogtati two cylinders arranged parallel to each other, one of these cylinders being of glass, wood, or other non-conducting material, and the other of metal. Grooves are cut upon the non- conducting cylinder, and in these grooves a brass or copper wire attached to the axis of the cylinder is wound. The other end of this wire is wound round the metal cylinder with its turns in an opposite direction to those on the insulated cylinder, so that if the wire be wound off one cylinder it is wound up on the other, and thus the relative proportions of the wire on the two cylinders may be regulated at will. The ends of the wire are attached to the axes of the cylinders, as may be seen from the figure. Connected with the cylinder A is the binding-screw n, and with the cylinder B is another binding-screw ; to the"se screws the battery wires may be attached, and thus the rheostat placed in the circuit. The action of this instrument depends upon the fact, that when a wire through which a current is passing comes into contact with a large mass of metal or other conducting material, the current passes directly through the conductor. Rheostat *** Consequently, when a current reaches the rheostat, it will only pass through that portion of the wire which is wound upon the non-conducting cylinder B. From B it will pass to m, and thence to , so that the only part of the wire which is in the circuit is that which is coiled on B, together with the portion which passes from B to A. It is evident that by uncoiling wire from B to A the resistance of the circuit will be decreased, while by the reverse process it is increased, This is effected by the handle shown in the figure, which 126 ELECTRICITY AND MAGNETISM. must be turned from right to left when the length of wire in th( circuit is to be increased, and in the opposite direction when the resistance is to be decreased. The length of wire uncoiled is denoted by a dial not shown in the figure. A modification of the rheostat is the rheocord, which is an instru^ ment used when the current is very intense, and where, therefore the rheostat cannot be employed because of the neces. The rheocord. sary tn i nness o f j ts w i re . It is shown in fig. 90, and consists of a board on which two uprights, E and F, are erected, and between these uprights a German silver wire a b c is stretched double, so as to lie horizontally between its supports. Upon it is threaded an iron thimble, G, through two holes in the top of which the wire tightly passes, and the thimble, which is thus made movable OE a b c, is filled with mercury. Now, in accordance with the principle upon which the rheostat depends, it will be easily seen that when a current enters the wire at a, it will pass along the wire until it reaches the thimble of mercury, G, through which it will pass and return to c, not traversing that part of the wire which lies between G and B. Consequently, the nearer G is to E the lees distance of wire will the current have to traverse in passing from A to C, and vice versa. In this way the rheocord, like the rheostat, may be em- ployed to regulate the amount of resistance in a circuit without breaking the connection. Before proceeding to describe a very important instrument for Resistance in measuring resistances, " Wheatstone's bridge," it will *" be necessary for us to briefly notice one or two points connected with the resistances of wires. In the first place, it is found that if we take a number of wires each having different resistance, and join them end to end, the resistance of the whole series will be the sum of the resistances of the individual wires. Thus, suppose we have six such wires, having astances represented by the numbers 2, 3, 6, 7, 4 and 8, the total resistance of the series, joined end to end, will be represented by M number 30. Again, if the wires instead of being joined end to end, are arranged side by side, and one eet of ends joined to the ELECTRICITY AND MAGNETISM. 127 positive and the other set to the negative pole of the battery, it will be found that the conductivity of the whole is the sum of the con- ductivities of the separate wires. Inasmuch as we may consider any prismatic conductor to be made up of an indefinite number of minute prisms lying side by side, it follows that if you have two wires of the same length and chemical nature as each other, their resistances will be inversely as their sectional areas inversely, that is, as the number of imaginary prisms of which we may conceive it to be made up. Putting the results of these two investigations together, we may say the resistance of a cylindrical or otherwise prismatic conductor varies directly with its length and inversely with its sectional area. It must be carefully remembered, in comparing the resistances of wires by noting their respective diameters, that the areas of circles vary directly as the squares of their diameters, consequently with two wires one having twice the diameter of the other, the resistances will be as 1 to 4, and not as 1 to 2. Another important feature in the resistance of a bundle of wires such as we have imagined above is the relation of the current pass- ing through each to their resistance. If we have, as Delation of just now supposed, a bundle of wires of different re- current to sistances lying parallel to each other, and joined up to ""stance, the poles of a battery, the current from the battery will divide, and will pass along the different wires in quantities varying inversely as their respective resistances, the greatest proportion of the current going along the wire having the least resistance. Having made ourselves acquainted with the foregoing facts con- nected with the resistances of conductors, we will next pass on to the consideration of the instrument by which the electrical resistances of different conductors may be ascertained. This instrument we have already referred to as Wheat- stone's bridge, or the electric balance ; we will first describe its con- struction, and subsequently investigate the principle of its action. A diagram of the bridge as usually constructed is given in fig. 91. Upon a piece of well seasoned board, M, are placed three strips of thin brass or copper about half an inch in width, which are fastened as shown at A, B, and D, a break being left at both ends of A, and also between B and D. From B to D a thin German silver wire, which should be uniform in thickness and free from flaws, is stretched, and soldered to the brass or copper strips at each end. Underneath this wire a paper scale accurately divided into a thousand parts is placed. Should the length of the wire be, as is usuaUy the case, a metre, the divisions will of course be millimetres ; but there is uo necessity for the wire to be of any definite length, all that ia 128 ELECTRICITY AND MAGNETISM. required is that it should be accurately divided, the measurements to to be taken from it being, as will presently be seen, not absolute but comparative. German silver wire is usually employed because its conductivity is but little affected by variations in temperature. An ebonite block c, provided with a metal pin, is made to slide along the board, and is connected with a wire, the other end of which may be attached to one pole of the battery or to a galvanometer. The metal pin is usually provided with a spring so that it may be pressed down upon the wire or not, at pleasure, thus forming a ready means of making or breaking the circuit. To the middle of the strip of copper A, a binding-screw is attached, and to this is fastened one of the battery wires. Binding-screws are also attached to the two ends of A, and to the adjacent ends oi the side strips B and D. In these binding-screws strips of wire, ? r 3 , can be placed so as to fill up the breaks between the side strips and the ends of the longitu- dinal strip A. B and D are connected with a delicate astatic gal- vanometer G. This being the construction of the Wheatstone's bridge, its mode of action will be perhaps better understood by a reference to fig. 92. Let A, B, c, and D, represent four conductors with breaks in them at a, b, c, and d, in which certain resistances may be introduced. Principle of Moreover, let the points A and c be connected with Wh brid St e ne ' 8 the terminals of a battery, and the points B and D with a delicate galvanometer G. Now, if we suppose in the first place that the four resistances a, b, c, d, are all equal, the current arriving at A will divide, one part passing round the gal- eter in the direction ACBGD, and the other in the direction SB, and these currents being equal and opposite will balance other, and therefore produce no deflection of the galvano- ELECTRICITY AND MAGNETISM. 129 meter needle. If, however, the resistances a and b are different, then the tensions at B and D being also different, a current will pass through the galvanometer, and the needle will be deflected, the current passing either from B to D, or D to B, according as the resist- ance a or b is the greater. We can, however, restore the equilibrium of the galvanometer needle by varying one of the resistances until the currents are again equal, and in this way, by noting the lengths say of b and c, we may determine their relative resistances. Further than this, however, we may prove that no deflection of the galva- nometer needle takes place when the four resistances are in the proportion a : b = d : c. By this we are enabled to obtain the value of the resistance of any conductor placed in one of the breaks for instance, c. If now our readers refer to fig. 91, they will see that in the ordinary form of Wheatstoae's bridge the two conductors AD, DC, are replaced by the German-silver wire, the breaks at c and d repre- senting the breaks between the side strips B, B, and the longitudinal strip A. In using the bridge it is usual to insert a wire of known resistance or a rheostat in one of these breaks, say 1\ ; the wire the resistance of which is to be ascertained is placed in the other break. The pointer c is then moved along the wire until a point is reached where no deflection of the galvanometer needle takes place, and the currents passing round the galvanometer must therefore be equal. If this result is obtained when the pointer rests on the middle point of the scale (500), the resistance of the wire to be measured, r 3 , is evidently equal to that of the known wire, r t . If however the pointer has to rest upon any other point of the scale than the middle, then the resistance of r v is to that of ?' 3 as r 2 is to ?- 4 . Suppose, for instance, the pointer to rest at one-third of the length of the scale from B, the resistance of r 3 would evidently be twice that of 1\. In addition to thus measuring the resistance of a wire by Wheat- 130 ELECTRICITY AND MAGNETISM. stone's bridge, we may perform the reverse operation and measure the lengths of different pieces of wire of which we know the resistance per unit length. Suppose, for instance, we have several lengths of copper wire, all of the same diameter and conductivity, by putting in one of the breaks a known length of the wire, and putting the other lengths successively in the other break, the points on the scale at which the index has to be placed to secure the equilibrium of the galvanometer needle will, as before, indicate the respective resist- ances, which being directly as the lengths of the wires, will give us the information required. The internal resistance of a cell may be determined without the use of a Wheatstone's bridge by joining up in circuit a rheostat and Determination of a galvanometer with the element whose internal internal resist- resistance has to be measured, and noting the deflection ano8 - of the galvanometer needle. A second element is then joined to the first so as to form one of double size that is to say, the two zincs and the two coppers are respectively united, which will, of course, halve the internal resistance. A length of wire is then added to the circuit by the rheostat until the same deflection of the galvanometer needle is obtained as at first. The length of wire added to the circuit obviously represents half the resistance of the single cell, and therefore double that length of wire will represent the whole resistance. The length of wire thus representing the internal resistance of a cell or battery is known as the reduced length of that particular cell or battery, in terms of the wire employed. Hitherto we have considered only the relative proportions of resist- ances to each other, but it is necessary for practical purposes to have some definite unit of resistance to which reference may resistance ^ e un i versa ^y made. Amongst the various units which have been proposed we may mention that of Siemens, which consists of the resistance of a column of mercury one metre high, and having a sectional area of one millimetre, which at C. is taken as the unit. The unit of resistance most generally adopted is, however, that fixed by a committee of the British Association specially appointed for the purpose, and which is known as the " British Association unit." This unit depends upon the relation between a current, the force it exerts upon a magnet, and the distance and strength of the magnet. The term " ohm," or " ohmad," is applied to this unit, which si equal to I'OISG of a Siemens' unit. ELECTRICITY AND MAGNETISM. 131 CHAPTEE XIV. ACTIONS OP CURRENTS ON EACH OTHER AND BETWEEN MAGNETS AND CURRENTS VOLTAIC INDUCTION. Laws of attraction and repulsion of currents Apparatus employed Use of the apparatus Repulsion of separate elements of a current Sinuous currents Laws of circular currents Important cases of the relation between currents Action of an infinite current upon a free current Currents capable of moving on an axis Rotation of currents by currents Apparatus to illustrate rotation of currents Rotation of magnets by currents Action of magnets on currents Rotation of currents by magnets Current reversere or com- mutatorsThe simple commutator, or rheotrope Another form of com- mutator Action of the commutator Berlin's commutator Action of Bertin's commutator Voltaic induction Faraday's researches Induced or secon- dary cutrente Direction of induced currents Induced currents on approach and withdrawal of secondary Effect of alteration in the intensity of the primary current Increase in the effect of induction phenomena Apparatus to exhibit induced currents Work done in moving the secondary circuit Theoretical explanation Induction with the Leyden Jar Use of Matteucci'a induction apparatus The extra current Induction by magnets Apparatus for showing magneto-electric induction Induced currents and lines of magnetic force Magnetism of rotation Apparatus to illustrate rotation of a magnet by copper Stoppage of rotation by magnets Heat produced by rotation Faraday's explanation of magnetic rotation Induction by the earth's magnetism Delezenne's circle Use of the apparatus Connection between electricity and magnetism Experimental illustration Magnets as groups of currents Magnetic attraction and repulsion explained Resultant polar currents Terrestrial currents Cause of terrestrial currents Expla- nation of former experiments. THE investigation of the action of currents upon each other was first undertaken by Ampere, shortly after Oersted's x, awg ofattrao discovery of the action of a current upon a magnetic tion and repul- needle, to which we have before referred. His researches <^ on o{ currents, soon led to the enunciation of two fundamental laws, which are as follows : 1. Currents which are parallel, and in the same direction, attract one another. Currents which are parallel, but in opposite directions, repel one another. 2. Two currents forming an angle attract one another if they are in the same direction rvith regard to the apex of the angle ; if they are travelling in opposite d irecti&ns with regard to the apex of the angle they repel one another. The apparatus usually employed for the demonstration of these laws is represented in fig. 93. Two brass columns, A and D, of equal 132 ELECTRICITY AND MAGNETISM. height, with a shorter column between them, are fixed on a stand, the column D being provided with a multiplier of twenty turns, M N, which greatly increases the delicacy of the instrument. Apparatus em- j he movable clamp by which the multiplier is attached ployed. ^ ^ e co i um n. D gives facility for its being adjusted at any height and in any position. In the short tube, which is hollow, is made to slide a brass tube carrying a mercury cup, c. The column A is also provided with a mercury cup, through the bottom of which is an extremely small aperture, into which a sewing needle is passed, the needle being attached to a small copper ball. The needle-point ex- tends into the mercury in the cup, and is freely movable in the aperture through which it passes. From the small copper ball pro- ceeds a bent wire frame, BC, re- volving in the mercury cup c, and balanced by two copper balls at- tached to the ends. IO yg The apparatus is usually worked with a battery of four or five Bunsen's cells, and we will suppose the current to travel in the direc- tion of the arrows, ascending by the column A to the mercury cup a, descending thence through B C to the cup c. From the central column it then passes to the multiplier by the wire P, and after traversing M N it returns to the TIG. 94. battery by the 'wire Q. If before the current is allowed to pass the ELECTRICITY AND MAGNETISM. 133 rame BC be turned so as to be in the same plane as the multiplier, with the side B opposite the side M, the movable frame will be re- pelled as soon as the connection is made, thus proving the second part of the first law of Ampere. To demonstrate the first part of this law the multiplier must be reversed, as shown in fig. 94. In this arrangement, as shown by the arrows, the current will be in the same direction both in the multiplier and in the adjacent part of the movable frame. Consequently attraction between the frame and the multiplier results ; and if the former be removed out of the plane of the latter it tends to return to it. The law of angular currents enunciated above may be proved by the modification of the apparatus just described, shown in fig. 95, in which it will be seen that the movable frame B c is replaced by another movable piece of different shape. To prove the first part of the law the apparatus is arranged as in the figure, when attraction ensues, if the movable part be moved when the current passes in the same direction through it as through the multiplier. The second part of the law is proved by arranging the apparatus as in fig. 96. Here the currents are in different directions, and repulsion between the multiplier and the movable frame ensues. From the preceding laws of Ampere, it follows that Re ^g^^ ge in a rectilinear current each element of the current parate elements repels the succeeding one, and is itself repelled,, of a current. An ingenious piece of apparatus to prove this was invented by Faraday. It consists of a piece of copper wire bent in the shape of the letter \J, the ends of which dip into two mercury cups. This 134 ELECTRICITY AMP MAGNETISM. wire is suspended from one end of a very delicate balance. When the mercury cups are connected with the poles of a battery, the wire perceptibly rises, sinking again when the current ceases to pass. FIG. 96. Here the mercury cups and the copper wire form two elements in the circuit, the former being fixed, the latter movable ; consequently the repulsion between these two elements of the circuit results in the raising of the copper wire. Another very important topic for consideration in dealing witii the ELECTRICITY AND MAGNETISM. 135 laws of currents is the action of currents passing through coiled wires. It has been demonstrated that these so-called sinuous (winding) currents are in their action equal to recti- Sinuous cur- linear currents, having the same length as their projec- tion. In other words, the action of a current passing through the coiled wire will be equal to that of a current passing through a straight wire of the length of the line joining the two ends of the coiled wire. This fact may be demonstrated by the same apparatus as that em- ployed for demonstrating the laws of the attraction and repulsion of currents. The apparatus is arranged as shown in fig. 97. The multi- plier is arranged vertically, and near it is placed a circuit half str; ijht and half sinuous. Upon allowing the current to pass, neither attrac- tion nor repulsion takes place, showing that the action of the sinuous portion of the current in n is equal to that of the rectilinear portion no. Concerning our consideration of the action of currents upon each other, we will now notice the laws which . ^ awsof govern the attraction and repulsion of circular currents. . If, as in fig. 98, we have two hoops of wire, A B, supported on a stand through which currents are allowed to pass in the direction shown by the arrows, it will be evident that if the hoops of wire be in the relative positions shown on the left of the diagram, the currents moving in the same direction will attract each other in all parts of their course. If, however, B were reversed with regard to A, the currents would then be going in opposite directions, and repul- sion between them would necessarily occur. Thus far our consideration of the mutual action of currents upon each other has been unaccompanied with much difficulty ; but there are one or two cases in which the relations of adjacent ImportantcaEes currents to each other will require more careful con -of the relation oe- sideration. These cases, however, deserve all the care tween currents, and attention we can bestow upon them, as they serve to illustrate 136 ELECTRICITY AND MAGNETISM. the important fact that, knowing the action between any two recti- linear parts of a current, we can easily solve the problem as to the nature of the action which the whole current will have upon an adjacent current, whatever may be its form. We will first examine the action of an indefinite, or _ Action of an ag ^ j g some ti m es termed an infinite, current, which is "upon a free" fixed and rectilinear, upon a portion of a second current current. f ree i n space and perpendicular in direction to the first. Let MN in fig. 99 represent the infinite rectilinear current travelling in the direction of the arrow, and let AB be a finite rectilinear current at right angles to MN and perfectly free in space. Let CD M 5f C ; - FIG. 99. be a perpendicular common to A B and M N : it is obvious that this perpendicular will vanish if AB and MN meet each other. Consider first the case in which the current AB approaches MN (vpper figure). The currents A Band MC approaching the apex of the angle, there will be attraction between them. This attraction we may represent by a force/, acting from a certain point m in AB towards a point p in MC. In the parts of the two currents AB and CN the currents are in the opposite direction to those on the other side of AB, that is to say, they are travelling- from the summit of the angle and conse- quently they will repel each other. As these currents are perfectly symmetrical with regard to those on the other side of the point c, ELECTRICITY AND MAGNETISM. 137 \ve may represent the repulsion between AB and CN by a force/ acting from m in AB towards a point/" in CN equal in distance from to p in CM; / will obviously be equal in amount to/. Now, according to the laws which govern the composition of forces for which we must refer our readers to the manual on "Mechanics" we may substitute for the two equal forces / and / a single resultant force F, bisecting the angle between them, and therefore parallel to MN. It follows, therefore, that this force v will draw AB in a direction parallel to itself and to MN, but in an opposite direction to the latter. In the second case (shown in the lower figure), where A B is tra- velling away from MN, the result will, by parity of reasoning, be easily understood to be that A B will be drawn away in a direction arpullel to itself and to MN, but in the same direction as that in which the infinite current is travelling. If A B be below M N instead of above, these directions will obviously be in each case reversed. Let us, however, suppose that instead of A B being perfectly free, it is only capable of moving on an axis parallel to itself, it will easily be understood that in the first case it would rotate on Currentg ca aMe its axis in a direction towards M, and would remain in of moving on an equilibrium when the plane of the axis and of the cur- *** rent have become parallel to M N, on the side from which the infinite current M N comes. In the second case equilibrium will result when the plane of the axis and' of the current are parallel to M N on the side towards which the infinite current is directed. It will also be seen that a system of two yerticaj currents rotating about a vertical axis above a horizontal current will be directed by this current in a plane parallel to itself, if one current be ascending and the other descending. If, however, both currents ascend or both 138 ELECTRICITY AND MAGNETISM. descend, no motion will result, the action on one branch of the system counterbalancing that on the other. We have next to turn our attention to the rotation of currents by currents. A reference to our diagram (fig. 100) will show how this is notation of H we have a circular wire in which a current currents by is passing, and have perpendicular to this a horizontal currents. current AB, it can easily be shown that AB is attracted by all the elements of the half-circumference CPC/, and is repelled by all the elements of the other half-circumference CP'C'. Conse- quently the resultant will be a force F parallel to the chord PP,' drawn from the point D at right angles to the diameter cc', and tending to cause the circular current to pass up A B. Directly A B is, by the influence of this force, slightly displaced, it will of course be perpendicular to another diameter of the circle, but its relation to the circumference will remain undisturbed. AB will therefore tend to turn continuously round -the axis O x so as to describe a cylinder. Should the rectilinear current travel from B to A instead of from A to B, rotation will still be set up, but it will be in the opposite direction. A piece of apparatus by which the rotation of a vertical current by a horizontal circular current may be experimentally verified is shown Apparatus to It consists of a brass vessel round which illustrate rota- about twenty turns of covered copper wire are coiled, >f currents. and through this wirc a current j g made to pass> The centre of this brass vessel is occupied by a pillar, having on its summit a mercury cup, into which dips the pivot of a copper wire, the two extremities of which are bent down at right angles so as to form vertical branches. These branches are soldered to a very light copper ring, -which i.s immersed in acidulated , which is connected with the central pillar. It then ascends the pillar and passes through the bent wire to the copper ring, to which the ends of the wires are attached. It then passes through the acidulated water to the sides of the -vessel, and returns to the battery by the binding- screw a'. When the current passes, the bent wire and its attached ring tend to rotate in a direction contrary to that of the fixed current. The reason of this is easily seen, for the branch on the right is attracted by one portion of the fixed current, while the branch to the left is attracted in the opposite direction by the other portion of the fixed current ; these two attractions will therefore coincide in setting up a continuous rotatory motion of the ring in the same direction. The horizontal part of the circuit in the bent wire would also be affected in a similar manner by the horizontal fixed current, but on account of the distance between these two their action is too small to re- quire notice. Not only, however, may currents be rotated by currents, but magnets may also have the same rotatory motion im- parted to them. The discovery of this important fact is due to Faraday, and in fig. 102 we have represented the ap- paratus by which it may be shown. A is a large glass vessel almost filled with mercury, on the edge of which is fixed a n otat j on of metallic ring, B, which just touches the mercury. In magnets by the mercury is "placed a small magnet, loaded with a currents, platinum cylinder, d, by which it is maintained in a vertical position. The magnet projects a little above the mercury, and has on its upper end a small cup containing mercury. A copper wire is attached by one of its ends to the metallic ring, and by the other to the binding-screw g. Attached to the binding-screws h and g are wires leading to a commutator, to which latter the battery wires are attached. As soon as the circuit is made, the current, entering we will suppose at A, traverses the pillar to which it is attached, passes to the magnet, thence into the mercury, and finally leaves by the binding-screw g. As long as the current passes, the magnet rotates with a rapidity depending upon the strength of the magnet and \f~' ; ntensity of the current. By reversing the direction of the 140 ELECTRICITY AND MAGNETISM. current the direction of the rotation of the magnet is also reversed. The explanation of this interesting fact depends upon a knowledge of the Amperian theory of magnetism, to which we shall refer later on, and we will then return to a theoretical consideration of this phenomenon, which the reader will find worthy of close attention. We have seen that, as shown by Oersted's experiments, currents have Action of a directive action upon a magnetic needle freely sus- magnets on peuded. If the conditions are reversed so that the currents. ma gnet is fixed, while the wire through which the cur- rent is passing is movable, it is seen that the magnet has a directive action upon the current. To demonstrate this fact the apparatus shown in fig. 103 is employed. As it is very similar to apparatus which has been previously fully described, the diagram will doubtless explain itself. The bent part of the circuit being movable, if a magnet is brought near as shown in the figure, the wire begins to move, and eventually sets itself at right anglej to the axis of the magnet. The directive action of magnets on currents may be extended to the rotation of the latter by the former, and shown by the apparatus Rotation of Devised by Faraday, a diagram of which is given in currents by fig. 104. Supported on a base with levelling screws is a copper rod B D, which along the greater part of its length is surrounded by a bundle of magnetized wires. On the top of is a mercury cup, in which rests a copper wire, E F, balanced on a steel point. The other ends of this wire dip into mercury contained m a circular basin. The current from a battery enters at the binding- ew * passes up the pillar, traverses the bent circuit E P, and passes i the mercury and metal framework to the binding-screw a, by which it returns to the battery. When the current is passing, if rJT C . t f iC .| )Undlc i of wires is > the circuit E P rotates, the M its rotation depending upon the nature of the polo ELECTRICITY AND MAGNETISM. 141 of the magnet which is presented to it. The explanation of this curious fact also depends upon Ampere's theory of magnetism. A solenoid or an electro-magnet may be substituted for the mag- netised bundle mentioned above, in whicli case a current is admitted by the binding-screws without letters shown to the right of the diagram, to traverse the solenoid or electro-magnet. In several of the experiments which we have described in the foregoing chapters we have had to speak of the reversal of the direction of cur- rents, and it will be obvious that a ready means of effecting this is a great convenience in many electrical investigations. It will be subsequently seen that a current reverser, or com- n imitator, forms an indispensable reversersor part of many electrical machines ; commutators, we will therefore briefly describe one or two of the more common forms, leaving the description of special commutators to be considered with the machines of which they form a part. The simplest form of commutator is that sho ,vn in fig. 105. Here a block of wood The simple is taken and four shallow troughs commutator, or are scooped out in it. These troughs rheotrope. being filled with mercury, are connected by wires arranged either diagonally, as in the right-hand figure, or parallel to the sides of the block, as in the left-hand figure. Two neighbouring troughs receive the battery wires, while from the other two troughs pass wires FIG. 104. conveying the current to the apparatus connected with the com- mutator. The current is at once broken by removing either of the connecting wires. The direction in which the current flows for each position of the wires is indicated by the arrows. A form of commutator which is much more efficient when a quick reversal of the current is required is that shown in fig. 106. A quadrant of a cylinder A is hinged as shown at a, so that it can be readily turned by the handle c. Upon the two convex edges are fixed 142 ELECTRICITY AND MAGNETISM. strips of brass, 1, 3, 2, 4, which are so arranged as to leave the middle of the curved surface uncovered. This uncovered Another form of p art j s occupied by pieces of ivory, ebonite, or other non-conducting material. The brass strips are joined with each other, as shown in the diagram, by pieces of brass which do not touch each other. As will be seen, the strip 1 is joined to 4, and the strip 2 to 3. Upon each end of the quadrant brass springs rest so as to be in contact with the brass strips, and the other ends of these springs are attached to supports at the ends of the commutator stand. To one pair of supports the battery wires are attached, while from the other wires pass into the apparatus with which the commu- tator is connected. When the springs rest, as shown in the figure, upon the non-con- ducting part of the commutator, no current caai pass ; Commutator but if by means of the nan(ile tne quadrant is moved so as to bring either pair of brass strips under the springs, the current will pass. If the strips 1 and 2 are brought under P and G, the current will pass from D to 1, then from 1 to P and G, and finally from 2 to E. If, on the other hand, 3 and 4 are brought under the springs F and G, the current will pass from 1 across to 4, from 4 to G and round to P, and then from P to 2 and E and back to the battery. We have, therefore, in this form of commutator a very ready means of making and breaking the circuit, and also of instantaneously changing the direction of the current. A very useful and ingenious form of commutator is that known as Berlin's, which is shown in fig. 107. It comprises a block of hard Bertin' W d UP n Whi h is fixed an ebonite P late movable by commutator. means of the handle m, the range of movement being limited by the two stops c and c'. On the ebonite disc two plates are fixed, and one of these, o, is always positive, being connected by the axis upon which the disc turns' and by the plate with the binding-screw P, to which the positive pole of the battery is attached. The second plate on the ebonite disc is in the shape of ELECTRICITY AXD MAGNETISM. 143 a horseshoe, and is connected with the plate, which is itself connected* with a binding-screw N, which communicates with the negative pole of the battery. At the opposite end of the board two binding-screws, B and B', are placed, and these support two elastic metal plates, R ands'. The action of this form of commutator is as follows: The ebonite disc being in the position shown in the figure, the current enters at P. passes into the plate o; from thence it passes to the ^ c ti ono f metal strip E and the binding-screw B, and then to the Bertin's apparatus which is being used, returning to the binding- commutator, screw B' and to the plate E'. From this ib passes to the horseshoe plate i.e, and by way of the binding- screw N back to the battery. Should the ebonite disc be so placed that the handle m occupies the mid-position between the stops, no current will pass, as the brass plates o and i e are no longer in contact with the pieces E, E'. If the handle is turned into the reverse position to that shown in the diagram, the current will pass first to FI. 107. B', and then will return to B ; this direction, it will be seen, is the reverse of that. described above. The next series of electrical phenomena which claims our attention is one of the most interesting and important in the whole range of the science which forms our study. The phenomena to which we refer are those of voltaic induc- tion, and their discovery is due to the great Faraday, whose name in connection with electrical researches has so often figured in these pages. In the year 1831 Faraday made the discovery that, when a current from a battery was sent through a wire, it had the power of evoking a temporary current in a neighbouring conductor un- connected with a battery, both when the circuit, was made and when it was broken ; to which he gave the name of induced or secondary currents. The current from the battery is termed the primary current, and the wire through i n a uce a or which it passes is called the primaiij wire. The wire secondary or other conductor in which the temporary currents currents, are induced is termed the secondary wire or conductor. If we have a primary wire through which a current may be sent from a battery, and near to this a wire unconnected . Direction of induced currents. with any battery, but in connection with a galvano- meter, the following phenomena may be observed : When the primary 144 ELECTRICITY AND MA ONETISM. circuit is made, the galvanometer needle is deflected, indicating the passage of a current through the secondary wire. The deflection of the needle is, however, only instantaneous ; it immediately returns to its normal position, and however long the primary current may be continued, the galvanometer needle is not again affected. When, however, the primary current is broken, another deflection of the needle takes place, indicating the passage of a current through the secondary wire. As before, this secondary current is only of momen- tary duration ; it, however, differs from the current induced at the making of the primary in being opposite to it in direction. This experiment shows us that when the primary is made, and when it is broken, induced currents of momentary duration are set up in the secondary wire, these currents being in opposite directions to each other. It may also be further shown that the induced current estab- lished at the making of the primary is in the opposite direction to tho primary current, while the induced current set up at the "breaking of the primary is in the same direction as the primary. Not only are these secondary currents produced when the primary is made and broken, but they are also produced by the approach and withdrawal of the secondary. If a current be passing through a primary wire, and a conductor, unconnected with a battery or other source of electricity, be made to approach the primary, a current is induced in the conductor, which continues only so long re^t** on*" as * ne approach continues, and is in the opposite direc- approachand tion to that in the primary. When the conductor is withdrawn from the primary wire, a current is set up in it, which is in the same direction as the primary current, and which lasts only during the time of withdrawal. Approach and withdrawal of the secondary therefore give similar results to those produced by the making and breaking of the primary when the secondary remains still. Not only are these induced currents produced by the making and breaking of the primary current, but they are also established upon Effect of altera- any alteration in the intensity of the primary. If the tion in the in- intensity of the primary is increased, a secondary tensity of the current is induced which is in the opposite direction to rent the primary. Should the intensity of the primary be decreased, a secondary current in the same direction as the primary is established, both these currents existing only while the increase or decrease in the primary current is taking place. These results might naturally be expected, making the current being only a maximum increase in the current strength; breaking the primary being a maximum diminution in the primary intensity. ELECTRICITY AND MAGNETISM. 145 If the single wires referred to in the last chapter are replaced by helices of wire, the strength of the induced currents is greatly in- creased. It is in fact found that the strength of the .. induced currents depends. (1) upon the strength of the effect of induc- primary current, (2) upon its nearness to the primary tion phenomena, current, and (3) upon the length of the opposed parts. The induced current, it has been further shown, varies directly with the strength of the primary. It varies also as the products of the lengths of the two currents. It therefore results that if both primary and secondary wires comprise several coils, the strength of the induced currents will be much exalted. The apparatus shown in fig. 10S is very useful to illustrate the in- duced currents which we have been describing. It consists of a small bobbin of covered wire, A, which is connected with a galvanometer and fits into a larger bobbin of similar construction, B. The latter is the primary wire, and is connected with the battery, while A is the secondary wire, and, as already said, is connected with a galvanometer. If A is allowed to remain inside B, the currents produced in it at the making and break- ing of the primary will be . evidenced by the move- exhibitinduced ments of the galvanometer currents, needle, which, as explained above, will be opposite in direction and momentary in duration. The currents induced upon approach and withdrawal of the secondary can also be illustrated by the gradual moving in and out of A, when, as before, deflections of the galvanometer needle will take place. These deflections, however, will only be maintained as long as the movement of approach or withdrawal is continued By the same apparatus the currents produced by alterations in the intensity of the primary may also be shown. It is noticed that when we approach a secondary wire towards a primary, a distinct effort has to be employed over and Work donc ^ above that caused by the mere weight of the secondary, movin* the The same is noticed when the secondary is withdrawn **<"k*y from the primary. The forqe which it is necessary to expend in moving a secondary either towards or away from a primary is explained by the laws of the attraction and repulsion of currents which we have previously 146 ELECTRICITY AND MAGNETISM. fully discussed. It will be remembered that currents travelling in the same direction attract, while those flowing in opposite directions repel, each other. Consequently, when a secondary Theoretical S pj ra i j s brought near a primary, repulsion between the induced and primary currents is set up, and we have, in bringing them together, to overcome this repulsion . On the other hand, when a secondary spiral is withdawn from a primary, the primary and induced currents attract each other, and there is consequently force required to overcome this attraction. The mechanical force thus ex- pended appears as heat in the secondary wire, and if the amount of heat be estimated, it is found to be the mechanical equivalent of that. Instead of a battery to produce the primary current, a Leyden jar may be employed ; and to illustrate the effects of induction by this means the apparatus represented in fig. 109 has been w devised b ? Matteucci. Two glass plates A and B, about ' twelve inches in diameter, are fixed vertically on two supports with movable feet. On the anterior face of the plate A ia coiled a copper wire c, about thirty yards in length and a millimetre in diameter. The two ends of this wire pass through the plate one in the centre, the other near the edge, and then are connected with two binding-screws, not shown in the figure, on the anterior face of A. The wires c, d, through which the primary current is passed, are attached to these binding-screws. A coil of very much finer copper wire is fixed on the face of the glass disc B, and its ends are connected with the binding-screws 1 *, to which the wires i and h are also attached. In order to insure the perfect insulation of the coils of wire on the glass plates, each * 1 o e il COvered with silk > and a Ia 7 er of shellac is interposed between In using this apparatus the wires c and d are connected respectively ' the outer and inner coatings of a charged Leyden jar. This ELECTRICITY AND MAGNETISM. 147 gives rise to a primary current in the wire C, which establishes a momentary induced current in the wire on the plate B, and a person holding the handles attached to h and i receives a snoc k.-p S eof Matteucci's The intensity of the shock increases with the diminution induction of the distance between the glass plates A and B. apparatus. In pursuing our studies in the domain of electrical induction, we have next to notice an important phenomenon which The extra occurs in the primary wire itself, and which is known current, as the extra current. We have already seen that when a current is started in a wire, it has the power of inducing a current in an adjacent conductor, which current is in the opposite direction to the primary. We have seen further that a similar secondary current is established when the primary is broken, but in this case the induced and primary cm-rents are in the same direction. As a further result of modern research, it has been shown that when a current is started in a wire, an induced current of momentary duration is set up in the primary itself, another current being also produced in the primary itself when the latter is broken. The current induced by the make is in the opposite, while that induced by the break is in the samt, direction as the primary. These currents are known as extra currents. When, therefore, the primary is made it raises an antagonist in its own path which momentarily reduces the strength of the primary ; while, when the primary is broken, a momentary current is set up in the same direction as the vanishing primary. We shall see hereafter how this extra current is dealt with in apparatus such as Euhmkorff's induction coil. The induction phenomena described above may all be produced without any battery whatever, magnets being substi- induction by tuted for it. The apparatus by which induced currents magnets, produced by magnets may be illustrated is shown in fig. 110. It comprises, as will be seen, a hollow bobbin of coiled covered wire, the ends of which are connected with a galvanometer. One pole of a bar-magnet A B is inserted into the coil, and immediately a deflection of the galvanometer needle indicates the presence of a current in the coil, which neto-electrio disappears as soon as the magnet ceases to move, induction. Upon withdrawing the same pole of the magnet from the coil, another 148 ELECTRICITY AND MAGNETISM. current parses through the coil, this second current being opposite in direction to the first. If now the other pole of the magnet is intro- duced, a current similar in direction to the last is generated in the coil ; while the withdrawal of the magnet results in a current in the opposite direction. In other words, the introduction of the north pole of a magnet into a coil produces in the coil an induced current in the same direction as that produced by the withdrawal of a south pole, and vwc versa. We have here the reciprocation on the part of the magnet of the magnetizing power which a current exercises upon iron and steel. Faraday, in his researches on induction, found that when a con- currentg ductor moves alon 9 the lines f ma nctic force (* ce andlinMo*" "article on Magnetism) no induced currents are pro- magnetic force.^ uce( j ; but that if a conductor moves across the lines of force, the induced currents appear. This affords an explanation of the phenomenon discovered by Arago, in 1824, that the number of oscillations which a magnetic needle makes under the influence of the earth's mag- Magnetism of ne tj sm } s vei y muc h decreased by the proximity of metallic bodies, especially copper. This led to the dis- covery by the same philosopher of the fact that when a copper disc is made to rotate beneath a magnet, the magnet is set in rotation also. In fig. Ill is represented the apparatus which is usually employed to illustrate this curious fact. It consists of a copper disc M, movable about a vertical axis, which is connected with a multi- a- plyi n g wheel A. Upon this wheel being turned by the tion of a magnet hand, the disc may be made to rotate with considerable by copper, velocity. Above the copper disc is a glass plate, and Balanced upon a pivot on this plate is a small bar-magnet. When the disc is rotated the magnet is attracted, and rotates with it. If the disc is rotated only slowly, the needle is only deflected from the magnetic meridian, and does not rotate ; but, as the rotation of the disc in- creases in velocity, the deflection of the needle is increased, and eventually rotation is set up. ELECTRICITY AND MAGNETISM. 149 The reverse of the previous experiment was demonstrated by Fara- day. A cube or sphere of copper was made to spin between the poles of an electro-magnet, by the untwisting of the string by which it was suspended ; but directly the magnet was st PP a & e f made the cube ceased to rotate. The experiment is a '^agnets. 3 '' very remarkable one, by reason of the instantaneous stoppage of the rotating body without any visible cause. A variation of the experiment is to take a bar of copper and pass it backwards and forwards between the poles of an electro-magnet ; a considerable amount of resistance is experienced by the operator, the sensation being a very curious one. not unlike the cutting of cheese. If a conductor is forcibly rotated between the poles of a power- ful magnet, it becomes very much heated ; the dis- covery of this fact is due to Dr. Joule, and was further Heat produced illustrated by Foucault, who caused his gyroscope to yr rotate between the magnetic poles. Faraday explained these phenomena of magnetic rotation by showing that when a metal disc is rotated so as to cut the lines of force, currents are set up in it, flowing from the centre to the cir- cumf crence and vice versa. Fig. 1 1 2 will make this explanation more magnetic rota- clear. Let A B be a magnetic faoni needle suspended over a copper disc, and oscillating from N to M in the direction .of the arrows. As the needle approaches M, it generates no. 112. a current which repels it, while at the same time by leaving N it induces there a current which attracts it, and both these actions tend to stop the oscillations of the needle : this explains Arago's first observation. Again, if the disc moves in the direction of the arrows, the point N approaching the magnet repels it, while the point M by moving away from the magnet attracts it, and therefore the needle follows the movement of the disc : this explains Arago's second experiment. Finally, supposing the magnet to be fixed -and the disc to turn in the direction of the arrows, N will be repelled by A as it approaches the magnet, while M as it leaves the magnet will be attracted by A, and both these actions tend to destroy the motion of the metal : these are the conditions in Faraday's experiments. The earth being itself, as we have already seen, a magnet, it will be readily understood that it is capable, b'ke other magnets, of pro- ducing induction phenomena. Faraday proved this by a long helix of silk-c"ovcrctl copper wire in the plane Of the magnetic meridian 150 ELECTRICITY AND MAGNETISM. and parallel to the dipping needle. The helix was then turned through 180 round an axis perpendicular to its length dearth's* in its middle. The ends of the helix being in connec- magnetism, ^ion w jth a galvanometer, it was noticed that a deflection of the needle occurred at each turn. For the examination of the induction phenomena due to the earth's action, the apparatus represented in fig. 113 may be Delezenne's em pi O y CC i A wooden ring about two feet in diameter 110 e< is fixed to an axis about which it can be turned by a handle, M. A square frame supports the axis, and this frame is also movable about a horizontal axis. A dial, b, is fitted to the frame, and needles fitted to the two axes of the apparatus mark on this dial the inclination towards the horizon of the frame, and therefore of the axis first mentioned. Upon a second dial, c, the angular dis- placement of the ring is registered. In a groove in this ring a quantity of insulated copper wire is coiled, and the ends of the wire are con- nected with a commutator, which causes the current always to pass in the same direction, in spite of the change in direction caused by the revolution of the ring. The commutator has on each ring two brass plates, which successively transmit the current by two wires to a galvanometer. In using the apparatus, the axis first mentioned is placed in tho magnetic meridian, and the ring is adjusted so as to be at right angles to the direction of the dippirv- n -vile. By turning the tfseofthe handle the ring is slowly routed, and a deflection of the galvanometer needle is produced, which of course indicates the presence in the coil of wire of an induced current. The intensity of this current increases until the ring has been rotate** ELECTRICITY AND MAGNETISM. 151 through a right angle ; after that it decreases, until, when the ring has completed half a revolution, the galvanometer needle returns to zero. Supposing the rotation to be continued, the current again makes its appearance, but in the opposite direction ; it attains a second maxi- mum when the ring has been rotated through a third right angle, and again disappears when a complete revolution of the ring has been effected. If the axis first mentioned be placed parallel to the dip. no current is produced. These results are extremely interesting, and tend to show that the natural magnetism of the earth (whatever may be the cause of that magnetism) has the same power of inducing currents in conductors as artificial magnets. They also open a wide field for speculative thought, and show us what a vast and complicated series of electrical phenomena constantly surround us, every accidental movement of a conductor of any sort, such even as the ring upon one's finger, may give rise to electric currents, which have none the less real existence because mostly unperceived We have already seen that, on the one hand, currents of electricity are capable of conferring magnetic properties upon iron and steel, and deflecting magnetic needles from their normal position, connection and on the other hand that magnets are capable of between elec- inducing electric currents in adjacent conductors, and tricity and of attracting and repelling movable currents. These maene ' and similar facts have led to the enunciation of a theory of magnetism, known as Ampere's theory, which supposes an intimate connection, between electrical and magnetic phenomena. Ampere's theory of magnetism will be more thoroughly understood by the consideration of the following facts. If, as in fig. 114, we have a spiral, P, through which a current is passing, as indicated by the arrow-heads, in the direction of the Experimental hands of a watch, and if we place upon this primary spiral a second similar spiral, s, there will be generated in this secondary spiral a current going in. the opposite direction to that in the primary, that is, in the opposite direction to the hands of 152 ELECT&ICITY AM) MAGNETIStf. a watch. Upon removing s from P, the secondary current will of course be, like the primary, in the same direction as the hands of a watch. Now, if we substitute for the primary spiral, P, the south pole of a permanent magnet, precisely the same effects are produced a current opposed to the direction of the watch hands upon approach, a current in the same direction as the watch hands upon withdrawal. Obviously, then, the south pole of a magnet acts in the same manner as a ring in which a current is passing in the same direction as the hands of a watch move. Upon substituting the north pole of a magnet for the south, the induced currents are respectively opposed in direction to those mentioned above. The north pole of a magnet, therefore, acts the same as a ring through which a current is passing in the opposite direction to the motion of the hands of a watch. From these considerations it is a natural and easy step to suppose that magnetism consists of groups of currents circulating in each molecule of the magnet in such a way that when the north pole is looked at by an observer, the currents appear to be going in the opposite direction to the hands of a watch ; while when the south <$> pole is looked at, the currents seem to be flowing in the same direc- tion as the hands of a watch. A little consideration Ma ou e i S f 8 wil1 show us that) thou g h the currents in the opposite currents. poles of a magnet appear to be going in opposite direc- tions, they are really all going in the same direction in xjwce. For instance, let fig. 115 be a cylindrical bar-magnet, having Amperian currents circulating round it as shown by the arrow-heads. It is obvious that when we look at each end, the currents will appear to be going in reverse directions ; this, of course, arises from the fact that, in looking first at one and then at the other end of the magnet, we have to turn round either the magnet or ourselves. The great test of any theory is the readiness with which it adapts itself to facts; and in this respect Ampere's theory is extremely Magnetic attrac- Satisfactory > as a11 known facts ma y be far better com- tion and repul- pared by its aid than by that of any other theory of sion explained, magnetism which has been propounded. The facts of the repulsion between similar magnetic pole?, and the attraction ELECTRTCITY AND MAGNETISM. 153 between opposite poles, flow naturally from Ampere's theory viewed in the light of the laws of currents which we recently discussed. If, as in fig. 116, the two north or the two south poles of two bar- magnets are presented to each other, the Atnperian currents of the two magnets will be opposed in direction, and repulsion will there- fore ensue. If however the opposite poles are adjacent, the Arnperian currents will be in the same direction as each other, and attraction between them will therefore result. The manner in which a series of molecular or atomic currents may cause resultant currents passing round trie entire magnet in the direc- tion to which we have referred, will be understood by reference to fig. 117. Here we have represented north and south poles of a magnet, and the small circles are intended to represent diagrammatically the molecules, having currents circulating in the directions indicated by the arrows. It will be seen that in the molecules in the central portions of the magnet, the neighbouring parts of the currents ofj adjacent molecules will! be opposed in direction, and will therefore neu- tralise each other. With the molecules near the cir- cumference of the magnet the case will be different. Here only the internal portion of each molecular current will be neutralised, while the outer portion will be free to exert its characteristic influence. The outer portions of the peripheral molecular currents will thus produce, as shown, a single resultant current in the same direction as the watch hands in the south pole, and in the opposite direction to the watch hands in the north pole of the magnet. Whatever theory of magnetism we may adopt, it must be evident that the explanation of the position taken up by a freely suspended 154 ELECTRICITY AND MAGNETISM. magnetic needle when at rest can only be caused by the influence of the earth upon the magnet. If we go no further than ^currents* 1 & merel y su P erficial explanation of this fact, we may content ourselves with saying that the earth is a large bipolar magnet, having one pole which attracts the (so-called) north pole of the magnet, and the other pole attracting the (so-called) south pole of the magnet. If, however, we go a step further, and endeavour to explain the origin of the earth's magnetic condition, we naturally recur to Ampere's theory, and suppose that, like all other magnets, the earth is traversed by a series of electric currents. The direction of these currents is from east to west, perpendicular to the magnetic meridian ; and they cause a resultant current traversing the earth's magnetic equator in the same direction. We have already had occasion to notice that there is a close con- nection between heat and electricity, and we shall shortly see that heat can generate electric currents. The heat of the sun is therefore thought to be the cause of the Amperian currents which circulate round our globe, the currents being caused by the constant variations in temperature occurring at every part of tte earth's surface. If this hypothesis be true and there seems to be very strong evidence in its favour we have to seek in the sun, the great centre of our system and the source of our light and heat, the origin also of that magnetism which, under the control of scientific minds, has been rendered so useful a servant to mankind. The explanation of some of the facts which we noticed in a previous chapter will now be evident to our readers. In the case of the rotation of magnets by currents, the current traverses the Explanation of magnet, and passing into the mercury, breaks up into '" an infinity of rectilinear currents, which proceed from the axis of the magnet to the circumference of the glass. Each of these rectilinear currents acts upon the Amperian current of the magnet in the same way as the rectilinear current upon the circular one in a previous experiment, and rotation of the magnet ensues. The currents being upon the surface of the mercury act only upon the upper part of the magnet ; and if the north pole be upwards, the rota- tion will be from west to east, while if the south pole be uppermost, the rotation will be from east to west. The action of a magnet upon a movable current will also be easily explained by Ampere's theory. In fact, by the adoption of this theory, all interaction between mag- nets and currents is reduced to an interaction of currents only, to which, of course, the usual laws of currents apply. _ ELECTRICITY AND MAGNETISM. 165 CHAPTER XV. ELECTRO-MAGNETS AND COILS. Polarity of electro-magnets Memoria tcchnica Effect of nature of tube of electro-magnets Laws of electro-magnets Solenoids Directive tendency of solenoids Action of currents on solenoids Mutual action of solenoids Ruhmkorff's coil Importance of insulation Action of the coil Automatic make and break The condenser Action of the condenser General sum- maryEffects of the Euhmkorff's coil. THE polarity of electro-magnets and of bars magnetized by the cur- rent affords a further proof of the truth of Ampere's theory. As we have already observed, the wire helices which surround bars which are to be magnetized may be either right- el *^[ of etg or left-handed. In a right-handed helix the coils pass in the same direction as the turns of an ordinary screw that is to say, when the helix is held vertically the downward direction of the coils is from the observer's right hand to his left. In a left-handed helix the direction of the coils when the spiral is similarly held will be from the observer's left to his right. In fig. 118, A is a left-handed or sinistrorsal helix ; B is a right-handed or dextrorsal helix. When a right-handed helix is placed round a bar, and a current is passed through the helix, the end of the bar where the current leaves the spiral will be a north pole, while the end at which the current enters will be a south pole. In a left-handed helix the polarity will, of course, be in the opposite direction. The polarity of the magnet may also be found by the following simple rule, whatever may be the direction of the coiling : If a person swimming in the current looks at the axis of the spiral, the north pole is always on his left hand. A little observation will show that these results naturally flow from Ampere's theory, and no difficulty can therefore be experienced in ascertaining the polarity of an electro-magnet : for if the current in the pole opposite the observer flows in the direction of tube of electro- magnets. 156 ELECTRICITY AND MAGNETISM. of the hands of a watch, that pole must be a south pole ; while if th current flows in the opposite direction, the pole must be north. The nature of the material composing the tube upon which helice are frequently wound is found to have some influence upon the strengt of the magnetism imparted to the core. Wood an Effect of nature glass have no appreciable effect, but, as will be imagine "from what has been previously said about inductioi copper, and also iron, silver, and tin, have a great effec in diminishing the strength of the magnetism. This effect is nc observed when the metal is slit longitudinally. In our first notice of the nature of electro-magnets we noticed th laws of electro- * aws governing their force of attraction and repulsioi magnets. The following statements embody what is known as t their other properties: 1. Taking the term electro-magnetic force to indicate either the ir duction current produced in the surrounding helix at the making an unmaking of the magnet, or its free magnetism acting upon a frecl FIG. 119. suspended magnetic needle oscillating at a certain distance, the dec tro-magnetic force is proportional to the intensity of the current. 2. Taking into account the resistance, tlie electro-magnetic force i independent of the nature and thickness of the wire. 3. The current being the same, the electro-magnetic force is indc pendent of the n-idth of the coils. The core must however projec beyond the coil, and the diameter of the coil must be small compare with its length. 4. The temporary magnetic moment of an iron bar is, within certaii limits, proportional to tlic. number of turns of the helix. 5. The magnetism in solid and hollow cylinders of the same dia meter is the same, provided that in the latter case sufficient iron i present for the development of magnetism. 6. As previously demonstrated (p. 108), the attraction of ai armature by an electro-magnet is proportional to the square of tb intensity of the current. ELECTRICITY AND MAGNETISM. 157. Solenoids. 7. For powerful currents, the length of the branches of an electro- magnet does not affect the weight which it can support. If further proof of the Amperian theory of magnetism be necessary, it is founded in the behaviour of those helices of wire which are known as solenoids (Gr. solos = a, mass of iron ; eidog= shape, form). In fig. 119 the form which the solenoid essentially assumes is shown. A piece of copper wire is coiled into a helix, and then one end of the wire is made to pass through the helix to its other extremity, or better, the two ends of the wire are passed through the helix and made to pass out of its centre. From the laws of currents it follows that the rectilinear current through the hori- zontal parts of the wire will counterbalance the action of the solenoid in a longitudinal direction, and consequently, when a current is passed through the solenoid, we shall be dealing only with a series of equal FIG. 120. parallel currents acting in a direction perpendicular to the axis of the solenoid. When a current is passed through a solenoid which is freely sus- pended, it sets itself in the magnetic meridian, and if moved out of it will return to it, oscillating precisely as a magnet would do. It is also found that that end of the solenoid Directive ton- which points towards the south pole has its current den cyof flowing in the direction of the hands of a watch, and ' vice verm. If by means of a commutator the direction of the current through the solenoid be changed, the latter immediately reverses the position of its ends. If. as in fig. 119, a solenoid is balanced on pivots in two mercury cups, above a rectilinear current P Q, and a current is passed through the rectilinear wire and through the currents solenoid, the latter will turn and set itself at right on solenoids, angles to the rectilinear wire. In this position of course its currents 158 ELECTRICITY AND MAGNETISM. are parallel to that in the fixed wire below, and the lower part of eac parallel current is in the same direction as the rectilinear current. To further prove the identity of magnets and solenoids, it is four that if the like poles of a magnet and a solenoid are brought near i each other, repulsion between them results ; while if the opposi poles are brought near, they mutually attract each other. In like manner, when, as in fig. 120, two solenoids, one of which held in the hand and the other is freely suspended, ai Mutual action brought near each other, precisely the same interactio takes place between them as between two magnets,- that is to say, "like poles repel, unlike attract." In solenoids we are of course dealing with groups of parallel cu rents, and inasmuch as the direction of these currents in the poles < the solenoids precisely agrees with the assumed direction of the hypi thetical currents in a magnet, we may rest assured that whethej tl theory of Ampere be, in its entirety, absolutely true or not, thei must be a considerable amount of truth in a hypothesis which s precisely coincides with the recognised facts. Full advantage has been taken of the laws and phenomena of indue tion in the construction of electrical apparatus, and we are now in position to enter upon a consideration of some of the most valuabl and ingenious machines which depend upon the principles we hav just been discussing. The first induction apparatus which we shall notice is that knowr Ruhmkorff's from the name of its inventor, as Ruhmkorff's coil. A coU. commonly constructed it is shown in fig. 121 : but better idea of its various parts will be obtained from fig. 122. 1 consists essentially of two coils one, the primary, made of stou copper wire; the other, which is the secondary coil, being compose* of very fine copper wire of great length. The primary wire, which may be forty to fifty yards in lengtl is coiled on to a cardboard cylinder, in which a bundle of soft iro wires, a little longer than the primary coil, is placed. The soft iro ELECTRICITY AND MAGNETISM. 159 wire forms the core of the machine. Over the primary a cylinder of some insulating material, such as vulcanite, is placed, and on this is coiled the secondary wire. This latter consists of a considerable length of very fine silk-covered wire ; the length of course varies with the size of the coil, but is rarely less than one or two miles, and in the largest coils may even exceed sixty miles in length. One of the most important things to be observed in the construc- tion of a Ruhmkorff's coil is the attainment of perfect insulation ; and we would advise those of our readers who intend mak- ing a coil to pay particular attention to this matter, as any failure in the insulation will greatly weaken, and may altogether destroy, the intensity of the induced current. Not only is it necessary, therefore, that the best silk-covered wire should be employed, but each individual coil of wire must be carefully insu- lated from the next by a thin layer of shellac or solid paraffin, which must of course be applied in a melted condition. In constructing a small-sized coil it will be found that the best wire for the secondary is No. 36. For the primary the wire should be about one-sixteenth of an inch in diameter. The thinner and longer the secondary wire the more nearly will the electricity produced resemble that of the machine that is to say, the greater will be its tension. The primary wire is in connection with the commutator or key of the coil, which will be subsequently described, while the ends of the secondary wire pass to the two glass pillars, o, o', and between these ends the spark-discharge passes. The binding-screws, a and c, being connected respectively with the positive and negative poles of a battery, the current enters the commutator from the binding-screw a, and Acti ""ji f the passing through this, reaches the primary wire ; and the current passing through the primary of course induces another current 160 ELECTRICITY AND MAGNETISM. in the secondary wire, while the primary returns to the battery through the binding-screw c. Now, supposing the current to pass without interruption in the manner just indicated through the primary wire, it is obvious from the laws of induction that only a single momentary Automatic curre nt will be induced in the secondary wire, and that ' 'the current must be constantly made and broken if a continuous series of sparks from the secondary be required. In order to effect this a very simple and ingenious arrangement is employed in this and many other induction machines. It will be best understood by reference to fig. 122, in which the commutator is, for the sake of simplicity, omitted. H is a piece of soft iron attached to a spring, by which it is forced to rest against a movable screw passing through a small metal pillar o, which latter is connected with the binding- screw m. A little consideration will show the reader that, supposing* the current to enter by m, it will pass through o H n to the primary wire ?p, and will return to the battery from the binding-screw I. But when the current passes through the primary, it will immediately convert the bundle of iron wires 1 1' into a powerful magnet ; this magnet will attract the soft iron hammer H, and will pull it away from the screw and break tlie circuit. The consequent stoppage of the primary current will, however, demagnetize the core, which will cease to attract H, and the latter will, under the influence of the spring to which it is attached, fly back, and again make contact with the screw, thus re-making the circuit. In this ingenious way the primary current may be constantly being made and broken automatically, and at each make and each break a current is of course induced in the secondary, and passes in the form of a spark between the ends s' s". The intensity and length of the spark produced by the Ruhmkorff coil may be very greatly increased by the addition of a condenser, which is usually placed in the stand beneath the coil. nser< It consists of a number of sheets of tinfoil joined to- gether and placed on opposite sides of a band of oiled silk, which forms an insulator, so that the arrangement in principle resembles a large Leyden jar. Alternate sheets of tinfoil are connected with the binding-screw m, which receives the current from the commutator, while the other sheets are connected with the screw by which the return current enters the commutator. The action of the condenser in increasing the intensity of the spark depends upon the " extra current," the nature of which we have alread y explained. When the battery current passes through the primary wire, an extra current is induced in the opposite direction to the primary, while at the ELECTRICITY AND MAGNETISM. 161 break of the primary an extra current will be induced in the same direction as the vanishing primary, and the latter will consequently be prolonged. Now, it is evident that when the primary is made the induced extra current in the primary wire will greatly reduce the strength of the primary, which in its turn, will induce in the secondary a correspondingly enfeebled current. On the other hand, when the primary is broken, the induced extra current adds itself to the vanishing primary, and a much more powerful secondary current is produced. The extra current produces a spark between the hammer and the screw point of the automatic contact breaker; the points where these touch should therefore be armed with small pieces of platinum. Let us suppose (fig. 122) the current to pass through the primary via ft, m, q, o, h, n, q, and return to the battery by 2>, I, and z. Now, if the two coils are in the same direction, the secondary current would at the make of the primary pass from 8' to s", while at the break of the primary it would pass from s" to s . Consequently, if the cur- rents thus produced were of equal strength, we should have a series of discharges alternating in direction between S and s '. We have, however, seen that in consequence of the extra current this is by no means the case. When the primary is made, it is greatly diminished by the opposing extra, and its secondary is therefore feeble. On the other hand, when the primary is broken, the extra current attempting to flow in the same direction as the primary fails to do so, as its path is broken. Consequently the secondary current produced by breaking the primary must be much more intense than that at the make, as the primary, being uninfluenced in any way by the extra current, would then produce its full effect upon the secondary. By the condenser these effects are considerably modified, as in it we are enabled to store up for future use the extra current produced at the break, which we have seen cannot pass for lack of path. This interrupted extra current passes at the break into the condenser ; and when the primary is again made in the manner explained, it is accompanied by the now released extra, by which it is of course strengthened, and its effect upon the secondary increased. In like manner the extra produced at the make also passes into the condenser, but in the opposite direction. When the primary is again broken, this last extra will assist the vanishing primary in its influence upon the secondary, inasmuch as the making of a current in one direction ia equivalent, as far as its inducing action goes, to breaking a current in the opposite direction. To sum up, then, we may say that the secondary current produced by the make is due to the influence of the primary, minus its extra, 162 ELECTRICITY AND MAGNETISM. and plus the extra of the last Ircali. The secondary produced at the General break is due to the effect of the undimi nished vanish- summary. in primary, plus that of the extra due to the last make. Therefore the make-secondary will always be of less importance than the break-secondary, and the discharge will con- sequently always take place in one direct im namely, from s" to ft The extra currents are condensed on opposite sides of the condenser' .nd are insulated from each other by the oiled silk. Paraffined paper will answer just as well for an insulating material. As we have already intimated, the electricity produced by the Kuhmkorff coil is by virtue of its high tension comparable to that Effects of the produced b ^ r the ordinary frictional machines, but is, of Ruhmkorff coil. cou rse, much superior in point of quantity. In fact, by . form of coil low-tension electricity is completely converted into high-tension electricity, and is therefore, like the latter when produced by friction, capable of leaping over considerable dis- tances and producing most powerful illuminating, physiological, and ELECTRICITY AND MAGNETISM. - 163 CHAPTER XVI. VOLTAIC ELECTRICITY (continued). The commutator or key Luminous effects Vacuum tubes Calorific effects of Ruhmkorff 'a coil Physiological effects Chemical effects- The medical coil The medical magneto-electric machine Principle of the magneto-electric machine The commutator Action of the commutator. THE last feature of the Ruhmkorff coil which remains to be described is the commutator. This in its usual form is shown in fig. 123. In it A represents a section of a piece of ebonite, which is a non-conductor. By the sides of A two brass The ^^y)** * plates c c' are fixed. Two strong brass springs a and c press against these brass plates, and are also connected with the two binding-screws which receive the wires from the battery. In the position in which the commutator is represented in our diagram, the current, as shown by the arrows, will enter at a, pass through G. and thence by the small screw y to the binding-screw b, which it leaves to enter the coil. On its return journey it enters the commutator by the plate K. which is connected with the hammer of the contact breaker ; from thence it passes by the screw x (opposite y) to C', and thence by the binding-screw C to the battery. A slight examination of the diagram will suffice to show that if the milled screw m * yiillltllllllllllliiiilllllllllllim be turned through 180 degrees, the current FIQ - 123 - will travel in the opposite direction. It will be equally clear that if the screw be turned only through 90 degrees, no current can pass at all. When the two ends of the secondary wire are separated, a series of brilliant sparks, similar to those produced by a Holtz machine, passes across the interval, with the usual snapping noise. In good coils, eight or ten inches is by no means an unusual length for these sparks, although only a small battery is attached to the coil, and in the largest coils sparks of far greater length may be obtained. Perhaps one of the most beautiful and striking effects which even electricity has to show is that produced by allowing the electric spark to pass through tubes containing either rarefied air or other gases. Under these conditions the spark passes a noiselessly, and produces a most beautiful chromatic appearance. Fig. 124 exhibits three examples of these luminous discharges : c represents the appearance of the spark as it passes through rarefied 164 ELECTRICITY AND MAGNETISM. air ; I as it is ,seen when the air contains a small quantity of the vapour of a volatile liquid ; a represents acurious deviation of the stream of light when the finger is brought near the tube. The luminous band is also acted upon by magnets in the same way as a conductor of an electric current would be. These tubes are always constructed upon the same plan, and consist of glass vessels usually shaped into some ornamental form, containing at or near each end small platinum wires fused into them. To these wires the ends of the secondary coil are attached ; or electricity of the frictional kind may be employed direct from a Holtz or other machine. Between the two inner ends of the wires a beautiful lumi- a PIG. 124. nous hazy light extends, which often exhibits stratification. Over he pos^ve pole the light is much more intense than over the nega pole rte r 1 r 117 f a r 1 * r 16SS bpight red Colour ' At the ne ^ pole the hght is generally of a beautiful violet, and is spread all over tiw Wire and not concentrated at its point, as is the case at the positive reversed tl *T ? commutator . *he direction of the current be T?e stl't-fi ! nstantaneous reversal of the discharge is very striking drcum^l n , 0f 1 the dl ' SCharge Varies in charact ^ according to ^ch gi iTt C 1 1 Ur f the dlSCharge is charac teristic for and ar^h^f T ^ manufactured b y Geissler of Bonn, Before somehmes known as Geissler's tubes ELECTRICITY AND MAGNETISM. 165 By means of the Ruhmkorff coil very surprising heating effects can be procured, the heat being sufficient to melt the most refractory metals. By placing a piece of thin iron wire between 5,^,^,, e ff ec ts the ends of the secondary coil, not only will the wire be of RuhmkorfTs fused, but it will burn with a brilliant flame. It has coU> been shown that the tension at the negative pole of the coil is greater than that at the positive pole, by attaching a piece of thin iron wire to each end of the secondary wire, when the wire joined to the nega- tive pole alone fuses as soon as the wires are brought near each other. The physiological effects obtained by means of a Euhmkorff coil are also very surprising. The discharge from a fair-sized coil is sufficient to kill a small animal. Great caution [ e ct s e in handling the terminal wires is therefore needful when the coil is in action, especial care being necessary that both wires are not touched simultaneously. As in the case of the constant current derived from a battery, t)* electrolysis of water and other compounds can be performed with tb* discharge from an induction coil ; and other chemical effects may be readily produced by its aid. The results, ^g^* 1 however, vary very sonsiderably, according to the tension of the electricity. A common experiment is that of causing a luminous discharge to take place between the electrodes under water without the decomposition of the latter. After the foregoing description of the Ruhmkorff coil but little need be said about the ordinary medical coil now in use. It consists of the usual primary and secondary coils and core, and is provided with an automatic contact-breaker precisely similar in principle to that described above. The core is, however, usually movable, so that a larger or smaller length of it may be placed inside the coil, and so the intensity of the current may be varied, within certain limits. In some induction machines, how- ever, the core is fixed, and a cylinder of vulcanite, ebonite, or other insulating material, is so arranged as to slide over it. As the cylinder is withdrawn, more and more of the core becomes subject to the influence of the current in the primary coil, and the strength of the induced current is correspondingly increased. The ends of the secondary wire are attached to two binding-screws on the stand ; and from these, conductors, with handles attached, pass to the person who is to receive the current. There is, moreover, no condenser in medical coils. As is well known, the electric current has a peculiar effect upon the nerves, not only of a living, but of a recently dead animal. Most people have felt the effects of the current upon the living body, and we will therefore not attempt the impossibility of 1(56 ELECTRICITY AND MAGNETISM. describing them. In the recently dead animal an electric current " stimulates " the nerves, causing the contraction of the muscles to which they are distributed. A more convenient apparatus, however, for generating currents suitable for medical use is a simple form of magneto-electric machine. The medical ** cons i sts f a l ar ge permanent horse-shoe magnet, magneto-electric mounted, usually in a horizontal position, on a stand, machine. Beneath the poles of this magnet two bobbins con- taining a considerable length of fine covered wire are made to rotate by means of an endless band which passes from them to a small fly- wheel, to which a handle is attached, so that it may be turned by the hand. (See fig. 126.) To those unacquainted with the phenomena of magnetic induction, Principle of the tne obtaining of a strong electric current without the magneto-electric assistance of a battery is extremely puzzling, but to machine. our rea( j ers the matter will present no difficulty. The principle of this form of coil will be readily understood by reference to fig. 125. Let N s be respectively the north and south poles of a permanent magnet, and let A and B be two soft iron bobbins round which two coils are wound in the direction shown. Now let" us suppose that the system A B begins to rotate in such a manner that A moves towards the reader and B sinks through the paper. It is obvious that a current will be induced in B, and will pass towards b. In A the current will of course be in the opposite direction namely from a. These currents, therefore, together produce a current from a to I. In like manner, as the two bobbins, on continuing their rotation, approach respectively the opposite poles of the magnet to those which they have just left, a current will be produced which will pass in the opposite direction. Consequently, unless some arrangement is introduced to modify the direction of the currents, they will be in each half-revolution alter- itely m opposite directions. For this purpose a commutator is employed. But before describing it we will briefly refer to a modi- f the ordinary medical magneto-electrical machine which ELECTRICITY AND MAGNETISM. 167 was invented by Clarke. This form of the machine is shown in fig. 126, and, as will be seen, corresponds in its principal features with the one we have just been describing. A is the compound permanent magnet, fixed vertically instead of horizontally ; B, B' are the bobbins, covered usually by about fifteen hundred turns of very fine silk-covered copper wire, the two outer ends being con- nected with a small brass or copper band, ^;. This is fixed upon the axis upon which the bobbins rotate, but is insulated from it by ivory. One coil is right-handed, while the other is left-handed, so that the current induced in each may be in the same direction. The commuta L or is shown separately upon an enlarged scale in fig. 127, and in section in fig. 128. J is a cylinder of ivory, and its axis is a copper cylinder of very much smaller diameter. This is fixed to the armature, V, of the bobbins, and Tbe J^ mU " rotates with them. The ivory cylinder has upon it a brass circular band, p. and also two similar semicircular bands, o and o', which are well insulated from each other ; the semi-band o is con- nected with p by the strip x, while o' is connected by a tongue r with the copper axis It. Below the commutator is a block of wood upon which two strips of metal, m, , arc fixed; attached to these plates are 168 ELECTRICITY AND MAGNETISM. two elastic metal springs, b and c, which, as the commutator is rotated, press alternately on the half-bands o and o'. The two commencing ends of the wire are attached to the copper cylinder k, while, as we have seen, the two opposite ends are joined to the metal band p. The commencing ends, therefore, are in metallic communication with o', while the terminals are in similar connection with o. As we have already shown, the currents produced in each semi-rotation are in alternate Action of the commutator. directions, and consequently o and o' are at one moment respectively positive and negative, and during the next respectively negative and positive. Suppose o' to be positive, the current will then descend the spring b, and return by the spring c, as shown by the arrows. In the next half -revolution, o will be positive ; but then, by the very fact of FIG. 128. half a revolution having taken place, b will now be in connection with o, instead of o' as before, and the current will consequently continue to flow through b in the same direction as in the previous half-revolution. - So far, we have supposed m and n to be joined by a connecting wire, I, but under ordinary conditions this is not the case. A third spring, , is therefore employed, in conjunction with two other metallic strips (one of which, i, is shown in the figure), to unite m and n. These pieces communicate respectively with o and o', and ELECTRICITY AND MAGNETISM. 169 are insulated carefully from each other. When the spring a touches either of these strips, the circuit is closed ; when a does not touch either strip, the circuit is broken. With this machine effects of extraordinary power can be obtained. For physiological purposes, two coiled copper wires, to which handles are attached, are employed. These handles being grasped, no appre- ciable effect is experienced so long as a does not touch i, but the moment contact is made between them, the current, no longer passing through the patient's body as at first, but passing only through b a c, induces a powerful extra current, which gives a violent shock to the person being the subject of the experiment. Rapid rotation of the bobbins produces a very rapid succession of these powerful shocks. CHAPTER XVII. VOLTAIC ELECTRICITY (continued'). Siemens' armature Wilde's machine Connection between electricity and heat Nature of combustion Distribution of heat in the voltaic circuit Relation of heat to resistance Effect of temperature on resistance Result of experi- ments by Siemens The resistance pyrometer. BESIDES the two forms of magneto-electric machines now described, there are several others of equal importance, but which differ from Clarke's only in minute details, the principles upon which their effi- ciency depends being quite identical. In those invented by Pixii and Saxton the magnet rotates instead of the bobbins. FIG. 129. An important improvement in the armature of the electro-magnet of these machines is that known, from the name of its well-known inventor, as Siemens' armature. It is represented in fig. 129, and will be seen to consist of a soft iron cylinder, A B, in the opposite sides of which a deep groove is cut; in this groove the wire is coiled in a longitudinal direction. E and D are brass discs attached to the ends of the cylinder. To the former a commutator, c, is fixed. This consists of two pieces of steel, which are well insulated from each other, and are M 170 ELECTRICITY AND MAGNETISM. respectively connected with the two ends of the wire. The bobbin rotates on the two pivots, and at the opposite end is a pulley, over which the cord passes by which it is set in motion. If a current be made to pass through the wire, the segments A and B of the core become endowed with opposite magnetism, one with north and the other with south polarity. If, on the other hand, the armature be made to rotate between the opposite poles of a magnet, the segments A and B are alternately magnetized and demagnetized, and currents alternately opposed to each other in direction will be induced in the wire. These currents being changed in direction by the commutator, will, as in Clarke's machine, produce a current con- stantly in the same direction between the terminals of the apparatus. Siemens' armature is employed in the magneto-electric machine invented in 1866 by Wilde. The principle upon which this ELECTRICITY AND MA GNETISM. 171 apparatus depends is simply this : The current obtained from a Siemens' armature is conducted round a very large electro-magnet ; and it was discovered by Wilde that the magnetism thus induced in the latter vastly exceeds that of the permanent magnets used with the armature to induce the original current. In one instance the supporting power of the permanent magnets was 40 lb., while the current they furnished produced an electro-magnet capable of sustaining 1088 lb. The armature to produce such an. effect requires to revolve with great rapidity, and the motive power employed to produce this rotation is that of the steam engine. In the improved form of his machine Wilde places between the poles of the large electro-magnet a second Siemens' armature, which, like the first, is made to rotate at a great velocity. This second armature produces currents of such enormous intensity that they readily fused rods of iron a quarter of an inch in thickness. With this machine, also, the electric light can be readily obtained. Some- times a second electro-magnet with a third armature is employed, and the current from the latter conveyed to the points at which it is to be utilized. In fig. 130, a view of Wilde's machine as constructed with one electro-magnet and two armatures is shown. The armatures are situated respectively at n in a piece of brass, O, placed between c, C, the soft iron keepers of the permanent magnetic battery P, and at m in a similar piece of brass, o, between the keepers of the electro-magnet K B. The remaining parts of the machine explain themselves, the current being of course ultimately derived from the wires r, s. The electro- magnet is formed of two large sheets of iron, usually 36 inches in length, 26 inches in width, and 1 inch thick. Upon these plates a wire about 1600 feet in length is coiled, and the magnet is com- pleted by the plate of iron at the top upon which the permanent magnetic battery stands. One cannot but be impressed with the obviously close connection between electricity and heat. Even at the very threshold of the subject we find that friction, which we know produces heat, also generates electricity, while in galvanism the connection of these two conditions of matter becomes electricity and still more apparent, the source of our electricity being heat> the oxidation, that is the combustion, of a metal such as zinc. It is now time that we should consider somewhat more closely than we have hitherto done the nature, so far as it is known, of this connection. It is beyond our province here to enter fully into the nature of 172 ELECTRICITY AND MAGNETISM. ordinary combustion, but we will just make brief reference to its principal features, referring our readers for fuller details Na * ur ^ of to manuals on Chemistry and on Heat That which we in ordinary parlance speak of as combustion is nothing more nor less than the union of certain substances with oxygen, which union, like every other chemical combination, causes an evolution of heat. When our fire, our gas, our lamp, or our candle is burning, the carbon and hydrogen of which these substances are mainly composed are uniting with the oxygen of the air and forming carbonic acid gas and water. The union of any substance with oxygen in this manner to form new compounds is termed by the chemist oxidation, and when in the voltaic cell zinc is oxidized, it is, in popular speech, burnt. In every battery, then, we have a certain amount of combustion going on and a certain amount of heat produced ; and we must now consider where that heat appears and how it affects and Distribution of j s affected by resistance and current strength. It must voltaic circuit. in tne first P lace be Carefull 7 borne in mind that the amount of heat produced by the oxidation of a given quantity, say a gramme or an ounce, is always the same, and all that can vary is the manner in which this heat is distributed in the circuit. If the two ends of a battery are connected by a stout copper wire, the resistance of which will, of course, be infinitesimally small, and the battery is placed in water of known temperature, a rise in the tem- perature of the water will indicate the amount of heat produced. Now let the battery be removed from the water, and its two ends be connected by a wire offering a considerable resistance to the current, in lieu of the good conducting wire at first used. If the battery be again immersed in an equal quantity of water, and the temperature noted at the commencement and end of the experiment, it will be found that the amount of heat imparted to the water is less than at first, although the current as measured by a voltameter is exactly the same as in the former case. This obviously shows that the amount of heat produced in the lattery by the oxidation of a given quantity of zinc is affected by the resistance of the connecting wire. If, how- ever, the resisting wire is enclosed in a separate vessel, and its heat imparted to a given amount of water is determined, it will be found that this heat added to that of the battery will be exactly equal to that produced in the battery itself, when, as in the first experiment, the poles were connected by a wire of nominal resistance. In like manner various resistances may be introduced into the circuit, and may affect the distribution of heat, but its total amount will in all cases be the same for the same amount of oxidation. ELECTRICITY AND MAGNETISM. 173 When we come to consider the amount of heat produced in a con- necting wire or in the circuit external to the battery, it is found that the heat produced is directly proportional to the resist- ance. That is to say, if the resistance of the external ^rwistance ** circuit is doubled, the amount of heat evolved there will be doubled, that of the battery being proportionally reduced. Not only, however, does resistance affect temperature, but the oppo- site effect takes place : increase the temperature of a conductor, and its resistance will be likewise increased. The ratio between these two conditions can hardly be said to Effect of have been accurately determined, so many disturbing esistance. n influences being at work to interfere with the results of experiments. A great difficulty in the way is the almost total impossibility of getting metals free from impurities ; for the least pro- portion of foreign matter has a considerable effect in the behaviour of the metal so far as its electrical resistance under the influence of heat is concerned. As the result of several experiments it would seem that the increase of resistance for every degree of the Centigrade thermometer is roughly about ^ (or 0-00366) of the entire resist- ance. This fraction represents the proportion of expansion of true gases when heated through [o.rie degree Centigrade ; we may there- fore conclude that the resistance of metals is proportional to their absolute temperature.* The following table exhibits the results of several careful experi- ments by Siemens on a platinum wire having a diameter Result of of 0'09 inch. The wire was heated in oil and also in experiments by Siemens > air. Heated in oil. Heated in air. lOO'O lOO'O CO 114-6 100 125-0 110 126-1 112-7 128-7 160 137-7 160-3 140'4 210 148-0 212-3 152-9 2-JO" 159-1 264-6 165'4 The figures on the left of each table indicate degrees Centigrade; those figures on the right of each table are intended to show the relative resistances. In order to measure very high temperatures, an instrument termed a resistance pyrometer has been constructed on the basis of the above and similar experimental results. It consists of an earthenware cylinder, round which is coiled a platinum Tnt > resistance wire, which is again covered by a protecting earthenware * Fcr the explanation cf tfce terms here employee 1 , see the Manual on Head 174 ELECTRICITY AND MAGNETISM. case. A Wheatstono's bridge is em ployed for the purpose of balancing the resistance of the pyrometer when cold against some other known resistance. Heat is then applied to the coil, and the increase of tem- perature is measured by the increase of the resistance. The instrument as in general use is graduated for different increments of temperature. CHAPTER XVIII. VOLTAIC ELECTRICITY (continued). Effect of alterations of temperature in the resistance of liquids Relation of heat to current strength Experimental proof of the foregoing statements- Explanation of certain experiments Currents caused by heat Thermo- electric couples Direction of the current Thermo-electric series The Peltier effect The thermopile. THE remarks we have made with regard to the ratio between tem- perature and resistance in the case of metals do not altefatfons of a PP^ to liquids which are decomposed by the voltaic temperature in current. In these cases an increase of temperature is the resistance of accom panied by a decrease of resistance. Melted metals, however, follow the same law as the same metals in the solid condition, while the resistances of compound bodies very rapidly decrease as their temperature is raised. Not only is the amount of heat produced in the external circuit proportional to the resistance of the conductor, but it is also affected by the strength of the current. It, however, does not Relation' of heat vary simply with the strength of the current, but directly as the sc l i;iare of tnat stren gt Q . Given, that is to say, the respective strengths of a series of currents as 1, 2, 3, 4, 5, the quantities of heat produced in each case will be respec- tively as 1, 4, 9, 16, 25. A little consideration will show that the estimation of variations of heat under the influence of current strength and resistance will prove a somewhat complicated affair, inasmuch as if by an ivoofofthe i ncreas e of current strength the heat in the conducting foregoing wire be increased, that very increase of heat will cause statements. a corresponding increase of resistance, and will thereby reduce the current strength. It is obvious that if the heat increased simply with the current strength, the increased resistance produced by it would tend to exactly negative its effect ; but inasmuch as the heat increases as the square of the increase of the current, the effect will be considerably modified, A method of verifying the statements ELECTRICITY AND MAGNETISM. 175 made above with respect to the ratio between heat and resistance, and heat and current strength, is by the use of the apparatus shown in fig. 131. S is a battery of three or four cells, one wire of which is connected with the voltameter V, and the other with A, one of two thick copper wires which pass through the cork of the flask F ; B, the other copper wire of P, is connected with one of the electrodes of the voltameter. The flask F is really a kind of thermometer, having a graduated tube passing, like the copper wires, air-tight through the cork, and being filled with spirits of wine up to a point in the stem. The graduation of this thermometer is effected in the usual way by placing it in hot water of known temperature, and noting the rise of the alcohol for a certain number of degrees, and then dividing the stem accordingly. The ends C, D, of the copper wires are united by a small spiral of platinum wire, Pt. When a current passes through the wires, the platinum becomes heated and raises the temperature of the alcohol. The weight of the alcohol in the flask and its specific heat being known, it is a matter of easy calculation to determine the number of heat units which correspond to a given rise of the alcohol in the stem. By performing two or three experiments with this appa- ratus under varying conditions, and noting the results, it will be found that the heat varies directly as the square of the current strength, and directly as the resistance. The discovery of this important law is due to Dr. Joule, who has worked out in so admirable a way the mechanical equivalent of heat. The foregoing explanation of the relation between heat and elec- tricity makes clear several important experimental results. One of the commonest of these consists in passing a current through a chain composed of alternate links of platinum Explanation of and silver, when it will be seen that while the silver ex p er i men t s . inks remain cool enough to be handled, the platinum inks glow with heat. This curious result is owing to the fact that 176 ELECTRICITY AND MAGNETISM. the conducting power of silver is about ten times as great as that of platinum. Another interesting experiment is the following : Through a long platinum wire a current is passed of just sufficient strength to make it glow feebly. If now a portion of the wire is heated, the remainder ceases to glow, demonstrating that its temperature is very consider- ably reduced. If, on the other hand, a portion of the wire is cooled, the remainder may be raised to a white heat, or even fused. In the first instance the heating of a portion of the wire increases its resist- ance, and thereby reduces the current strength ; in the second case the cooling of a portion of the wire decreases the resistance and allows an increased amount of electricity to pass along it, thereby greatly aug menting the temperature of the rest of the wire. Another phase of the same experimental facts is the fusing a platinum wire by shorten- ing it. If we have a long and thin platinum wire, it will of course ofter a considerable resistance to the current, and a corresponding amount of heat will be evolved. Let us halve the wire : what will be the result 1 Obviously the resistance will be halved and the curren strength doubled, and as the heat increases with the square of the current there will be a considerable rise in the temperature of the shortened wire ; and by further reducing its length, the heat may become so great as to fuse the platinum. As we have already shown, (he electricity which we obtain from an ordinary battery is closely connected with the heat that is developed ; Currents ^ ^ might tnerefore be supposed possible to produce caused'by'heat. electriciT y by heating two dissimilar metals, without having recourse to chemical action. This was first con- clusively shown to be the case in 1821, by Professor Seebeck, of Berlin, who found that if plates of two different metals were soldered together and heated at the point of junction an electric current was produced. A piecs of apparatus sometimes employed to illustrate this important fact is shown in fig. 132. A plate of copper, m n, has its ends soldered to a plate of bismuth, while a magnetic ueedle is placed between them and balanced on a ELECTRICITY AND MAGNETISM. 177 pivot. The apparatus being placed in the magnetic meridian, the soldering is heated, when the magnetic needle is deflected in such a manner as to indicate the passage of a current from n to m, that is from the heated to the cool soldering in the copper. If instead of heating the soldering it is cooled, as by the application of ice, the deflection of the needle will indicate the passage of a current in the opposite direction. The strength of these respective currents is dependent on the difference between the temperatures of the two junctions. A more usual way of illustrating the existence of these currents is to take a plate of antimony and a plate of bismuth soldered together end to end, and to fasten a copper wire to each of the plates. The copper wires are then joined up to a deli- cate galvanometer, the needle of which indicates the existence and the direction of the currents which are produced upon heating or cooling the point of junction between the two metals. The currents thus produced between two metals by variations of temperature are known as thermo-electric currents (Greek thermos heat), and such a couple as that just described is termed a thermo- electric couple. When the junction of a thermo-electric couple is heated, the current passes from the bismuth to the antimony across the point of junction, but from the antimony to the bismuth through the con- necting wire. The bismuth is therefore the positive Dir c e ^ t f tho metal, but the negative pole ; while the antimony is the negative metal and the positive pole. In order to fix thoroughly in the mind of the student the direction which the current takes when the soldering is heated it maybe useful to remember that if we repre- sent the antimony and bismuth by their initial letters, the direction of the current through the junction of the metals is in the opposite direction to the sequence of the letters of the alphabet that is, from B to A. Of course, when the soldering is cooled the direction of the current will be from A to B. Although antimony and bismuth are usually employed to exhibit thermo-electric phenomena, any two metals will, if similarly arranged, exhibit like phenomena, though generally in a less degree. From a number of experiments a thermo-elec- Th*" 00 ' 6160 *" trie series of the metals has been arranged, which corresponds to the voltaic series given in an early chapter on gal- vanism. The metals in this list are so set down, that on heating the junction between any pair the direction of the current is through the junction from the one above to the one below. 178 ELECTRICITY AND MAGNETISM. 1. Bismuth. 15. Molybdenum. 2. Nickel. 16. Rhodium. 3. Cobalt. 17. Indium. 4. Palladium. 18. Gold. 5. Platinum (a). 19. Silver. 6. Uranium. 20. Zinc. 7. Copper. 21. Tungsten. 8. Manganese. 22. Platinum (c}. 9. Titanium. 23. Cadmium. 10. Mercury. 24. Steel. 11. Lead. 25. Iron. 12 - Tin - 26. Arsenic. 13. Platinum (J). 27. Antimony. 14. Chromium. 28. Tellurium'. xt will be observed that the metal platinum occurs three times in the foregoing list : this is owing to the fact that its thermo-electric property depends on the nature of its manufacture. In the list the letter a 81 gn lfi es cast, b rolled, and drawn. The amount of eleetro- motive power obtained from any two metals will depend u * in the above list ; and * wil1 be fr m 6ach ther ' Tdlurium - and much less easily btained than ntimony and this latter metal is therefore more usually employed between^ " I" - ed , that ^ & ^ is P assed i SS between two dissimilar metals, the junction rises or lowers in tern- The Peltier perature acc ording to the direction of the current. It effect. ma y be set down as a general principle, that if we ELECTRICITY AND MAGNETISM. 179 enclosed in the bulb of an air thermometer, and the current arranged so that it can be regulated preferably by means of a rheostat. The current produced by warming the junction of one couple of antimony and bismuth is, as may be supposed, extremely slight, and a considerable difference of temperature is necessary to produce an appreciable effect. By the union of a number of couples, however, a battery may be formed which will produce sensible currents with but small variations of temperature. Such an arrangement is shown in fig. 133, a"nd is termed a thermo-pile or tliermo-multiplier. It was devised by Nobili, and in its construction a number of bars of antimony and bismuth are soldered together alternately, as shown at a b. A series of couples is thus formed ; and the bismuth at b is then soldered to the antimony of another similar series, the bismuth of this second series being soldered to the antimony of a third series and so on, until a large number of couples are thus united, and a pile is formed having antimony at one end and bismuth at the other. The couples are insulated from each other by varnished paper, and the whole enclosed in a brass or copper case, P, so as to leave only the solderings exposed at each end of the pile. The first antimony and the last bismuth communicate by fine wires with the binding-screws in, n, and from these wires pass to a delicate galvanometer when the pile is in use. The ends of the pile are also usually provided with hoods when the pile is used for experiments with radiant heat. A well-constructed pile will, with a good galvanometer, give indications of the slightest amount of variations in the heat of bodies brought near its ends, and the thermopile is therefore a very valuable adjunct to the physical laboratory. 180 ELECTRICITY AXD MAGNETISM. CHAPTER XIX. VOLTAIC ELECTRICITY (continued). Causes of thermo-electric currents Currents with one metal Pouillet's thermo- pile Animal electricity Nerve currents Natural nerve currents Negative variation Natural muscle currents Currents in oblique natural transverse sections The rheoscopic frog. WHEN we come to inquire into the causes of these thermo-electric currents, we are at once reminded that it is not the mere contact Causes of ^ *^ e two meta ^ s employed, for they are united not thermo-electric directly to each other, but by an intervening layer of currents. so lder ; and, moreover, as we shall presently see. these currents can be produced with only a single metal. It has been moreover conclusively shown that they are not caused by chemical action, as they can be produced in an atmosphere of hydrogen, or even in a vacuum. The existence of such currents would seem to be caused by the unequal propagation of heat in different parts of the circuit, for it has been found that if a metal is perfectly homogeneous, so as to conduct heat equally in every direction, no current can be produced, but that if the metal is treated in any way so as to destroy its homogeneity, it will give rise to currents when warmed. An illustration of the foregoing statement may be easily found. Take a straight piece of platinum wire, and heat it in any part while it is in connection with a galvanometer ; it will be Tne e metol found that the needle of th e latter is in no way affected, indicating the absence of any current. Now twist one part of the wire into a spiral, or make a tangle in it ; upon applying h?at near the tn, and the most negative towards a. One of the most striking proofs of the existence of natural nerve currents is found in the fact that the natural nerve current may be utilised as a galvanic battery for stimulating other nerves or muscles. In this experiment a preparation is made of two leg muscles of the frog with the nerve attached. The nerve of one preparation is t lien allowed to fall on the muscle of the other, when the latter contracts in the same way 184 ELECTRICITY AND MAGNETISM. as if stimulated by an electric shock. The preparations are placed, for the sake of insulation, on a glass plate, and the nerve of the one preparation must come in contact with the equator and one end of the muscle of the other. The nerves and muscles must also be perfectly fresh. This experiment is known as that of " the rheoscopic frog." Many other interesting facts concerning animal electricity might be discussed ; but for these, we repeat, we must refer our readers to any good treatise on Physiology, to which branch of science the subject more properly belongs, CHAPTER XX. RECENT APPLICATIONS. BY H. CHAPMAN JONES. The travelling of sound Toy telephone Action of telephones Transmitter ana receiver Reiss's telephone Reiss's transmitter Page's discovery Capa- bilities of Reiss's telephone Gray's first telephone Bell's telephone. THE practical application of certain known facts concerning elec- tricity and magnetism has made such rapid strides during the last few years, that it has been thought well to reserve the consideration of telephones, microphones, the electric light, and one or two kindred subjects, until the end of this series of articles. These applications have been at once so striking and useful, that it will suffice to state, by way of introduction, that they naturally classify themselves in two divisions : namely, (1) those relating to sound, and (2) those connected with the utilization of powerful currents of electricity for illuminating purposes, for the transmission of force, etc. When we see a flash of lightning, we know that the thunder was produced at the same instant, and that the few seconds we wait for it are taken up by the sound travelling from the cloud Th of sound! 1118 to our ears> But sound is merely motion in the case of thunder a motion of the air ; and this motion of the. air travels through the air in every direction from the point at which the disturbance is produced. Where there is nothing to move there can be no sound, and it is obvious that a travelling movement must cease to exist when it reaches a region that is empty of matter. The transmission of sound may be controlled by confining the move- ELECTRICITY AND MAGNETISM. 185 ment (or sound-wave) within prescribed limits ; and so by preventing in great measure the dispersion of its energy, it may be caused to travel to a much greater distance than would otherwise be possible. Ordinary speaking-tubes have this action upon aerial sound-waves. But if, instead of producing the sound-wave in the ai^ as we do in speaking, we make the wave in a solid substance, say a wooden rod, we shall find that the wave will pass along the rod, being guided by it just as effectively as the aerial wave is guided by the speaking tube. This leads us to a passing mention of what has been called a toy telephone, though it has no more right to the name telephone than any piece of tube or a common staff. If we speak into , a small drum that has only one end to it, like a box without a lid, the end will be set in motion by the sound of our voice ; and if this end is connected by means of a tight string with another drum, the sound-waves will be transmitted along the string, and will be audible to any one who applies his ear to the second drum. In the instruments more properly called telephones, the sound, waves themselves are not transmitted at all, but merely cause varia- tions in an electric current which are reproduced at the distant station, and there lead to sound-waves which Action of tele- are similar to the original ones. Thus the telephone. if perfect, would reproduce sounds at any distance to which an electric current can be sent, and as at present constructed works well over wires from one hundred to two hundred miles in length; but if the sound-waves themselves are transmitted, as in the apparatus described above, the ordinary human voice will be found scarcely audible when more than one hundred yards away. To transmit sounds telephonically, we must have a wire between the speaker and the hearer arranged so that the return current may pass through the earth as in ordinary telegraphy, or the circuit may with greater advantage be completed by a second wire. The speaker must have an instrument capable of receiving sound-waves and send- ing them away as variations in an electric current, while the listener must be provided with the means of reproducing the sound from this varying current. The instrument spoken into is the transmitter, while that which re-forms the sound is called the receiver names that may be more easily remembered if they are associated with the persons communicating, rather than the instruments used. Now, if the transmitter and re- ceiver are identical in construction, it is obvious that two persons can converse together without making any change in the disposition 186 ELECTRICITY AND MAGNETISM. of the apparatus an advantage that is realized in the commonest forms of telephones now in use. but not in the first instruments that were constructed. As early as 1857, the Count du Moncel suggested that sound-waves might be caused to interrupt an electric current, the interruptions corresponding exactly with the vibrations of the sounding substance, and that by this means a note might be reproduced at a great dis- tance. This suggestion, however, remained practically unfruitful, and it was not until 1861 that the first telephone was constructed by Philip Reiss of Friedrichsdorf. Before this, single musical notes had been reproduced at a distance by interposing a tuning-fork in an electric circuit so as to telephone. FIG. 137. break and make the connection by its vibrations, but by such means it was only possible to transmit the note peculiar to the fork em- ployed. Reiss's telephone dispensed altogether with tuning-forks, and transmitted any note without making any changes in the ap- paratus. It is shown in fig. 137. If a musical note is sounded near the tube marked T, the vibrations of the air pass into the box K, which prevents them in great measure Reiss's f m . getting dls P ersed . and. sets the membrane in transmitter. v ibrating in unison with the air-waves. This mem- brane is made of caoutchouc, and is stretched over a circular opening in the upper part of the box, as shown. It is the vibrations of this membrane that make and break the current. The wire from the battery is fastened to the screw marked 2, and the current passes thence along the thin slip of copper, i, which extends ELECTRICITY AND MAGNETISM. 187 to the middle of the membrane, and rests upon it so as to partake of its motion. This slip is protected at its end by a thin plate of platinum, immediately over which, and nearly touching it, is a fired platinum point carried by the arm a b c. It is obvious that when the membrane is raised, the platinum plate o comes into contact with the point under b and the current is " made," to be broken, how- ever, as soon as the membrane regains its original position. This intermittent current passes from the screw marked 1 to the telegraph wire, and so to the screw 3 of the receiver shown in fig. 138. This apparatus consists essentially of a soft iron bar about the size of a knitting needle, d d, surrounded by an insulated coil of wire, ff, through which the current passes from the screw 3 to the screw 4. Page discovered in 1837 that by rapidly magnetizing and de- rm. 1-38. magnetizing a piece of iron a note was produced corresponding in pitch to the number of makes or breaks in the current that is employed to form the magnet. Thus the electro- magnet in the receiver formed by the rod d d and the coil a will give a note exactly corresponding in pitch with the note acting on the transmitter, when the intermittent current produced by the transmitter passes through the coil ff, as has been described. The box B and lid D serve to strengthen the note produced. The circuit is completed from the screw 4 by connecting it to earth, the battery joined to 2 (fig. 137) having its other electrode carried to earth, as is usual in telegraphic apparatus. It is important to notice that, though Eeiss's telephone will transmit any note, or any rapid succession of notes (thus, any tune), it does not reproduce either the intensity, that is the Capabilities loudness, or the quality of the notes ; for however ofReiss's violently or gently the membrane in the transmitter telephone, vibrates, it merely makes and breaks the current, and so produces 183 ELECTRICITY AND MAGNETISM. the same effect in the receiver. For this same reason it cannot transmit speech, and however carefully the instrument is made one can only just recognize words that are already agreed upon. If, however, a drop of acidulated water is placed upon the platinum plate so 'that the point is always immersed in it, we have at once a considerable improvement. But this small addition entirely alters the principle of the apparatus, for the current is no longer inter- mittent, but continuous, decreasing in strength when before it was stopped altogether, and the variations of current strength becoming greater when the membrane is caused to vibrate more vigorously by an increase in the loudness of the sound. The employment of continuous but rapidly undulating currents is Q FIG. 139. the fundamental principle of modern telephones. The amplitude of the vibrations in the sound-wave (speaking familiarly, the loudness) here finds what we may call its electrical equivalent, and is therefore reproduced by the receiver. Elisha Gray of Chicago demonstrated, in 1874, the advantage of undulating instead of intcr- G tele' hone* m i ttent currents by the performance of his telephone which is represented in its earliest form in fig. 139. The mouthpiece has at its lower end a diaphragm, from the centre of which hangs a metal rod. This rod dips into slightly acidulated water, and within a little distance of its lower end there is another rod, fixed into the bottom of the water vessel, and in communication with a binding-screw that is connected with the battery. It will be obvious from the engraving that a current is always passing, but that, as the upper rod moves with the vibrations of the diaphragm, the ELECTRICITY AND MAGNETISM. 189 amount of resistance introduced by the water (that is, tLe space between the ends of the two rods) will vary exactly with those vibrations. The receiver consists of an electro-magnet placed near to a thin iron diaphragm, so that the varying current makes a similarly varying magnet, which produces vibrations in the dia- phragm of the receiver like those taking place in the diaphragm of the transmitter. This apparatus of Gray's certainly substantiates its claim to be called an articulating telephone, though it must be allowed that it is a failure so far as its practical utility is concerned. One of its greatest drawbacks is that the current passes through a decomposable liquid, and the gases evolved at the ends of the two rods must be continually varying the resistance, and so interfering with the desired action of the instrument. The telephone invented by Graham Bell obviates this difficulty. But the reader must bear in mind that though it is convenient in explaining the principles of these instruments to consider Gray's before Bell's, the latter was in no BeU ' s ^P 110116 ' way a development of the former, but an independent invention. CHAPTER XXI. RECENT APPLICATIONS (continued). Construction of Bell's telephone - Principle of BeU's telephone Telephonic currents Action of the iron disc Gower's telephone Ader's telephone Wheatstone's microphone Bell's modification Hughes' discovery Micro- phone as a transmitter Construction of microphones. IN the close of the last chapter we pointed out what appears to be the greatest drawback in Gray's telephone, a drawback from which Bell's instrument is free. But it must be understood, in saying that one obviates a difficulty standing in the way of the other's success, that reference is made only to the instruments themselves, inde- pendently of the experiments or the reasoning that guided each of the inventors. In Gray's instrument the sonorous waves produce undulations in the electric current by varying the resistance, while the electro-motive force remains constant ; but in Bell's apparatus the resistance is unvaried, the undulations in the current being produced by causing the sound-waves to affect the electro-motive force. Moreover, it may be observed that the telephone we are about to consider requires no battery to work it, and as each instrument 190 ELECTRICITY AND MAGNETISM. acts either as transmitter or receiver, the whole arrangement is much simplified ; and a conversation may be carried on between two persons, each speaking in turn, without making any change in the apparatus. Professor Graham Bell, of Boston, who had been working at the subject of the transmission of sounds electrically since 1872, exhi- bited an articulating telephone at the Philadelphia International Exhibition in 1876. In Bell's early experiments he took the human ear as his model, and constructed an artificial tympanum of mem- brane, to the centre of which he secured a piece of iron to re-act with the electro-magnet employed. By increasing the size of the disc of iron improved effects were obtained, until at last the membrane was dispensed with, and only the iron disc remained. The battery power was reduced from fifty cells to none at all, without altogether losing Line. the desired effects ; for the residual magnetism was found sufficient for the working of the apparatus. Permanent magnets were therefore introduced instead of the electro-magnets, and by these and other similar steps towards perfection the instrument assumed its now well known form. A pair of telephones are shown in section in fig. 140 without their outer cases. In each, NS is a permanent magnet and b a soft iron core which forms a prolongation of the magnet. Eound this core is C H a bobbin which holds a coil of v ery fine silk-covered Bell's telephone. c PP er wire, one end of which is connected with the other instrument and the other end to earth ; though it is preferable when possible to complete the circuit by a second wire. The mouthpiece a has an iron diaphragm fixed as near as possible, so as to avoid contact, to the end of the core b. The listener applies the concave surface with its hole at a to 'his ear. Such, so far as its construction goes, is this simple apparatus, that will convert sound-waves into electrical undulations, and these back again into ELECTRICITY AND MAGNETISM. 191 sound-waves after they have travelled almost any number of miles, with a certainty that has brought many thousands of these instru- ments into daily use. But if we seek for the method of its action in the light of the experiments that have been made upon it, we find that we have a much more difficult task than would at first appear. The simplest theory of its action is that the disc of the transmitter is caused to vibrate by the sound-waves thus alternately approaching and receding from the core J, so strengthening and weakening the magnetism induced in it by the permanent magnet, and producing magneto-electric currents in the coil C. These currents then pass to the coil of the other telephone, and there vary the magnetic intensity of its core, and so set its iron disc vibrating, and produce sounds which are the exact facsimile of those acting upon the transmitter, except that they are uniformly weaker. And we should reason that the louder the sound, the greater would be the amplitude of the vibrations of *he disc and the stronger the currents produced, leading of course to a correspondingly greater vibration of the disc of the receiver. Moreover, the complex sound- waves containing many waves superimposed that are produced by ordinary speech, we can easily imagine to impart their motion to the disc and so be exactly reproduced by the disc Of the receiver. In support of this theory, experiments have been made that prove the production of currents by the motion of the disc of the telephone. By connecting a telephone with a Thompson's reflecting galvanometer, the needle is deflected when the disc is pressed by the finger, and again in the opposite direction when the finger allows the disc to pass back to its original position. It has also been shown that telephonic currents will produce an audible effect when reversed five hundred times per second, though their strength ia no greater than about one thousand million times less than the currents used in ordinary telegraphy. But with regard to the vibrations of the iron diso. experiment seems rather to contradict the idea that it is needful for it to vibrate at all. Attempts to measure the amplitude of it 3 vibrations have failed, and if the cavities on either side of the disc are packed with wool, or if the disc itself is pressed by a finger (so long as it does not touch the magnet), or if the disc is glued to a block of wood an inch thick, the action of the telephone is not impeded. Or again, if the disc is connected with a sensitive mem- brane by a short tight thread, no sound can be detected from the membrane. There can be on doubt that under certain circumstances the disc 192 ELECTRICITY AND MAGNETISM. does vibrate as we have imagined; but the experimental results referred to above seem to indicate that this vibration is not neces- sary, and that we must seek further to discover the real action of the iron disc. Professor Bell himself states that he inclines to the opinion that the vibrations are molecular that is, presumably, similar to the motion induced in a solid body when a sound was passed along it but it is impossible at present to do anything more than suggest an explanation of the phenomena. We know very well that the rapid magnetization and demagnetization of iron pro- duces sound, and it is possible that Bell's telephone, when used as a receiver, reproduces the sound merely in this way, without there being any vibrations of the iron disc as a whole. Indeed, it is stated that the magnet may be removed without detriment to its action as a receiver. Now, it seems at least possible that the reverse of this action takes place when Bell's telephone is used as a transmitter viz., that the sonorous waves bring about a vibrating condition of the magnetic state of the disc, which would of course be as efficacious in producing currents in the coil as the oscillations of the disc itself. But, whatever may be the true explanation, it is quite certain that the telephone does not work in so simple a way as was at first supposed. The sounds given out by Bell's telephone are so weak, that many attempts have been made to improve the instrument in this par- ticular. Mr. Gower uses a horseshoe magnet both poles le ' of which are provided with bobbins, and a thicker dia- phragm. The Ader telephone, which was used with great success at the Electrical Exhibition at Paris in 1881, is very similar to Gower's in form. The magnet is ring-shaped, and is provided at each end with a soft iron pole-piece, which te ep one. can>ics a coil . a b ove ^ Q d j SC) or diaphragm, is a soft iron ring, which is the' peculiarity of this instrument, and serves to concentrate the force of the magnet upon the disc. There is no need to dwell further upon these modifications, for it is evident that they are in principle the same as Bell's original telephone : we will therefore now pass to a consideration of the microphone, which will in turn lead us to the subject of the carbon telephone, and a fcw remarks upon the applications and methods of using these various instruments. One word here will be sufficient to warn the reader against interpreting this order of treatment into a statement of priority of invention. The word microphone was first employed by Sir Charles Wheat- stone, to describe an instrument used by him, which consisted mo.rely of a pair of plates to bo pressed upon the ears, each having a rod ELECTRICITY AND MAGNETISM. 193 fixed to its centre. These ods were bent conveniently, so that they met at a point, and this point was brought into contact with the sounding body. The apparatus may be com- pared to a double-acting stethoscope, and merely pro- vided a solid conductor for the sounds. Professor Bell has lately modified this instrument, by fixing a diaphrag.n exactly as in his telephone, and joining to the side of it that is not presented to the ear a short straight rod, which makes Bell'smodifica- contact between the sounding body and the diaphragm. But the word microphone, as now used, invariably refers to an instrument that renders faint sounds distinct or inaudible sounds audible by the use of an electric current. Early in 1878, while Professor Hughes was experimenting on the effects of sound-waves passing along wires with regard to their electric conductivity, he had occasion to include a stretched wire in a battery circuit with a telephone, a clock being placed near the wire as a source of sound. No effect was observed till the wire broke, and then a momentary rushing sound was produced by the telephone. In endeavouring to repeat the experiment by bringing the broken ends of the wire together again, and pressing them together by a small weight, sounds made near the imperfect join were markedly reproduced by the telephone, and the arrangement had all the essential parts of the more perfect microphone. The general disposition of the apparatus is shown in fig. 141, where B is the battery, T the telephone, and 8 the microphone in this case the loosely joined broken wire. Of course the distance between the microphone and the telephone may be increased at pleasure without impeding the action of the apparatus. But if, instead of merely pressing the one wire end on to the other, each was made to terminate in a nail, and these nails were laid parallel to each other with a third nail resting across them, the re- production of sounds was much more perfect ; and if the circuit were completed between the wire ends by heaping around them a fine metallic powder, the .arrangement acted perfectly as a transmitter, 194 ELECTRICITY AND MAGNETISM. the peculiarities of the speaker being faithfully reproduced by the telephone. It should be observed that the apparatus improved as the number of loose junctions in the conductor was increased, and that the sound-waves shake these touching contacts, never breaking the circuit, and so produce those variations in the current that affect the telephone. We have therefore in the microphone a transmitter which Micro hone acts ^ e ^ e ^ ss ' s ^ v var yi Q g the resistance of the circuit, as a but free from its two principal drawbacks namely, its transmitter. i ac k o f sensitiveness, and the employment of an elec- trolyzable liquid. But the microphone is much more than this, for it can be made so sensitive that the vibrations caused by a fly walking produce quite a loud tramp in the telephone. Microphones may be made after an almost endless variety of patterns; but the material that is generally best suited for their construction is charcoal, which is a sufficiently good Construction of conductor and not liable to change by oxidation. The charcoal may be made white-hot and quenched in mercury, so that its pores are filled with that metal ; but this treat- ment ia not necessary. CHAPTER XXII. RECENT APPLICATIONS (continued). Theory of the microphone Edison's carbon telephone History of Edison a telephone Principle of Edison's telephone The carbon telephone is not a receiver Edison's receiver Principle of Edison's receiver Dolbear's receiver. PKOFESSOE HUGHES thus describes the arrangement to which he first gave the distinctive name of microphone. " It consists simply of a lozenge-shaped piece of gas carbon, one inch long, a quarter-inch wide at its centre, and an eighth of an inch in thickness. The lower pointed end rests as a pivot upon a small block of similar carbon ; the upper end being made round plays free in a hole in a small carbon block similar to that at the lower end. The lozenge stands vertically upon its lower support. .... The form of the lozenge- shaped carbon is not of importance, provided the weight of this upright contact piece is only just sufficient to make a feeble contact by its own weight," In such an instrument the wires that make the circuit would be joined respectively to the upper and lower blocks ELECTRICITY AND MAGNETISM. 195 of carbon, so that the current has to pass through the two touching connections. There is another form of microphone in which the two carbons are kept lightly touching by means of a spring, which can be tightened or loosened to suit loud or feeble sounds, for if the vibrations are strong enough to separate the carbons, and so break the circuit, we get merely an intermittent current, and the action of the microphone as such ceases. If a galvanometer is included in the circuit, there should be no variations in the deflection of its needle ; for when the microphone is acting properly, the oscillations in the resistance of the circuit, and therefore in the current, are so rapid that no galvanometer can be affected by them. The principle that underlies the action of the microphone was for some time a matter of dispute ; but it is now generally allowed to owe its efficacy merely to the fact that an increase of pressure between two conductors produces better con- Theory of the tact and therefore a less resistance. It is quite certain "" that mechanical movements will produce these changes of pressure in the microphone, and so cause sound in the receiving telephone ; but whether all sounds act on the microphone only by the production of mechanical vibrations is a question that it is not easy to answer. When one has an instrument capable of registering the jerk caused by a fly when it puts down its foot, it is very difficult to tell where mechanical effects end and molecular begin. It will have been observed that the microphone may be used in telephony as a transmitter ; by arranging it in a suitably shaped box with a mouthpiece it may be made as convenient for this purpose as Bell's telephone. Edison's car telephone is neither more nor less than a microphone specially asranged to transmit vocal sounds. This invention was made, however, quite independently of Hughes', and was led up to by entirely different experiments ; but while Edison can fairly claim the honour of inventing the carbon telephone, it is to Hughes that we owe the discovery of the simple principle that governs its action, and the perfect generalization of this principle. Edison, like Bell, uses an iron disc as a diaphragm, but in working out his telephone he followed in the footsteps of Gray, endeavouring to replace his decomposable water by elementary sub- Siaior O f stances like carbon and platinum. He assumed that Ed son's the disc vibrated as much as the string of a violin or telephone, harp, and sought to make these mechanical vibrations move a thin piece of plumbago and so vary the resistance of the circuit. It was in 1876 that Edison made this instrument, which was quite successful 196 ELECTRICITY AND MAGNETISM. enough to encourage the inventor to work further, though, as we shall subsequently see, its principle of action was probably entirely different from what he imagined. In the following year he varied his apparatus a little, intending to take advantage of the supposed fact that the electrical resistance of carbon varies under varying pressures. (The resistance of carbon is not changed by any ordinary amount of pressure ; but if a circuit is completed by a piece of carbon inserted between two electrodes, pressure upon the carbon through the electrodes will diminish the resistance of the circuit, because the carbon and the electrodes are thereby brought into more intimate contact.) To this end he made his battery wires to end in two small platinum plates between which was inserted a piece of plumbago. One of the platinum plates was fixed, and a gentle pressure was maintained upon the outside of the other by a piece of indiarubber fitted in between it and the iron disc. The arrange- ment will be understood by reference to fig. 142. The two platinum plates are shown at D D, and between them the carbon block c. The diaphragm, A A, causes pressure upon the upper platinum plate by means of the piece B, which in this case was a ring of indiarubber placed edgeways. Using this instrument as a transmitter, its results were fcoblo ; and Edison tried many substitute? for the plumbngo ELECTRICITY AND MAGNETISM 197 block among others, compressed lampblack but he eventually re- turned to the use of ordinary hard carbon. It was not till he devoted his attention to the indiarubber between the iron disc and upper metal plate, that any substantial improve- ment was made. For permanence' sake he replaced it by a short spring, the result of which, and his subsequent steps, he thus de- scribes : " I found, however, that this spring gave out a musical tone, which interfered somewhat with the effects produced by the voice ; but with the hope of overcoming this defect, I kept on substituting spiral springs of thicker wire, and as I did so I found that the articulation became both clearer and louder. At last I substituted a solid substance for the springs, that had gradually been made more and more inelastic, and then I obtained very marked improvements in the results. It then occurred to me that the whole question was one of pressure only, and that it was not necessary that the diaphragm should vibrate at all. I consequently put in a heavy diaphragm, If in. in diameter, and T \th in. thick, and fastened the carbon disc and plate tightly together, so that the latter showed no vibration with the loudest tones. Upon testing it I found my surmises verified : the articulation was perfect, and the volume of sound so great that conversation carried on in a whisper three feet from the telephone was clearly heard and understood at the other end of the line." Thus, although Edison did not rightly know the principle of the apparatus that he had contrived, by dint of repeated experiments he improved each part of it, eliminating what was not p,^ le necessary; until by just about the same time that O f Edison'* Hughes announced his discovery of the practical appli- telephone, cations of loose connections, Edison exhibited his carbon telephone, which, as before stated, was merely a microphone specially adapted to a particular purpose. It may seem strange to call a loose connec- tion what Edison speaks of as " fastened tightly together," but of course the fastening was nothing more than a pressing contact (that is, not cemented in any way), and " tightly " is merely a comparative term. We must remember that if a fly walks on the platform of Hughes' original microphone, its every footfall is sufficient to jerk a piece of carbon of very considerable weight; it is but reasonable, therefore, to suppose that the sound-waves caused by the human voice shall prcduce proportionately greater effects. Moreover. Hughes has shown that to adapt the microphone to the transmission of speech, it is necessary to keep the movable carbon in its place by a spring, the aerial disturbances produced having so great an effect upon the instrument. Edison has since taken another step in the direction of the simple 198 ELECTRICITY AND MAGNETISM. construction of the microphone, by making contact with his carbon by means of a light spring faced with platinum. It is obvious that the carbon telephone is only a transmitter ; as a receiver we may employ the original electro-magnetic arrangement of Page (already described in connection with Reiss's telephoned telephone), or more conveniently a magneto-telephone not a receiver. Qfa e Bell's), or some one of the receivers that have been recently invented with the special purpose of working with Edison's telephone. Thef-e last are very similar in principle, and consist of a diaphragm held in a state of tension, which tension is varied by the changes in the current. The one devised by Edison himself has a spring connected with the diaphragm, the free end of the spring being armed with a piece of platinum which presses upon a rotating chalk Edison's cylinder. This chalk cylinder is soaked in a solution of sulphate of sodium, and so arranged as to revolve continuously away from the diaphragm, so that the diaphragm is drawn towards the cylinder by the friction between it and the spring. The current passes across the connection between the spring and the cylinder, and diminishes the friction between them in proportion to its strength. Thus the variations in the current regulate the pulling force of the spring upon the diaphragm, and produce corresponding vibrations in it. To understand the action of this apparatus, we must bear in mind that the solution of sulphate of sodium is an electrolyte, and that _ . . . this solution occupies the position of a lubricant between of Edison' i the cylinder and the spring. As the passing current receiver. varies in strength, so will the amount of decomposition vary ; the stronger current producing more gas, which of course must tend to raise the spring and diminish the friction of the connection. But there is a considerable difficulty on the face of this explanation. The current must vary in strength many hundred times in a second, and it is scarcely possible to imagine under these circumstances anything else than the accumulation of the products of electrolysis at a rate proportional to the current strength. But any such accu- mulation would of course be fatal to the action of the apparatus. It was pointed out by Ampere that subsequent parts of a rectilinear current repel each other, and Professor Barrett has suggested that we may find here the true explanation of the action of this receiver. That is. the mere passage of the current between the spring and the cylinder must cause repulsion between them ; and this repulsion increases, friction therefore diminishing, as the current becomes stronger. It is easy to imagine this repulsion varying as rapidly as ELECTRICITY AND MAGNETISM. 199 the current, but if this really is the action, the employmer;* of an electrolyte is a superfluity. Professor Barrett has replaced the chalk cylinder by one of polished brass scrupulously dried, and he finds that under these circumstances sounds are still reproduced by it. Professor Dolbear has constructed a receiver which he dignifies with the name " Eataphone," its action depending upon the changing current varying the strength of an electro-magnet. The current passes through the coil of a short straight electro-magnet, the core of which is kept uniformly rotating. A horseshoe-shaped armature has its ends resting on the poles of the magnet, and is connected with a diaphragm. Inasmuch as when the current passes the core of the magnet attracts the armature, the revolving core must exercise a continual pulling force upon the armature and thence upon the diaphragm ; and this pulling force will of course vary with the strength of the current. We have therefore again here all the apparent essentials of a receiver : namely, a method of exactly converting variations of current strength into a proportional physical force, and an arrangement for causing this varying force to induce vibrations in a diaphragm. But it will be observed that in DolVrv.-'s receiver there is r.'so all that is required for its action after the manner of that used by Reiss (merely an electro-magnet on a sounding board), and it seems at least possible that by suitably combining these two principles the sound reproduced might be considerably augmented in intensity. CHAPTER XXIII. RECENT APPLICATIONS (continued.) Telephones compared Induction by neighbouring currents Advantageous use of induction Prevention of detrimental induction Special arrangements of telephones The telephone as a scientific instrument Telephonic communi- cations without a wire Signalling by means of light The photophone Photophone transmitter The selenium cell. SUCH are the chief forms of telephones (using the word to include transmitters, microphones, and receivers) that have been laid before the public during the last few years ; and we propose now to say a few words as to their comparative powers, l Telephone* their most important differences, and the special uses oompare to which they may be applied. 200 ELECTRICITY AND MAGNETISM. For mere convenience of manipulation, rothing can excel the magneto-telephone ; for it is complete in itself, is either transmitter or receiver, requires no battery, nor is there any motion to maintain; while on the other hand, the carbon telephone requires a battery and can act only as transmitter, so that it needs a special receiver, which in some cases requires a continual and regular motion to be kept up. But the very fact that the magneto-telephone requires no battery, the currents being generated by the sonorous vibrations, is somewhat of a drawback ; for the currents are necessarily small, and small currents produce small effects and are interfered with by small dis. turbances. The carbon telephone, on the contrary, employs a battery current perhaps a thousand million times greater, the sound-waves having merely to vary its intensity. The greatest drawback to the Induction b use ^ or( linary telegraph wires for telephonic corn- neighbouring munications is that neighbouring wires if in action currents. i n( j uce ever-changing currents in the wire employed, which make so confusing a noise in the receiver that with a magneto- telephone it is impossible to interpret the message. With the carbon telephone these disturbing influences become proportionately smaller. The latter instrument is indeed more powerful in a general way ; with Edison's receiver it is known as the " loud speaking telephone," because the sounds are reproduced so as to be audible to many persons at the same time, the voice being reproduced aloud rather than, as in the magneto- telephone, in a whisper. Returning to the subject of induction, it may be observed that the transmitter and receiver need not be in the same circuit, for if sufficiently long portions of the two circuits are arranged ^tflmUwtion s ^ e ^3" s ^ e ' ^ ie ^ucing action of the one upon the ' other will be quite sufficient for the purposes of trans- mission. Taking advantage of this fact, it is possible to considerably magnify the effects of telephones. The variations of resistance brought about by the sound-waves are small, and in an ordinary long circuit these variations form but a very small factor of the total resistance. But if the circuit containing the transmitter and battery is completed by the primary of a small Ruhmkorffs coil, the re- sistance of the circuit is greatly reduced, and the variations produced in it by the sonorous vibrations are proportionately increased. By connecting the line with the secondary wire of the coil, these increased effects are transmitted to the receiver. This arrangement is stated to have another advantage : namely, that the electricity being changed from what may be called a .quantity current to a tension current, (that is, at least partially, from voltaic to frictional,) it is less affected by changes of resistance in the line and connections. But this is ELECTRICITY AND MAGNETISM. 201 not an unqualified advantage, for the same property that enables it to overcome faults in the line, etc., also enables it to escape with greater ease where the insulation is but indifferent. The accidental production of currents by induction may be com- pletely obviated by enclosing the wire in an iron sheath electrically connected with the earth, but such a precaution is p reventi hardly practicable on the large scale on account of its detrimental expense. A simpler and quite practicable method is to induction, use a wire for the return current, and to twist this with the trans- mitting wire so that induction from without shall be equal in each, n i i Dpp vrc L^JI tJf J ftttir~flp TELEfHOtXS FIG. 143. the equal and opposite inductions neutralizing each other. From the establishment of so many telephonic exchanges, and the wide appre- ciation that the instrument has received all over the civilized world, including China,* it must be allowed that it has already made for itself a firm footing as a ready means of transmitting speech. Where a mutual conversation is not required, but only the repro- duction of sounds whether talking or otherwise made Special in a given place, it is possible to secure very great ad- arrangements of vantages by special arrangements. At the Electrical telephones. Exhibition at Paris, in 1881, the Opera House was telephonically connected with the Exhibition buildings, and it is stated that the * The ordinary telegraph is useless to the Chinese, as they have no alphabet. O 202 ELECTRICITY AND MAGNETISM. singing was positively heard to greater advantage at the Exhibition, than in the Opera House itself. Ir- this case there were a series of transmitters arranged along the front of the stage ; the connections of these with the receivers being so managed that each of the listeners had two receiving telephones the one for his left ear reproducing the sounds from the transmitters on -the left of the prompter's box, while the one applied to his right ear did the same for the other side of the stage. One thus gets a sort of stereoscopic effect which is very pleasing. The transmitters were microphones consisting of two parallel horizontal bars of carbon attached to the under side of a sounding board, connected by six cross-bars which rested loosely in holes made for them in the two principal bars. The battery wires terminated in the two main bars, so that the arrange- ment might be described as a six-fold microphone. (See fig. 143.) But the telephone has many uses for the scientific experimentalist, which render it to him an invaluable instrument, whatever the public may think of its capabilities. : The fundamental property of the magneto-telephone as a receiver is that of rendering audible minute changes in electric currents ; but The telephone we ma J a ^ so use ^ * detect the minutest currents, if as a scientific those currents are rendered intermittent. By using a instrument magneto-telephone instead of the galvanometer in Wheatstone's bridge, it is possible to detect inequalities in the balance of resistances that would otherwise escape notice. Given a rapidly intermittent current, we can at once tell within a little the number of breaks per second, by the note produced in a magneto-telephone. For use with a thermopile, it is more sensitive than the best mirror galvanometer, and far more convenient and simple in management. Microphones, as receivers, have been used to investigate earthquake noises, and have been proposed as a means of discovering minute sounds under many different circumstances, as in surgical examina- tions, and the mining operations of a military enemy ; but when put to such uses, it cannot be too carefully borne in mind that these instruments are not mere sound-detectors, but that they are also affected by mechanical disturbances. So far as we have yet considered the uses of the telephone, we have seen the necessity for a wire connecting the transmitter with Telephonic ^e rece i ver > ^ u * seeing that communications are some- communications times desired between two points where a wire could without a wire. no t ^ e maintained, it is only natural that inventors should seize upon the shadow of a possibility of doing away with the need of a wire. Experiments have shown that watercourses may be utilized as conductors in telephony, but it is obvious that their use ELECTRICITY AND MAGNETISM. 203 must be very restricted, for we can imagine with amusemeut the confusing results of more than one pair of communicators happening unwittingly to use the same course at the same time. The employment of a beam of light to convey information is a custom probably co-existent with man himself ; for the modern lighthouse is simply a development of the lamp put in the window to guide the benighted traveller. By employing a suitably intermittent light, one may signal to a distant station by means of the ordinary telegraphic code, the dot and the dash being respectively indicated by a short and a long flash. This method of signalling is actually employed in military practice. Now, it cannot but be observed that we have in. these flashes the exact counterpart of the intermittent current of ordinary telegraphy ; and by carrying this analogy a little further, we shall see that speaking along a sunbeam is an operation not altogether beyond the practical. Just as by making the current undulating instead of intermittent, not musical sounds only, but the human voice may be reproduced ; so, by making undulations in the intensity of the light-beam comparable to sonorous vibrations, we might hope to be able to reproduce the sound at a distance by means of these undulations. Such has indeed been done by Professor Graham Bell, in conjunction with Mr. Tainter ; and although the apparatus is too cumbersome, and the results too uncertain, to justify the hope that the "photophone" will ever be of any practical utility, the principles involved in its working are of great interest to the scientific student. A sketch of the photophone, which will serve to make its descrip- tion more clear, is given in fig. 144. The transmitter consists of a mirror and lens, which concentrate the sun's ray upon the reflector u, and a lens K, to render the reflected rays approximately parallel. The reflector B is the essential part of the transmitter, and consists of a disc of thin glass silvered on the front, much resembling, both as to size and manner of fixing, the disc of an ordinary telephone. There is a small air- chamber behind this mirror, from which a flexible tube leads to the mouthpiece. The use of the mirror B is not difficult to conceive. Any sound produced near the mouthpiece produces sound-waves which pass into the air-chamber referred to above, and here com- municate their vibrations to the mirror itself. The mirror B, being fixed at its edges, can only vibrate at its centre, and the oscillations of the centre will produce alternately a concave and a convex surface. But it is obvious that any changes in the mirror will affect the intensity of the beam of light, the least convexity scattering the 204 ELECTRICITY AXD MAGNETISM. light, while a concave surface will concentrate it and throw a greater quantity upon the lens R. Thus we see how sonorous vibrations may be made to produce proportional disturbances in the intensity of a beam of light, and it remains to reproduce sonorous vibrations from these disturbances. It is the construction of the receiver that alone justifies the intro- duction of the photophone in the present place, for so far neither electricity nor magnetism has taken any part in the action of the apparatus. The problem is to produce sound-waves from variations in a light-beam and from variations, be it remembered, that arc very slight. There is no known method by which this change can be brought about directly ; but there is a substance resembling sulphur more than any other element, viz. selenium, which has the TRANSMITTER RECEIVER peculiar property of suffering changes in the resistance that it offers to an electric current according to the amount of light that falls upon it. Now, we know very well how to produce sound from variations in an electric current, for that is the essential property of the magneto-telephone when used as a receiver Thus, by passing from light to electricity, and thence to sound, the problem is solved which till lately was considered hopeless. The arrangement of the receiver is shown in fig. 144. The current from the battery P passes through the two telephones T T in the ordinary way, the circuit being completed by a mis- called "selenium cell" at s, but which is merely an apparatus to interpose selenium in the circuit. Th beam of light is concentrated by the p>>-,vbolic mirror c c upon the surface of the selenium. ELECTRICITY AND MAGNETISM. 205 CHAPTER XXIV. RECENT APPLICATIONS (continued). Theory of selenium cell Uses of powerful currents Davy's experiment: Magndfco-electricity Dynamo machines Limits to dynamo machines. THE construction of the "selenium cell" will be understood by reference to fig. 145, the black portions representing the selenium itself, interposed between the metallic pieces, which are numbered. These metal conductors are connected alternately to the two wires of the circuit as shown, so that the circuit is completed only by the interposed selenium. The "cells" are generally constructed of a cylindrical form; and it is important to notice that the selenium must be heated and then very slowly cooled, BO as to convert it into t a 0456789 ro the crystalline modification which is alone available for this purpose, and that as large a surface of the selenium as possible should be exposed, for the action of light upon it does not penetrate its substance. The conductivity of selenium is increased by light, and diminished when the light is cut off, the changes produced being instantaneous ; but as its conductivity is always comparatively small, it is necessary that the surfaces of contact should be as large as possible. Such is the generally accepted theory of the part played by selenium in the photophone receiver ; but it would not be right to leave this subject without drawing attention to the fact that this theory requires us to ascribe to selenium a property not observed in any other substance. The caution necessary to scientific progress demands that new properties shall b3 accepted only with the greatest reserve. The necessity for slowly cooling the selenium, so that it shall be crystalline, was stated above ; and it is well known that crystallized selenium forms very 206 ' ELECTRICITY AND MAGNETISM. bad contact with metallic surfaces, so much so that it has been shown experimentally that a large part of a supposed selenium resistance may be due to the bad connections that it makes. Dr. James Moser has pointed out that, if viewed in this light, the selenium receiver may be nothing more in action than a microphone, the selenium playing the part of the loose piece of carbon. He further remarks that the microphonic effect may extend to the selenium itself, its loose and varying texture (for it caa exist in three or four different forms) conferring this possibility. According to this view, which Dr. Moser has supported by many experiments, the changes in re- sistance caused by varying the illumination of a "selenium bell" are due merely to the heating effects of the beam of light which disturb the contacts of the selenium, and perhaps even its constituent parts. The currents employed in using the instruments that we have so far dealt with are in some cases excessively minute, and the most powerful are not stronger than those used in ordinary te l e o ra phy- But the uses of powerful currents have long been known, and their applications, and in general the methods of dealing with and producing them, have of late years made rapid progress. One of their most important applications is seen in the electric light. Davy was the first to experiment with very powerful currents. In quite the early part of the present century he decomposed many metallic oxides that had till then resisted all attacks u P on them ! an( l showed that by passing the current from a large battery between two pointed rods of carbon, and then separating them to a small distance, a light of surpassing brilliancy was obtained. Voltaic batteries were the only means known by which to produce electric currents, until, in 1831, Faraday discovered that a current ;.s induced in a conductor by moving it across the magnetic lines of force. This electricity. sub J ect has been already referred to. But the steps from the discovery of a principle to the practical application of it are often long and tedious ; and we find that Faraday, in 1854, when called upon to report on Watson's (voltaic) and Holmes' (magneto) electric lights, recommends that they be tried for other than lighthouse uses first. He said that he " could not put up in a lighthouse what had not been perfectly established beforehand, and was only experimental." In 1859, however, the Corporation of the Trinity House started an electric light at the South Foreland; and in 1871 this method of lighting was regularly established at the South Foreland and Souter ELECTRICITY AND MAGNETISM. 207 Point lighthouses. Improvements in apparatus were gradually made, until the great success of electric lighting in Paris at the Exhibition of 1878 made the uses of electricity the absorbing topic of the day. The first division of this extensive subject which must engage our attention is that of apparatuses for generating the current. The action of the voltaic battery has already been discussed ; it will 208 ELECTRICITY A.VJ) MAGNETISM. therefore be sufficiet here to say that the current is proportional to the zinc oxidized, and that it is quite out of the question, on the score of economy, to make zinc our fuel. (Zinc is just as really burned when it is oxidized in a liquid as when it is set fire to by heating it in the air.) But by using a magneto-electric machine in conjunction with a steam engine, we burn coal instead of zinc, and are in a position to compete with any other power-producer so far as cost is concerned, provided only that our apparatus is sufficiently perfect. Batteries are economical where a small current is needed for a long time with the minimum of attention, as in telegraphy ; but they must altogether give place to machines where great quan- tities of electricity are required. The simple form of magneto-electric machine with Clarke's im- provement on it, and Wilde's machine with a Siemens' armature, have already been described. Clarke's machine was considerably TIG. 147. enlarged by Nollet of Brussels, in 1849, who designed thereby the machine now known as the "Alliance," the first magneto-electric machine of a practicable size. A view of this machine is given in fig. 146. Its construction is worthy of note, as an illustration of the method by which a small apparatus, like Clarke's original, may be arranged so that its essential parts may be multiplied to any desired extent. In the machine represented the axis carries four bronze wheels, each of which carries sixteen bobbins. This system rotates within a battery of permanent horseshoe magnets, the number of poles corresponding to the number of bobbins. The bobbins are wound with twelve separate wires, so as to diminish the resistance ; and these wires are joined from bobbin to bobbin throughout, as shown in fig. 147, where the junctions on the face of the wheel are represented in full, while the dotted lines show the junctions at the other ends of the bobbin made on the other side of the wheel. From m a conductor is led to the axis of the machine, and n is connected ELECTRICITY AND MAGNETISM. ' 2C9 with the first bobbin of the next wheel. We therefore may describe the machine as consisting of a continuous twelve-fold wire wound on sixty-four bobbins, rotating within a battery of magnets so arranged that the magnetism of the core of each bobbin is reversed sixteen times during a single revolution. Each reversal of the magnetism of a core of course produces a current in its coil, and these currents will alternate in direction. The bobbins all work together, so that each rotation gives eight currents in each direction, each current being produced by the full force of the machine. This machine is found to yield its best results when the rate of rotation is about 230 turns per minute, thus yielding 3680 alternate currents per minute. The machine is not fitted with a commutator, as the rapidly alternating current is just as suitable for the production of the electric light as a continuous current in one direction ; indeed, it has certain advantages not possessed by the latter. The above details must be regarded merely as an example of this class of machine ; it is obvious that the number of bobbins and magnets may be increased as far as the strength of the frame will aljow, and the number and length of the wires on the bobbins may be varied within wide limits. The first machines made in this country, by Holmes, were copies of the "Alliance." Since it was shown to be practically possible to produce currents of great strength by the movement of coils in magnetic fields, there have been two discoveries made which probably surpass in importance any others in connection with this subject of current generators : namely, the principle of dynamo machines, and the Gramme ring. As these discoveries are applied to many machines which differ in other particulars, it will be advantageous to consider them here rather in the abstract before proceeding to a detailed description of the machines most commonly in use. In February 1867 Drs. Werner and C. W. Siemens and Sir Charles Wheatstone announced almost simultaneously the essential principle involved in all so-called "dynamo" machines. This principle was included in a specification of Hjorth's of so early a date as 185 J, describing inventions for which he obtained " provisional protection " from the English Patent Office ; but this matter was forgotten, if ever generally known. The advantage of using electro-magnets instead of permanent ones had been recognized and practically applied by Wilde in 1863 ; but in his apparatus he still employed a battery of permanent magnets as his starting point, using the current produced by their means to excite the electro-magnets of the machine itself. The dis- covery at present under consideration consisted in dispensing with 210 ELECTRICITY AND MAGNETISM. the permanent magnets altogether, and using the current induced by the machine itself to excite its own magnets. By this means it will be seen that as the current increases in strength, so also do the magnets that produce it ; and by this action and reaction the limit to the intensity of the current is fixed only by the mechanical power at command, the strength of the framework of the machine, and, most important of all, the development of heat in the coijs of the machine, which increases with the current, and would dynamo eventually destroy the insulation of the coils. This machines, arrangement is made clear in fig. 148, which shows by a mere sketch the two poles N and s between which the armature revolves. Connected with the axle of the armature are the two conductors from which proceeds the wire that forms the circuit. If the soft iron poles were entirely free from magnetism, the revo- lution of the armature could make no current ; there must be a certain amount of magnetism in the poles to start with, and this necessary initial charge was at first produced by passing a battery current through the coils, and converting the poles into an electro- magnet. On removing the battery, the residual magnetism was always sufficient to start the machine. But even this little assistance was soon found unnecessary, the induction by the earth's magnetism being all that was needed. ELECTRICITY AND MAGNETISM. 211 CHAPTER XXV. RECENT APPLICATIONS (continued). Ladd's machine Wheatstone's improvement The Gramme ring Theory of the Gramme ring Action of the iron core Wallace-Farmer and Lontin machines Gramme machine De M^retens machine Brush machine Siemens machine. Now, we know that the intensity of a current is inversely propor- tional to the resistance of the circuit, other things being equal. By bringing this law to bear upon the apparatus figured on opposite page, it will be obvious that the greater the resistance introduced into the external circuit that is, [the circuit outside the machine the more will the current be reduced ; or, what is the same thing, the more /^~~K^\ FIG. 149. work the apparatus has to do, the less able will it be to do it. To remedy this* supposed defect, Ladd reverted to Wilde's principle, and used two bobbins, the current from one passing round the coils of the machine, while the current from the other was utilized for the external work. But this arrange- ment is practically not so advantageous as the original one. A considerable improvement, however, was made by Sir Charles Wheatstone, who suggested completing the circuit of the machine through its own coils only, and introducing the ex- ternal resistance upon a shunt circuit. This arrange- ment is illustrated in fig. 149, where the external resistance, or work to be done, is represented by a lamp. It wil be seen that, in this case, an increase in the external resistance will 212 ELECTRICITY AND MAGNETISM. cause a greater proportion of the current to flow through the coils of the machine, and so enable it the better to overcome the increased resistance. The other important invention that we have to deal with, is the Gramme ring. The armatures that is, the iron cores of the coils that we have had occasion to refer to so far have been The Gramme straight, but in 1870, Gramme and d'lvernois patented the use of a ring. Their object was to obtain a con- tinuous effect, for the revolutions of a wheel that is symmetrically disposed can produce no change in its position relatively to other objects. Fig. 150 will serve to render clear the construction and action of Gramme's invention. The ring of soft iron covered with its many coils is shown at R, and by means of the pulley P and band K it is caused to revolve rapidly between the poles of the magnet A B. The coils are formed by a continuous wire which, between each coil and the next, is soldered to an insulated brass plate extending to the axle of the apparatus. The ends of these conductors are shown at c, and it is from these that the currents induced in the coils are collected by the two discs m and , which are pressed by springs against the ends of the brass plates. A more modern form of the Gramme "ring" is shown in fig. 151, in which the ring itself, A, is formed of a bundle of iron wires instead of a solid bar. It will be observed that the lower part is represented as unfinished, so that the disposition of the coils may be more clear ; but that the upper portion is drawn com- plete, and shows the connections of the coils with the plates B ; and the extensions of these plates from which the induced currents are ELECTRICITY AND MAGNETISM. 213 collected. The revolving discs at first used as collectors are now always replaced by bundles of wires in the form of brushes, which make better contact and so lessen the resistance and in great measure avoid the production of sparks. In order to explain the changes taking place during the revolutions of a Gramme ring, we must refer again to fig. 1 50. Let us suppose that a coil on the middle of the outside of the ring is rising towards the pole B ; a current will be induced Theory of the in it, and this current will be maintained while it passes the pole and recedes from it, for as it recedes from the pole the position of the coil is reversed when compared with its position during its approach a different end of the coil being presented to a< FIG. 151. the pole in each case. The same coil, as it approaches, passes, and recedes from, the pole A, will have a current induced in it in the opposite direction. So that, taking all the coils collectively, the upper half produces currents in one direction and the lower half gives currents in tbe opposite direction. Now, it must be remembered that these coils are formed of a continuous wire ; we may therefore repre- sent this wire concisely by the full line in fig. 152, where the arrows show the direction of the antagonistic currents. It is obvious that under these circumstances the two currents must neutralize each other ; and such is practically the case, for by working a Gramme machine there is no current whatever produced until the external circuit is completed. The dotted line shows the external circuit, and the arrow the direction of the current in it, and makes it clear that the two antagonistic currents are both here working together. 214 ELECTRICITY AND MAGNETISM. So far, we have merely considered the action of the poles upon the moving coils, but the iron core of the ring also has important func- tions. In order to make clear this rather complicated Action of the p ro blem, let us first suppose that we have a common bar-magnet, with a short coil capable of sliding along its whole length, and that the ends of this coil are connected, with a distant galvanometer. If, with the apparatus so arranged, We move the coil along the magnet, we shall see that a current is produced ; but as soon as the coil passes the middle of the magnet this current will suddenly stop and be replaced by one in an opposite direction. These results may be expressed in a complete way by saying that the action of a magnet upon a coil that surrounds it and moves along with it, is the same as if all the Amperian currents were concentrated at the equator of the magnet that is, the neutral line between the two poles. We may therefore, so far as the inducing action of the magnet is concerned, disregard the magnet altogether, and imagine only a flat spiral or ring through which a current is passing, encircling its neutral point. To return to the core of the Gramme ring. Although when the machine is in action the ring is continually revolving, we may regard it as practically stationary so far a& its magnetic condition is concerned, for it is of soft iron, and the inducing poles are fixed. The condition of the ring is therefore constantly as shown in fig. 153a, in spite of its motion. It is a simple and obvious step to regard the ring as two curved bar- magnets, as shown at fig. 163* ; and these magnets, so far as their inducing action is concerned, may be resolved into the two rings in which Amperian currents flow, shown in fig. 153 Edison's domestic meter contains a domestic meter* solution of a copper or zinc salt, through which a defi- nite part, Bay one-thousandth of the current, passes by means of copper or zinc electrodes. These are weighed from time to time, and the weight of metal transferred from the positive to the negative gives an exact indication of the current passing through the meter, and therefore of the whole current used. But it must be borne in mind that, although we grant that the resistance of the meter and its shunt is exactly one thousand times as great as that of the main conductor (and it is not easy to secure this), the error of observation is multiplied one thousand times ; and a small error may become great by such multiplication. Dr. C. W. Siemens has constructed a current meter which avoids this evil, in which the cuirenTme 8 te wn l e current passes through a long thin strip of copper. The heat developed in a conductor has been shown by Joule to be proportional to its resistance and to the square of the current ; it remains, therefore, only to measure the heat to be able to find the proportional current strength. The heat is registered by the expansion of the copper band, which is considerable on account of its great tenglh. Its changes in length actuate a lever carrying a pencil under which a strip of paper is drawn by clockwork. A fixed pencil also marks the paper, and the divergence of the two lines so drawn gives a continual register of the current strength. A current regulator of Dr. Siemens' is also based upon the above principles. The lever that is moved by the copper band is so arranged that as the current increases it breaks a touching contact > and is so obliged to pass through a resistance coil, which is merely a shunt when weaker currents pass, and a series of similarly arranged coils gives the necessary gradations in the resistance. The uses of the electric light is so interesting and important a subject that it may not be passed over in silence, though space forbids much more than the cataloguing of them. The eleotoriclight. light given ^ incandescent lamps is markedly yellow sometimes even more yellow than gas in appearance but still it seems to show colours of all sorts well. The facts that they do not vitiate the atmosphere, nor are dependent upon it, and that there is no exposed fire, render them especially suitable for use in mines ; but. the necessity for conducting wires appears at present to preclude their use. This difficulty may perhaps be overcome by portable secondary batteries. The arc differs from its domesticated ELECTRICITY AND MAGNETISM. 239 neighbour in its unparalleled intensity, and is able to demonstrate its usefulness in ways not dreamt of before. Dr. Siemens has shown that the growing of plants and the ripening of fruits might continue through one long day if night's darkness were dispelled by electric lights, and that the plants and fruits so grown are as healthy and luscious as if they had been allowed their usual periods of rest. Photography is now independent of daylight, so far as portraits are concerned ; and machinery may, like Dr. Siemens' plants, be kept in continual activity, if required, without any of the drawbacks that artificial lighting used to entail upon the workmen. It has been remarked before, that intense light means intense heat ; and Dr. Siemens has shown that it is practically possible to utilize the heat of the electric arc in metallurgical operations. The temperature of the arc is by far the highest that Siem f ^ a f e ectrio can by any means be obtained, and it has besides the advantage of rendering its heat available in the immediate neigh- bourhood of the metal to be fused, without introducing any sub- stance that can exercise a chemical action upon it. Dr. Siemens sets a plumbago crucible of the necessary size in a bed of charcoal, to avoid the loss of heat by conduction, and arranges the poles so that the positive passes through the bottom of the crucible and the negative through the lid. It is important that the metal to be fused should form the positive pole, as it is at this pole that the heat is the most intense. Thus a practically unlimited temperature can be brought to bear upon the contents of the crucible, at the same time that the economy of the arrangement is nearly equal to that of a regenerative furnace so far as the fuel is concerned. When the small portions of carbon that split off the negative pole exercise an injurious reducing effect, this pole may be made of a copper tube through which a continual current of water flows to prevent its fusion. But probably the most important application of electricity in the future will be found in the transmission of power ; for the amount of energy available for man's use, if he could only take advantage of it, is incalculably enormous. Waterfalls, tides, and flowing rivers, are the chief of these natural sources of energy ; and their force might be partially utilized to generate electric currents by means of magnets or dynamo machines, these currents being carried by conductors to the spot where thi rower is required, and there made to do work. The apparatuses by which electric currents are made to produce mechanical motion are called electric motorg. Their action is very simple, being exactly the reverse of the Kot rs. current generator. To obtain a current, coils of wire are forcibly 240 ELECTRICITY AND rotated in a magnetic field; but if a ready-ma^e current is made to traverse the coils of a similar machine, its reaction upon the mngnels will cause the armature to revolve. Thus, a magneto or dynr.mo machine supplied with mechanical energy gives an electric current ; or if supplied with a current, gives mechanical energy. The earliest motors were constructed by Henry in 1831, and Ritchie in 1833, but these were mere toys compared with those that have been used ELECTRICTIY AND MAGNETISM. 241 recently for farming operations. One of the bast of recent motors for working small lathes or sewing machines is that of Griscom, an American, for which he obtained a gold " medal at the Paris Electrical Exhibition. It is only four and a ha'f inches long, and weighs a little over two pounds. It consists of a simple Siemens' armature entirely surrounded by the poles of an electro-magnet, which serve also as the framework of the machine. It is stated that this motor will attain a velocity of 3,000 revolutions per minute by means of the current from six bichromate cella. It is not difficult to take one step further and imagine an electric locomotive, and to set forth to ourselves the advantages of an absolute freedom from steam and noxious gases in our under- ground railways. A most successful electric railway, a mile and a half in length, has been constructed near Berlin by Messrs. Siemens and Halske. The rails are laid upon wooden sleepers, and convey the current from the fixed machine to the motor on the carriage. The electric railway at the Paris Exhi- bition was a quarter of a mile long, and had overhead gear to conduct the current, so avoiding the waste produced by the necessarily bad insulation of the rails when they are utilized as conductors. M. Olovis Dupuy, chief engineer at the bleaching establishment of M. Paul Duchesne-Fournet in France, in March 1882 completed an electric railway 2,040 metres in length, to facilitate the hauling in of the bleached linen. The locomotive is shown in fig. 165 ; it is worked by a Siemens' dynamo machine used as a motor, and the current is supplied by sixty Faure's cells (weighing about half a ton) carried by a tender. The charging of these cells by a Gramme machine absorbs three horse power for from five to eight hours, and this charge will work the train at a total weight of six and a half tons for three hours. In dealing with so important a subject as the present, it might be supposed that comparative costs would at least find a place among the other branches of the subject, and it would be very easy to quote vast masses of figures, compiled chiefly by advocates of the universal employment of electricity, for the purpose of demonstrating its economy. But these figures are in almost every case calculations based upon partial experiments, and not the actual working expenses. Moreover, very many of the prac- tical demonstrations of the uses of electricity have been chiefly for the purposes of advertisement, the cost being many times greater than the amount charged. The most important points for outsiders to consider are the results attained, and the possibilities demon- strated ; the contractors themselves * ill soon discover whether their business is a profit or a loss, UC SOUTHERN REGIONAL , SCIENCE AND ENGINEERING LIBRARY University of California, San Diego 1-tD 10 I9V3 384 If Ml A A *r\ JvN 4 19B4 SE 16 UCSD Libr.