ig BY 828 Broadwa REESE LIBRARY UNIVERSITY OF CALIFORNIA. Received ... / Accessions No. ^ f c yfy rate, it is clear that the latter will vibrate under the influence of the electro-magnet, whenever the transmitter is controlling the line currdni/ It is also evident that a good degree of synchro- nism must exist in the two vibrating reeds, in order that the vibrations may be reproduced. If their periods are identical, whenever the transmitter throws a current into the line, which it does at a definite instant in each vi- bration, the receiver will be in a position to be favorably acted upon by the electro-magnet then excited. If, on the contrary, the two do not vibrate alike, it will frequently occur, that, when the receiving reed is drawn by the mag- net, it is being urged in the opposite direction by its own elasticity, and its motion may then be nearly destroyed. Imagine, now, that two transmitters and two receivers are connected to the same line, and that the vibration frequency of one set is 300 per second, while that of ^the other is 500. When both reeds are connected to the line, a series of impulses will be trans- mitted, compounded of these two frequencies, resulting in a sort of composite fluctuation in the current strength, similar in many respects to the fluctuations in the density of the air, when transmitting two sounds differing in pitch. But the receiving reeds possess the power of analysis and separation. If the vibration fre- 132 A CENTURY OF ELECTRICITY. quency of 300 is present in the composite, the reed tuned to that number of vibrations per second will respond to it, and the same is true of that whose vibration frequency is 500 per second ; but neither will respond when the vi- brations corresponding to the other are alone transmitted. By manipulating the keys at the sending station, the continuous tones may be broken up into " short " and u long," or dots and dashes ; and each operator at the receiving station has only to attend to these interruptions in the motion of his own reed. But the number of pairs of vibrating reeds is plainly not limited to two ; several more may be employed, and the number of messages simultaneously transmitted increased ; and this system may also be com- bined with the duplex, as. has been done by Mr. Gray, with excellent results. Numerous attempts have been made to ac- complish multiplex telegraphy by means of the synchronous motion of cylinders, disks, etc., at the ends of the line. The production of two motions which shall be rigorously synchronous is, of course, practice 'y unattainable. Unfor- tunately the applications of this principle to telegraphy demand a degree of approximation to perfect synchronism which has been found difficult to realize ; but a considerable advance seems to have been made recently by Mr. P. B. MULTIPLEX TELEGRAPHY. 133 Delany, whose multiplex telegraph, based on the synchronism of two revolving arms, .has attracted much attention within the last two or three years. Distributing disk for multiple telegraph by means of synchro- nous rotation. The principle of this form of multiplex work- ing is exceedingly simple. 134 A CENTURY OF ELECTRICITY. A single wire connects the two stations, and at each end is a distributing wheel. A flat, circular disk of wood or other insulating mate- rial is provided with radial metal strips let into its surface, not extending to the centre, and in- sulated from each other. There may be any number of these strips, say, one hundred in all. Suppose that it is desired to work five distinct circuits over one line wire : the first strip will then be joined, by means of a copper band or wire running around the disk, to the sixth, the eleventh, the sixteenth, and so on. The second will be joined in a similar manner to the sev- enth, twelfth, seventeenth, etc. ; and this opera- tion will be continued until five groups have been formed, each containing twenty wires elec- trically joined to each other. Each group is then connected with a suitable relay or receiv- ing instrument and a key, as in an ordinary telegraph. The same battery may supply the current for all of the circuits. Moving about an axis passing through the centre of the disk, and at right angles to it, is an arm which con- nects through the axis with the line, and upon the outer end of which is a spring contact-piece which presses lightly upon the face of the disk, so that by the rotation of the arm this spring touches successively upon each of the radial strips described above, and thus puts each MULTIPLEX TELEGRAPHY. 135 group of them, one after another, in connec- tion with the line. During one revolution of the arm, any one group, with its receiver and key, will be electrically connected with the line twenty times ; and, if the arm is driven at the rate of three revolutions per second, such con- nection will exist sixty times in that interval of time. Now, imagine a precisely similar appa- ratus at the other end of the line : call the two station A and B, and the circuits No. 1, No. 2, etc. If the movements of the two revolving arms at A and B are absolutely synchronous, it will be easy to arrange so that circuits No. 1 A and No. 1 B will be in electrical connection with each other through the line sixty times in every second ; similarly, No. 2 A and No. 2 B will be joined sixty times per second ; but at no time will No. 1 A be in communication with any other than No. 1 B, nor will No. 2 A ever be connected with any other than No. 2 B ; and so on with all of the five circuits. It has already been shown, in the considera- tion of Gray's harmonic telegraph, that a con- tinuous current is not essential to the working of a telegraph system ; and it is found, that with an ordinary Morse receiving instrument, if the breaks in the current occur with sufficient rapidity, the effect is similar to a continuous but weaker current. In the present instance 136 A CENTURY OF ELECTRICITY. currents are transmitted through each circuit at the rate of sixty per second, and this would enable the operators to work with each other with nearly the same ease as if the current were continuous. The number of circuits may be considerably increased ; and, in fact, as many as seventy-two have been provided for in some of Mr. Delany's instruments, and have been successfully operated. As thus far described, the system is really nothing more than that suggested by Farmer in 1852. The same principle has been worked over by many other inventors since that time, but the real difficulty has always been the seem- ing impossibility of maintaining synchronism of motion at the two ends with a sufficient de- gree of approximation. Although he has im- proved the method in many of its details, it is in surmounting this difficulty that Mr. Delany has been most successful. The mechanism made use of for this purpose is somewhat com- plicated, and it will be desirable to consider it only in a general way. The nearest approach to synchronism of mo- tion which is suitable for this purpose, is found in two vibrating reeds, or tuning - forks, care- fully adjusted to agree with each other in pitch. Accordingly the revolving arms are driven by electric motors, the operation of which is con- MULTIPLEX TELEGRAPHY. 137 trolled by vibrating forks. For this purpose the electro - magnets of the motor may be ex- cited by currents which are regularly thrown on by the vibrating fork, as was explained in the discussion of the harmonic telegraph. But the vibration period of a tuning-fork is influ- enced by the temperature to which it is sub- jected ; and it is found that, however perfectly two forks may be made to agree at one place and time, they cannot be depended upon to be perfectly synchronous when separated from each other, and under different conditions. Without some method of correction, the revolving arms would soon stray away from synchronism, and dire confusion in the communications would re- sult. To surmount this difficulty, Mr. Delany has introduced extra radial strips in the distrib- uting disk, which are not connected with any of the branch circuits. They are arranged in such a way that, as soon as one arm gains a little upon the other, a correcting current is thrown into the line, which, by its effect upon the magnets of the motor, reduces the speed of that which is in advance, and thus a sufficiently approximate synchronism is maintained. This system is well suited for the distribution of telegraph facilities in the neighborhood of the terminus of a main line. A dozen or more business men in New York may be connected 138 A CENTURY OF ELECTRICITY. with as many correspondents in Boston through, one line connecting the two*" cities. As the number of branch circuits increases, the rapid- ity of transmission diminishes ; but, in what may be called " private telegraphy," speed of working is generally not of primary impor- tance. It is stated that a line between Boston and Providence, on which Delany's system was used, worked at the rate of forty words per minute over each of six separate circuits ; when divided into twelve, the rate was reduced to twenty words per minute ; and that seventy- two printing circuits were worked at the rate of from two to three words per minute. In the application of electricit}' to the trans- mission of signals through submarine cables, so many modifications of the methods already de- scribed are found necessary, that the subject of submarine telegraphy is worthy of special con- sideration. Although at the present time the laying of a cable, even across one of the great ocean-beds, is a mere business matter, involving only the expenditure of a certain amount of money, it has become so only after a vast num- ber of failures and the loss of an immense amount of capital feebly represented by the material lying useless at the bottom of the sea. The conditions necessary to success were not SUBMARINE CABLES. 139 at first understood ; for, in the earlier years of cable-laying, it was not considered necessary or desirable to utilize the knowledge and skill of scientific electricians of the first rank. Success was finally reached only after such were in- duced to study the complex problems involved. The lesson was costly but unavoidable. The attempt to lay insulated conductors un- der water naturally followed upon the practical introduction of the telegraph. Covered with hemp or cotton, which was saturated with tar, asphaltum, or other insulating materials known at the time, they soon lost their insulation, often after only a few days' exposure. Owing to the great importance of being able to connect land lines across rivers and lakes, every effort was made to discover a suitable material for insu- lation, and it was even proposed to use short glass tubes connected together by universal joints, to give flexibility. While various materials were being tried, an English surgeon, stationed at Singapore, was experimenting upon the properties of a sub- stance which was destined to afford the solu- tion of the problem. In 1842 he recommended gutta-percha as useful in making splints and other surgical appliances, and shortly afterward forwarded specimens of the substance to the Society of Arts in London. It soon attracted 140 A CENTURY OF ELECTRICITY. the attention of the commercial world, and numerous patents for its preparation were taken out. One of its most striking properties is its high insulating power ; and its value as a covering for submarine conductors was soon recognized. During the past forty years innu- merable attempts have been made to discover something which will take its place, but without great success. It is extremely probable that the wide-spread use of submarine cables would have been postponed many years, had this sub- stance remained unknown. One of the first cables insulated by this material, and possibly the very first, was laid in 1848 across the Hud- son River, from Jersey City to New York. In 1850 a cable was laid across the Channel, from Dover to Calais; but it was unprotected by any sheathing or armor, and it lasted but a single day. In the following year the experiment was re- peated, this time with a cable protected by a number of heavy iron wires. The operation was successful, and permanent telegraph communi- cation was established. During the next few years the number of submarine cables increased rapidly, as did also their length, although, on account of ignorance in regard to many condi- tions necessary to insure the best success, fail- ures were numerous. Many people began to SUBMARINE CABLES. 141 consider the feasibility of a line connecting the continents across the Atlantic Ocean. A few sanguine capitalists combined to further the enterprise, and through the undaunted courage and faith of an American, Mr. Cyrus W. Field, the purely financial obstacles were surmounted. Unfortunately the electrical and engineering problems to be met with were not understood ; and the memorable first cable of 1858, after gasping for breath for a few short weeks, lay dumb forever at the bottom of the sea. Something of the character of this cable may be learned from the following brief description by Sir William Thomson, to whom, more than to any other one man, the world is indebted for the success of submarine telegraphy : In the year 1857 as much iron as would make a cube of 20 feet side was drawn into wire long enough to extend from the earth to the moon, and bind sev- eral times around each globe. This wire was made into 126 lengths of 2,500 miles, and spun into 18 strands of 7 wires each. A single strand of 7 copper wires of the same length, weighing in all 110 grains per foot, was three times coated with gutta-percha, to an entire outer thickness of .4 of an inch ; and this was " served " outside with 240 tons of tarred yarn, and then laid over with the 18 strands of iron wire in long, contiguous spirals and passed through a bath of melted pitch. 142 A CENTURY OF ELECTRICITY. An attempt to lay this cable in 1857 resulted in the loss of 400 or 500 miles, by breakage from the stern of the ship from which it was run. After some further experimentation, it was determined to employ two ships to lay it in the following year ; and accordingly, on the 29th of July, 1858, the Niagara and the Aga- memnon, each loaded with half the cable, met in mid ocean, joined the ends, and started, the Niagara for the West and the Agamemnon for the East. . On the 5th of August the ends were successfully landed on the opposite shores of the Atlantic. The cable was known to be in bad condition before the laying was completed, and the ear- nest but ill-advised efforts which were made to force it to work during its brief period of ac- tivity, only tended to shorten its life. Com- munication of a very irregular and unsatis- factory character was maintained for several weeks. The admirable mirror galvanometer, which had just been devised by Sir William Thomson, was for the first time in use at the Valentia end,. while for a time the attempt was made to use the ordinary receiving apparatus, which had been provided by the company at Newfoundland. The result of this was that signals were received with little difficulty at Valentia, while much trouble was experienced SUBMARINE CABLES. 143 at Newfoundland. Later the mirror galvanom- eter was put in use on this side, but not be- fore very powerful currents had been used on the cable, tending to increase existing faults. In fact, Sir William Thomson has declared his belief, that, if proper methods of handling the cable electrically had been in use from the be- ginning, its performance would have been last- ing, and in the main satisfactory. Owing to the fragmentary character of many of the messages transmitted, a single sentence from that of the Queen to the President having been received on August 16, and the remainder twenty-four hours later, many persons in both Europe and America became sceptical as to the transmission of signals, and not a few even doubted that the cable had been laid. As a matter of fact, four hundred messages, contain- ing over four thousand words, were sent. On September 1, interchange of messages ceased; but on October 20 the cable spoke its last words, " two hundred and forty," which were read at Valentia, being part of a message giving the number of battery-cells then on the line. From that date the " splendid combina- tion of matter lay at the bottom of the sea, for- ever useless." But it had not lived in vain : the possibility of the thing was demonstrated, and it only remained to surmount certain obstacles, the existence of which this trial had proved. 144 A CENTURY OF ELECTRICITY. During a few years succeeding this first at- tempt, the problem was studied in the light of the experience which it had afforded. Another trial was made in 1865, this time by the Great Eastern, a vessel which offered many advan- tages for cable-laying. After about two thirds of the distance was run the cable broke, and further operations were postponed until the fol- lowing year, when a complete cable was suc- cessfully laid, and that of 1865 picked up, spliced, and finished. Since then other lines have been placed across the Atlantic ; and now the operation of laying an ocean-cable attracts no attention, save from those who are directly interested in the enterprise. In the construction of a cable, it is essential that the wire which is used as a conductor should be surrounded by a sufficient thickness of as perfect insulating material as is available. A line suspended in the air may be insulated with comparative ease, for the medium by which it is surrounded is itself an almost per- fect non-conductor. The conducting power of water, however, is extremely high, compared with that of air, and the water in which the cable lies must nowhere come in contact with the copper conductor. The cable must also possess sufficient strength to survive the opera- tion of laying, and to be uninjured by whatever SUBMARINE CABLES. 145 disturbances it may be subjected to when in place. Consisting, as a cable does, of a long conduc- tor surrounded by a thin layer of insulating material, and then again by a conductor, it is electrically similar to a great condenser, or Ley den jar, as was observed by Faraday in connection with some of the earlier short ca- bles. Reference has already been made to what is called the " static capacity " of a land line : that of a cable of the same length is generally many times as great. If a well-insulated wire, a few hundred or a thousand feet in length, be coiled in a vessel of water, with one end pro- jecting into the air or sealed over with gutta- percha, the current from a single battery-cell, in rushing in to charge the wire, will cause a violent deflection of the needle of a sensitive galvanometer. A considerable length of time will be consumed in completely charging a ca- ble, and, of course, time will be occupied in dis- charging it. The result of this is a great re- tardation of signals and a correspondingly less speed of transmission. It is said, that, if the attempt were to be made to use ordinary Morse instruments on one of the Atlantic cables, hardly more than one word per minute could be transmitted. The signals are not only re. tarded : they are altered in character, becoming 10 146 A CENTURY OF ELECTRICITY. less sharp and distinct as the length of the ca- ble increases. As already remarked, the use of strong cur- rents is extremely objectionable, and thus there are several reasons why ordinary methods of operating prove insufficient when applied to ocean cables. Scarcely any modification is re- quired in the sending apparatus : a single key for closing the circuit may be used, or a double Ocean cable system ; a a, condensers ; g, galvanometer or re- ceiving instrument. key, by means of which either positive or nega- tive electricity may be sent to the line. It is found to be advantageous, however, not to con- nect the battery with the line at all ; that is to say, not directly, but only indirectly through a condenser, one branch of which is connected with the line, and the other with the earth through the battery, key, and receiving instru- SUBMARINE CABLES. ment. The condenser is prepared by instating sheets of tinfoil from each other, as already De- scribed. The surface of foil used in one of these condensers is only slightly less than one acre, although it occupies a space of less than three cubic feet ; and, for the purpose of " du- plexing " the cable, a condenser of more than two acres of surface has been used. The use of a condenser increases the speed of transmis- sion, besides offering other advantages. The signals are received by an extremely sensitive galvanometer, devised for the purpose by Sir William Thomson, to which reference has al- ready been made. In this the wire is very fine, and the number of turns very great. The needle is extremely small, consisting of several short magnets fastened to the small circular mirror, the whole often weighing less than half a grain. This needle is suspended by a single fibre of silk in the centre of the coil, which is wound as closely to it as the necessary freedom of motion will allow. A beam of light falls on the mirror, and is reflected upon a screen, where a spot of light is seen. The movements of the needle are indicated by and magnified in the motion of this spot, and the alphabet is made up of to-and-fro movements. This beautiful instrument has been used on many cable lines, but it has been largely super- 148 A CENTURY OF ELECTRICITY. seded by the " siphon recorder/' devised by the same distinguished electrician. In this a light, rectangular coil of fine wire is suspended be- Thomson's siphon recorder for ocean cables. tween the poles of a powerful electro-magnet. Advantage is taken of the fact that a coil of wire through which a current is passing tends SUBMARINE CABLES. 149 to place itself in a particular position in a mag- netic field. A fine glass siphon tube is attached to the coil, and moves with it. The short arm dips into a vessel of ink, which is insulated and capable of being electrified. The long arm has its open end very near to a small plate or table, over which a strip of paper is moved regularly by clock-work, as in the Morse register. The whole system (tube and coil) moves with great *JV^j^ * ft I 4 r m. * * * | ft < Specimen of message written by siphon recorder. freedom, and is deflected from its normal posi- tion by very feeble currents. The electrifica- tion of the ink causes it to be projected from the end of the tube in minute drops, so that the movements of the coil are recorded on the mov- ing slip of paper in very fine dots very near to each other. An actual record of the message is thus made, which can be read at leisure and preserved. As noticed by Mr. Prescott in his valuable work on the telegraph, it is curious to 150 CENTURY OF ELECTRICITY. see, that, in the evolution of telegraphic meth- ods, that feature of the Morse system which was at first thought to be of the highest impor- tance, namely, its capacity for recording the message on paper, has been almost wholly dis- carded in practice ; while the needle telegraph, which in the beginning made no record, now finds almost its only representative in the si- phon recorder. CHAPTER VI. FARADAY'S DISCOVERY OF INDUCTION AND THE DEVELOPMENT OF THE DYNAMO. AMONG innumerable contributions to the world's knowledge of electricity, three splendid discoveries stand incomparably above all others. More than all others, these opened new fields for research, and created new possibilities of ap- plication. The discovery of the " new electric- ity" by Galvani, and of a means of generating it by Volta, and Oersted's memorable experi- ment in which its influence upon the magnet was revealed, have already been described. Magnificent results which have sprung from these two discoveries have been briefly consid- ered, although it cannot justly be affirmed, that, in their development, nothing has been due to the third member of the triad. But the cata- logue of the accomplishments of human genius, as worked out along the line of electricity, would be very incomplete if terminated at this point. Nearly all of the more recent and more 152 A CENTURY OF ELECTRICITY. striking applications of the electric current, in which almost daily it is being made to serve man in some new capacity, rest upon the last of the three great discoveries, that of electro- magnetic induction, by Faraday. The son of a blacksmith, for a time a news- paper-carrier, a bookbinder's apprentice at the age of thirteen, Michael Faraday, as a youth, enjoyed few facilities for the acquirement of an education. In a common school he learned the rudiments of reading, writing, and arithmetic ; but his apprenticeship, which lasted for eight .years, afforded some opportunities for satisfy- ing his keen thirst for knowledge. He eagerly devoured scientific literature which fell in his way, and his attention was especially drawn to electricity by the perusal of an article in an encyclopaedia which he was employed to bind. A customer of his master's shop, who was a member of the Royal Institution, afforded him the opportunity of attending four lectures on chemistry, given by Sir Humphry Davy in 1812. Of these lectures he made an admirable series of notes. These he neatly transcribed, illustrated, and sent to Davy, with the request that he might be given some employment in the Royal Institution which would enable him to indulge his taste for experiments and study. To this Davy replied, praising the notes, and FARADAY'S DISCOVERY. 153 promising an interview. At that interview he advised young Faraday to stick to his book- binding, and "promised to give him the work of the Institution, as well as his own, and that of as many of his friends as he could influence." Shortly after this, however, Davy dismissed his assistant, and, remembering Faraday's desire, he employed him to fill the place. Thus at the age of twenty-two years, and under these not very promising circumstances, he began a ca- reer which, for usefulness as well as brilliancy, has perhaps never been eclipsed. Before entering Davy's laboratory, he had experimented largely in electricity ; but, almost of necessity, for several years afterwards, his attention was mostly given to questions of a chemical nature, and indeed, as he declared himself, he was "for nearly twenty years a student," during which time he was laying the foundation for the remarkable series of re- searches which he afterward carried out. Reference has already been made to his suc- cess in producing continuous rotation of a con- ducting wire around a magnet, and also the reverse. A little later, about the year 1825, the scientific world was puzzling over an ex- periment by the famous Arago. It consisted in rotating a copper or brass disk underneath a freely suspended compass-needle : the latter 154 A CENTURY OF ELECTRICITY. was deflected, and might, indeed, be made to rotate about the axis of suspension, provided the metallic disk was turned with sufficient ra- pidity. No one was able to offer a satisfactory explanation of these rotations ; but Faraday conceived the idea that they were due to elec- tricity induced in the revolving disk, and so re- corded his belief in his note-book. Even earlier than this he was convinced, that, as an electric current affects a magnet, and may even produce magnetism, a magnet, in turn, must be capable of exerting an influence upon an electric cur- rent ; and from 1825 he occupied himself more or less in the experimental study of the ques- tion. He failed to discover anything like an induction effect, and he failed again and again during the next few years. The subject was again taken up in 1831, and on the 29th of August he began that wonderful series of ex- perimental researches in electricity which at once placed him in the front rank of living philosophers, and established his position as the finest experimentalist of the present age. Faraday's failures arose from the very natural belief that induction in " voltaic " electricity ought to resemble the induction so long known, which occurs when a body charged with electric- ity is brought near an insulated conductor, in which case a permanent state of electrification FARADAY'S DISCOVERT. 155 is set up. Again : as an electric current pass- ing through a conductor produces a permanent deflection of a magnetic needle, that is, a deflec- tion which lasts as long as the current flows, it is natural to infer that, if the presence of the magnet produces a reactive effect upon the cur- rent, that effect will last while the magnet is present. That generalization known as the principle of the conservation of energy, which Faraday's ring ; from cut in Phil. Trans. 1832. has since been established, and to the proof of which Faraday himself contributed so much, shows that neither of these things could hap- pen. But what Faraday afterwards pronounced to be " the highest law in physical science which our faculties permit us to perceive," was then but dimly outlined in the minds of a few men, and could not be depended upon, as now, to put a check upon hypothesis. 156 A CENT UR Y OF ELECTRICITY. Knowing that magnetism was produced from electricity, he attempted to produce electricity from magnetism. For this purpose he used an iron ring about which two or three helices of wire were wound. A current being passed through one of them, the ring became mag- netic, and he looked for the production of a cur- rent of electricity in the other helix. To detect this current, he connected the extremities of this helix with the poles of a galvanometer. The faculties which enabled Faraday to turn a series of failures into success, as he did on this occasion, are thus aptly described by Tyn- dall : He united vast strength with perfect flexibility. His momentum was that of a river, which combines weight and direction with the ability to yield to the flexures of its bed. The intentness of his vision in any direction did not apparently diminish his power of perception in other directions ; and when he at- tacked a subject, expecting results, he had the faculty of keeping his mind alert, so that results different from those which he expected should not escape him through preoccupation. Thus, although expecting permanency of ef- fect, failing to get it, he did not allow the mo- mentary movement of his galvanometer to es- cape his attention. Indeed, he soon discovered that this was the effect for which he was search- FAR AD A PS DISCOVERY. 157 ing; that a momentary current was produced in his coil when the ring was made magnetic, and another when its magnetism ceased. He developed and expanded his experiments with wonderful rapidity, and was soon able to pro- duce electricity from magnetism at will. By using bits of charcoal or fine wire, lightly in contact at the extremities of his secondary he- lix, and jarring them a little at the moment of the passage of the current, so that separation took place, he was able to produce a spark. This, to many, was the most striking evidence of his success in getting, electricity from mag- netism ; and the general interest in the subject caused the experiment to be repeated under va- rious conditions. Faraday at once saw in these phenomena the explanation of Arago's rotations, believing that the metal disk, when revolved under the mag- netic needle, was the seat of induced currents, which, by their reaction on the needle, caused it to deflect. On October 28, he mounted a disk so that it could be revolved between the poles of an electro-magnet, and connected the axis of the disk and its edge with his galva- nometer. When the disk was turned, the needle moved, showing the presence of induced currents. This was the first dynamo-electric machine, the parent of all that are to-day flood- 158 A CENTURY OF ELECTRICITY. ing with light the cities and towns of the civil- ized world. In the mean time he endeavored to induce a current by means of a current, without the use of a magnet. Here again, after many failures, he was successful. The results of his work The first Dynamo. from August 29 to November 4, during which time only ten days had been spent in actual ex- perimenting, he collected into the first series of "Experimental Researches," presented to the Royal Society on November 24. This paper contains all the general propositions pertaining to electro-magnetic induction, and in its prep- aration his " rate of discovery " can only be FARADAY'S DISCOVERY. compared with the work of Ampere his receipt of the news of Oersted's expe It was followed by other papers, in which, in connection with the first, a new and rich field of research was opened to electricians. Fara- day's discoveries during these few months, from both the scientific and the practical stand-point, must rank with those of Galvani and Volta in discovering current electricity, and providing the means for its production. Indeed, his dis- covery might have been made had the existence of current electricity and the battery been en- tirely unknown ; aqd all of the modern applica- tions of electricity might have followed. As a matter of fact, the method of generating elec- tricity which he first gave to the world, in addi- tion to supplying new demands which it itself created, is rapidly being substituted for the voltaic battery, although it is not likely ever to entirely take its place. The fundamental principles of electro-mag- netic induction, as discovered by Faraday, may be thus briefly stated, and almost in his own words : 1. When an electric current is passed through one of two parallel wires, it causes at first a cur- rent in the samedirection in the other ; but uuu / this current is only momentary, notwithstand- y-./^ ing the inducing current is continued. When 160 A CENTURY OF ELECTRICITY. this current is broken, another current is pro- duced in the wire under induction, of about the same intensity and momentary duration, but in the opposite direction to that generated at first. 2. If a coil of wire whose ends are joined, through a galvanometer or otherwise, so that a current can pass, be brought up to a magnet, or if the magnet be made to approach the coil, a current will pass through the coil. This cur- rent will not be permanent, but will exist only during the motion of approach. If the magnet and coil be separated, a current will again be induced, but, as in the previous case, its direc- tion will be opposite to that of the first. In endeavoring to express the conditions un- der which induction takes place, Faraday intro- duced the conception of " lines of force," being lines along which a free magnetic pole would move, which has since played so important a part in the theory and literature of electricity and magnetism. Doubtless no one appreciated the value of this work more than Faraday himself, although he failed to protect his right in it by letters patent. It was easy to foresee the immense value of the numerous applications which might be made of his discoveries, but, after having laid the foundation for the electricity of the future, he left its development to others. " I FARAD ATS DISCOVERY. 161 have rather been desirous," he said, " of discov- ering new facts and new relations, than of exalt- ing those already obtained, being assured that the latter would find their full development hereafter." The general principle which he had estab- lished was that mechanical energy might be converted into electricity in motion, which, in- deed, was but another form of energy, capable itself of being reconverted into other varieties ; for he had shown that a current of electricity could cause motion in a mass of matter, and that the movement of a mass of matter could produce an electric current. In his experiments, it was only while work was being done that the current flowed ; but it was not then as clearly recognized as at present, that a real expenditure of work was necessary to move a magnet to- wards or away from a coil of wire, that is, work in excess of that required to make the move- ments in relation to a coil of rope or other non- conducting material. This work is the equiva- lent of the energy of the electric current, and, in his experiments, was so small as not to be perceptible except as an electric current. When he turned his copper disk between the poles of an electro-magnet, he did not observe that it was more difficult to move than when the mag- net was absent, although that is really the case ; 11 162 A CENTURY OF ELECTRICITY. an apparent " friction against space " existing when the magnet is present and a current is being produced. This friction may raise the temperature of the moving conductor, precisely as if it were ordinary friction, as was first shown by Joule. In order to understand more fully the con- ditions under which induction takes place, it will be necessary to recur to the simple laws of electro-dynamics, as discovered by Oersted and Ampere. It will be remembered that Oer- sted found that a current of electricity passing through a conductor in the vicinity of a magnet tends to move the north pole of the magnet in a certain definite direction ; and that Ampere discovered that two parallel conductors attract each other when traversed by currents in the same direction, but repel each other when the currents are in opposite directions. In 1833, Lenz, a Russian philosopher, announced the simple and beautiful law, that currents induced either by the motion of a conductor traversed by a current, or by the motion of a magnet, are always in such direction as to produce forces opposing the motion generating them. To over- come these opposing forces, the expenditure of energy is necessary, and this energy is the equiv- alent of the currents generated. The problem the solution of which Faraday FARADAY'S DISCOVERY. 163 bequeathed to others was to construct a machine by means of which electricity could be produced in sufficient quantities to be useful, and in which the largest possible percentage of the total energy consumed should be converted into available electric currents, and as little as possi- ble lost in friction and in heat arising from non- available currents. Fifty years have been spent in eliminating the difficulties in this problem, but at the present time its solution is astonish- ingly near perfect. During these fifty years, innumerable inventors, in both the old and the new world, have contributed to the develop- ment of Faraday's principle ; and it will only be possible to refer to a few of the principal advances that have been made from time to time, and which have combined to give the dynamo - electric machine of to - day a nearly ideal efficiency. Perhaps the first realization of the new prin- ciple was in a machine devised by Pixii of Paris in 1832. It consisted of an electro-magnet of the horseshoe form, the wire of the coil being long and fine ; and of a permanent steel mag- net, also of the form of a horseshoe, so arranged that it could be rapidly rotated about an axis parallel to its length, and in such a manner that its poles passed, at each revolution, very close to the poles of the electro-magnet. The soft 164 A CENTURY OF ELECTRICITY. iron cores of the latter were thus rendered mag- netic for an instant by induction, reaching their maximum strength when the poles of the revolv- ing steel magnet were nearly opposite. The rapid changes in the intensity and character of this magnetization induced currents in the coils, first in one direction, and then in the other. The extremities of the coils were joined to the external circuit, and currents were made to pass through that circuit in the same direction by means of a device called a " commutator," which, in some form or other, appears in nearly all modern machines. The simplest arrange- ment for a commutator, and the form used in Pixii's machine, consists of a brass cylinder re- volving on the axis of the rotating magnet, which is divided symmetrically into two parts insulated from each other. Metal springs, con- nected with the ends of the coils of the electro- magnet, rest with slight pressure against this cylinder, as do also similar springs connected with the external circuit. These springs, and parts of the divided cylinder, are so arranged, that, when the rotation of the magnet reverses the direction of the current in the coils, it also reverses the connection between the contact springs belonging to the electro-magnet and those attached to the external circuit ; and thus the current, though discontinuous, is always in the same direction in that circuit. FARAD ATS DISCOVERY. 165 Saxton improved upon this by rotating the coils instead of the magnet, on the principle that the lighter part should be the moving part. A London instrument-maker named Clarke in- troduced many modifications into the machine, and especially in the method of winding the coils, finding that very different electrical effects could be produced by varying the length and thickness of the wire. He was probably the first to distinguish between what is now tech- nically known as winding for " tension " and for u quantity ; " that is to say, for machines of high or low electromotive force. The earliest discoveries of some of the phe- nomena relating to induction were made by Charles G. Page, a young physician of Salem, Mass. At the age of ten years he had con- structed an electrical machine, and soon after his graduation from Harvard University he de- vised a magneto-electric machine, which differed in form and somewhat in principle from those already described. He placed coils of wire about the poles of a permanent steel magnet which was fixed in position. Variations in the strength of the magnetic field were produced by rotating a soft iron armature before and very near these poles. The weight of the moving part of the machine was thus diminished, and the strength or character of the current pro- 166 A CENTURY OF ELECTRICITY. duced could be easily regulated by adjusting the distance of the revolving armature from the poles of the magnet. Both Clarke's and Page's machines are still very common. The effects produced by them are generally very feeble ; but, when they are properly wound, currents of considerable elec- tromotive force can be generated. They soon came into general use for medical purposes ; and, in fact, no method of producing electricity has yet been devised which has not been as- sumed to furnish a current of peculiar value as a curative agent. The use of these machines for this and similar purposes has greatly les- sened since the invention of the induction coil, the earlier forms of which must also be attrib- uted to Page. They are still very generally used, however, as convenient substitutes for the voltaic battery, especially in the transmission of signals, as in the ordinary call-bell of a tele- phone system. Other forms of magneto-electric machines, not differing greatly from Clarke's, appeared during this period ; but all were small in dimensions, and, generating little electricity, their applications were limited. At last, in 1849, Nollet of Brussels under- took the construction of a Clarke machine on a large scale, combining many coils and magnets. At first an attempt was made to maintain con- FARAD AT 8 DISCOVERY. 167 stancy of direction of current in the external circuit by commutating, as in the simpler forms of Clarke machines. After many failures, this was given up, and the machine was converted into one giving alternate currents. In this method of working, the current in the external circuit is reversed as often as it is in the re- volving coils, which, in some machines of this class, is as often as one hundred times per second. For 'many purposes, however, such a current is as useful as one continuously in the same direc- tion. An Anglo-French company, known as the Compagnie de I 'Alliance, was formed to carry out Nollet's ideas, from which fact the generator has been known as the Alliance ma- chine. It is said that this machine was at first intended to form a part of a chimerical project to obtain oxygen and hydrogen in large quanti- ties by electrolysis, these gases to be afterward utilized in the production of heat and light. The amount of heat expended under the boiler of the engine which ran it exceeded so much that which could be obtained at the paying end of the machine, that the company soon failed, and deservedly. Its re - organization, for the purpose of manufacturing machines for electric lighting, was an event of real importance in the history of that industry. It was demon- strated that a powerful current of electricity 168 A CENTURY OF ELECTRICITY. could be generated by induction, and that elec- tric lighting by this method was possible, if not profitable. Up to this time the battery had been looked upon as the only means of producing powerful currents, and there was no good reason for be- lieving that the battery could ever be made a convenient and economical source ; so that the practical utilization of strong currents did not appear to be close at hand. One of the forms of this machine, designed by Holmes in 1856, was submitted to the English Light - House Board, with a proposal for its introduction into light-houses. After severe tests, in which Fara- day himself participated, the machine was de- clared satisfactory, and it was permanently installed in 1862. Others were introduced in France ; and, although great improvements were seen to be possible and necessary, their success was such as to encourage inventors everywhere in the belief that the difficulties were not insurmountable. A very considerable advance was made by Dr. Werner Siemens of Berlin, in the invention, in 1856, of what is known as the "Siemens Armature.'' Recognizing the importance of ro- tating the coils in a magnetic field of the great- est possible intensity, he planned the armature as a means of accomplishing this end. It con- FARADAY'S DISCOVERY. 169 sisted of a long cylinder of soft iron, in which two parallel longitudinal slots were cut at op- posite extremities of a diameter. In these slots the wire was wound, thus forming a long and Skeleton armature of the Siemens type. narrow coil, which could be rapidly rotated be- tween the opposed poles of a magnet or of a series of magnets. This form of armature, with slight modifications, is found in many of the best modern machines. In all of these machines the magnetic field was produced by permanent steel magnets. It is easy to make an electro -magnet of vastly greater power than that possessed by any per- 170 A CENTURY OF ELECTRICITY. manent magnet ; and this fact was taken ad- vantage of by Wilde, who constructed What was really a double machine, consisting of a smaller one with steel magnets and a Siemens armature, the current from which was passed through the large electro-magnets of the other, thus exciting a magnetic field of much greater intensity than was before possible. In this field another armature revolved, and furnished the current utilized in the external circuit. This was hardly accomplished when a most interesting discovery was made, almost simulta- neously, by Wheatstone and Siemens, which enabled them to dispense with the permanent magnets entirely. It consisted in utilizing the minute traces of magnetism which exist in all iron, for the production of feeble currents, which, in their turn, excite a more intense mag- netization, and in this way the cores of the magnets are qivickly charged to saturation. The first machine involving this principle was exhibited by Wheatstone to the Royal Society, on February 14, 1867, and it is not necessary to say that it excited the greatest interest. By a remarkable coincidence, such as occurs now and then in the development of a scientific principle, on the same day a paper was pre- sented to the society by Siemens in which the same improvement in construction was de- FARAD ATS DISCOVERY. scribed. The coils about the electro-i are either apart of, or a "shunt" main circuit. When the armature is started either the whole or a part of the current cir- culates about the field-magnets ; and, although it may be feeble at first, by its effect in increas- ing the magnetism of the cores, it very soon reaches its maximum. The discovery of ^his method of developing and maintaining the " field " must always be regarded as of the highest importance. The production of dynamo-electric machinery received a fresh impetus from the construction of a novel machine by Gramme of Paris, in 1870 ; from which time, in fact, it passes from the experimental to the industrial and commer- cial stage. Gramme's generator produced prac- tically continuous currents of constant strength, and its merits were due to the introduction of a new form of armature. In the form and con- struction of this armature, however, Gramme was anticipated by Pacinotti, an Italian, who had constructed a machine on this principle in 1860, but it had remained undeveloped. This armature is the basis of those used in several modern dynamos. Its operation will be better understood after a little further consideration of the conditions under which induction takes place, as discovered by Faraday. 172 A CENTURY OF ELECTRICITY. Reference has already been made to the fact, that, in defining these conditions, Faraday made use of the expression " lines of force ; " and the study of electro-dynamic induction is greatly facilitated by an understanding of this far- Curves formed by iron filings in the field of a bar magnet. reaching conception. Every one is familiar with the experiment of sprinkling iron filings upon a sheet of paper, card-board, or glass un- derneath which a magnet has been placed. The arrangement of these filings, especially when the card - board is lightly tapped to facilitate their movement into lines and curves about the FARAD ATS DISCOVERY. 173 poles of the magnet, is a very striking and in- structive phenomenon. In the first of his " Ex- perimental Researches," Faraday speaks of mov- ing a wire so as to " cut " the magnetic curves, and explains in a foot-note as follows : By magnetic curves, I mean the lines of magnetic forces, however modified by the juxtaposition of poles, which would be depicted by iron filings, or those to which a very small magnetic needle would form a tangent. From that time these hypothetical lines have been usefully employed in the development of the principles of electro-dynamics. According to this theory, every region in which a magnet would be in any way acted upon or influenced is to be considered as a field of magnetic force. The region immediately surrounding the earth is such a field, as is shown by the tendency of a magnetic needle to rest in a certain direction. All fields of force are pervaded by lines of force, which are lines along which the force acts, and are defined as above. If a very small magnetic needle, suspended so as to be free to move in all directions, be brought into the field of force surrounding a magnet, it will come to rest in, or, more accurately, tangent to, a line of force. If it be moved somewhat from its first position, its direction will, in general, change, showing that the lines of force are not parallel. Such 174 A CENTURY OF ELECTRICITY. a needle may be used for exploring a field of force ; and when the lines are traced out, it will be found, as shown in the experiment with the iron filings, that the lines of force appar- ently spring out of the poles of the magnet, or, as is often convenient, they may be imagined to come out of the north pole, and to reunite on entering the south pole. They will be found to be most numerous in the immediate vicinity of the pole ; so that they may conveniently represent the two essential elements of a force, direction and intensity, the latter being meas- ured by the number of lines cutting through a given area, as a square centimetre, taken at right angles to their direction. Experiment with the iron filings, or with the small exploring needle, shows that the position of the poles of a magnet in relation to each other determines the form, and to some extent the number, of lines of force at a given point ; and the introduction of a third pole in the immediate neighborhood will be found to mod- ify them materially. If two magnets have the opposite poles placed near to each other, or if a magnet be bent so as to bring its poles near together, it will be found that, in the region directly between them, the lines of force are very numerous and nearly straight. Now, Faraday's investigations proved that, FARAD AT 8 DISCOVERY. 175 in order to induce a current of electricity in a wire by means of a magnet, it must be moved so as to cut these lines of force. If a wire is moved in a field of force in a direction parallel to the lines, so as to cross none of them, no cur- rent will be induced. Furthermore, it was found that the electromotive force produced in the wire (on which, together with the resist- ance of the circuit, the strength of the current depends) increases with the number of lines of force cut in a given time. This is on the sup- position that a single linear portion of the cir- cuit is moved in the field ; and it is important to note, that if a complete circuit, in the form of a loop or ring, be moved directly across the lines of force in a uniform field, no current will be induced as long as the loop remains parallel to its original plane. It is easy to see that this is due to the induction, on opposite sides of the circuit, of currents opposed in direction, of equal strength, which therefore completely neutralize each other. If, however, such a ring or loop be twisted out of its plane, a current is at once produced. This leads to an extension of the principle stated above, so that it becomes finally the following : the movement of the whole or part of a conducting circuit in a magnetic "field will induce a current in that circuit, provided that during that movement the number of lines 176 A CENTURY OF ELECTRICITY. of force passing through the circuit is increased or diminished. It will be understood, of course, that relative motion is here referred to ; that is to say, the conducting circuit may be fixed in position, and the number of lines of force pass- ing through it may be altered by moving the magnetic pole from which they spring, and in- duction will follow, as before. Any operation, therefore, which changes the number of lines of force passing through a cir- cuit, will induce a current in that circuit ; and, since this is equivalent to a modification of the nature of the field (as defined by direction and intensity), it may be said that, in general, any modification of the magnetic field in the vicin- ity of a conductor gives rise to an induced cur- rent. A thorough understanding and apprecia- tion of this statement will greatly facilitate the study of induction machinery. The core of what is known as the " Gramme Ring Armature " consists of a soft iron ring, or often a bundle of soft iron wires bent into a ring and the ends then joined. It is mounted so that it can be rapidly rotated about an axis through the centre of the ring, perpendicular to its plane, precisely as the rim or tire of a carriage- wheel turns about its axis. Around this ring, coils of wire are wound so that their planes pass through its centre ; that is, they are FARADAY'S DISCOVERY. 177 situated exactly as if they had been originally wound in a helix around a straight cylindrical bar, which was afterwards bent so that its ends joined, thus forming a ring. A number of separate coils are thus wound ; but they are joined together by connecting the proximate ends of every pair of adjacent coils to a strip of metal, generally copper, there being as many Skeleton Gramme armature. strips as there are separate coils of wire. These strips are arranged in the form of a cylinder around the axis of the armature, to which they are parallel ; but they are insulated from it and from each other. At the opposite extremities of a diameter of this cylinder the collecting brushes are placed, consisting generally of thin pieces of copper, which are pressed against the cylinder by means of springs or other devices 12 178 A CENTURY OF ELECTRICITY. which afford some elasticity. The cylinder of bars, with the collecting brushes, constitutes the commutator. The magnetic field in which this armature revolves is produced by a powerful electro- magnet, generally excited by the current which the machine itself produces, the opposite poles of which lie at the extremity of a diameter of the armature. The operation of the machine is not difficult to understand. The soft iron ring core of the armature is constantly rendered powerfully magnetic at two opposite points, very near to the poles of the stationary or field magnet. At points a quarter of a circumfer- ence from these it is neutral, so that the action is nearly the same as if the coils were rapidly moved around a ring magnet whose poles were at the ends of a diameter. The result is, that the number of lines of force passing through any particular coil is continually and rapidly increasing and decreasing, and the induced cur- rents are correspondingly strong. These cur- rents are carried to the external circuit through the commutator bars and collecting brushes, and the coils are so numerous that the current is practically continuous. Since the construction of the Gramme ma- chine, improvement in dynamo-electric genera- tors has been extremely rapid, and their practi- FARADAY'S DISCOVERY. 179 cal use has correspondingly extended. Innu- merable patents have been issued for machines involving improvements upon, or modifications of, the Gramme armature. Other forms have been highly developed, and some entirely novel systems have appeared, to compete for popular favor. Among those commonly in use in this country, one of the earliest to achieve success was that of Mr. Charles Brush of Cleveland, O. The Brush armature is similar in many respects to the Gramme, involving, however, important modifications in the form of the iron ring core, and an ingenious arrangement for connecting the coils, which is unlike that adopted in the Gramme armature. The disposition of the field -magnets is also different from that of Gramme. Another well-known machine is that invented by Mr. Weston. This involves several novelties, especially in the construction of the armature. While this resembles the Siemens armature in general form, in detail it differs from it very much. The commutator of this machine is also different from that already described, in that the metal strips are laid on spirally around the cylinder. The object of this is to secure contact of the brushes with two of the strips at all times, so that the current may be more nearly uniform. The Thomson Houston dynamo, which is largely in use for 180 A CENTURY OF ELECTRICITY. arc lighting, is unique in the form of its arma- ture, which is nearly spherical, and consists of three coils. Mr. Edison, although entering this particular field of invention more recently than many others, has produced a machine original in many respects, and especially re- markable for the tremendous size and power of some individual specimens that have been built. Many other machines whose success has been practically demonstrated might be mentioned. Each machine doubtless possesses some points of superiority over all the others. In fact, the business of building dynamo-electric machinery has come to resemble very much that of manu- facturing steam-engines. Hardly more than ten years ago, the phrases " Gramme machine " and " dynamo - electric machine " were almost synonymous. To-day there is no one machine there are many ; and in making a selection the ruling questions relate to expense of construc- tion, adaptability to special uses, dimensions, weight, etc., precisely as with the steam-engine. Reviewing briefly what has been said concern- ing it, a dynamo-machine is seen to consist of two principal parts. One of these is at rest, a huge electro - magnet of some form or other, which, when excited by the current through its coils, creates a magnetic field of intense power. FARAD ATS DISCOVERY. 181 Within this field is the armature, a coil or collection of coils, and often with magnetic cores, which is whirled around its axis at a speed varying from 350 revolutions per minute in Edison's giants to 3,000 or 4,000 per minute in small laboratory machines. The field of force in which it rotates is thick with imagi- nary lines of force, through which the swift- moving coils plunge, and in virtue of which one may conceive the induction to be brought about. But though the " lines " are imaginary, and th coils touch nothing as they revolve, it must not be forgotten that they meet with great resistance; unless, indeed, no current is being induced. It cannot be too often repeated that the energy of the current which comes out of a machine is never more, and in fact is al- ways slightly less, than the energy put into it. It must be said, to the credit of those who have contributed to the perfection of the dynamo, that already it is able, in some instances at least, to return, in the form of energy of the electric current, more than ninety per cent, of that which it draws from the steam-engine or other motive power. That this remarkable re- sult has been reached in so short a time is un- questionably due to the fact that the science of electro-dynamics was greatly in advance of the art; so that the latter, by taking advantage of 182 A CENTURY OF ELECTRICITY. the former, could accomplish as much in a few- brief years as has been done in other directions through generations of vague and unsatisfactory empiricism. It is certainly not one of the least of the achievements of the present age, that out of the small copper wheel and steel magnet of Fara- day, producing currents which required for their detection the most delicate and sensitive devices at his command, there has been evolved the gigantic dynamo-machine, requiring an engine of hundreds of horse-power to put it in motion, and producing a current of electricity which re- quires, for its safe and economical transmission, conductors nearly as large as a man's wrist. CHAPTER VII. THE ELECTRIC LIGHT. THE well-established relation between supply and demand would alone establish the fact that the invention and development of electric gen- erators was the result of a wide-spread recogni- tion of their prospective utility. In a general way, electricity is useful to man as a convenient means of transmitting energy from one point to another. In the early stages of its applica- tion, the quantity which could be easily pro- duced was small ; but it possessed the valuable qualification of being easily transmitted to con- siderable distances. It was therefore utilized in operations in which only a very small amount of work was required to be done at the distant point, only enough to produce a vis- ible or audible signal ; and out of its adapta- bility to this end grew the telegraph. One of the forms under which it could be made to appear, or into which it could be trans- muted, was the energy of chemical action ; and it was at an early period made use of in electro- 184 A CENTURY OF ELECTRICITY. lysis, electroplating, etc. That it could also be transformed into heat, and, through heat, into light, was quickly recognized, and the first at- tempts to utilize it in this way have already been described. The amount of energy re- quired, however, in operations of this kind, is so considerable, that the voltaic battery, when built upon a scale necessary to the production of sufficiently powerful currents, was soon found to be unreliable and unsatisfactory, besides be- ing extremely costly. The complete solution of the problem was therefore necessarily deferred, until the devel- opment of the dynamo-electric machine gave promise that the economical production of pow- erful currents of electricity was something to be certainly anticipated. Along with this de- velopment, and, indeed, sometimes in advance of it, was the working- up of the other part of the problem; that is, the determination of the best means of utilizing these currents in the production of light. This part of the sys- tem is generally called the " lamp," the use of the term arising out of an analogy which is obvious. Although of almost infinite variety, electric lamps are easily classified under two species ; in a few rare cases, however, the separation is not quite perfect. The most extensively used THE ELECTRIC LIGHT. 7 is the well-known arc lamp. Of this; ber of varieties is so great that it possible to consider general features, m which are common to all. The electric light, as exhibited by Davy and the early experimenters, was the arc light, al- though nothing deserving the name of lamp then existed. Two pieces of carbon in contact formed a part of the circuit ; and when they were slightly separated, if the current was suffi- ciently powerful, a brilliant light was produced, often taking the form of an arc between the two carbon poles. These poles were consumed rapidly, and the light was suddenly extinguished by the interruption of the current. To provide against this, it was necessary to contrive some device by means of which the carbons could be made to approach each other as they were grad- ually consumed. Foucault was probably the first to undertake this, and in 1844 he produced a sort of hand-regulator ; and at the same time he diminished the necessity for the movement by introducing the use of hard coke, such as is taken from gas-retorts, instead of ordinary char- coal. This was only a few years after the in- vention of the Grove and Bunsen batteries, by means of which fairly powerful currents could be generated and maintained for some hours. The performance of these batteries gave rise to 180 A CENTURY OF ELECTRICITY. the hope that the electric light might be eco- nomically employed, provided a suitable lamp could be devised; and numerous inventors at- tacked the problem from this time on. It was soon recognized that a successful lamp must be automatic ; that is, it must be capable of ad- justing itself, and the control of the force ac- complishing this adjustment, if not the force itself, must originate in the variations of the strength of the current. A brief consideration of the principles in- volved in the production of heat in any part of a conductor conveying a current will be of use at this point. Reference has already been made to the fact that a current, in passing through a short piece of fine wire, will, if the strength of the current be sufficient, raise it to a red heat, or to incandescence, or will fuse it. It has been clearly proved, that, if the same current be passed through two wires, the heat generated in each will be proportional to the resistance which it offers : hence an iron wire becomes hot or fuses where a silver or copper wire of the same dimensions will be scarcely warmed, be- cause the latter offers much less resistance than the former. Again: it will be remembered, that, all other things being equal, the greater the resistance in a circuit, the less the strength of the current will be. The relation is accu- THE ELECTRIC LIGHT. 187 rately and beautifully expressed in the law first announced in 1827 by G. S. Ohm. Guided by Fourier's classic investigation of the flow of heat in conductors, Ohm, from purely theoret- ical considerations, arrived at the conclusion that in any circuit the strength of the current was directly proportional to the electromotive force in the circuit, and inversely proportional to its resistance. In 1841, Joule, in a magnifi- cent experimental research, proved that the heat generated in any part of a circuit was pro- portional to the square of the current strength, and also to the resistance of that part. A consideration of these well-established prin- ciples will at once show that, in the production of light (heat) at any point of a circuit, it will be desirable to lead the current to that point through conductors offering as little resistance as possible, so that useless generation of heat shall not take place ; and to make the resistance at that point just sufficient to produce the re- quired result. Carbon does not rank high as a conductor, and may be made to offer the neces- sary resistance in a convenient form, and, be- sides, it does not fuse when brought to a high temperature. In an arc lamp, if the two car- bons be in contact and a current is passed, they will not, in general, become greatly heated. On account of more or less imperfect contact where 188 A CENTURY OF ELECTRICITY. the two rods touch, considerable resistance is there offered, and they may be heated to red- ness. But if they be now separated slightly, the "arc "will be established. As they are with- drawn from each other, the surface of mutual contact diminishes with great rapidity, and the resistance increases enormously ; so that at once the heat generated becomes very great, the tips of the rods are heated to incandescence, and even, when complete separation takes place, the circuit is still completed through the arc of minute particles which are driven off from the poles, and also by the intensely heated air, which becomes a good conductor when raised to a high temperature. The arc continues, un- til, by the disintegration of the carbons, the interval between them increases, and the re- sistance becomes so great that the current can no longer be made to bridge over the interval. It is therefore necessary to make them ap- proach each other, but not so far as to reduce the resistance below a certain limit. The principles involved in the construction of a regulator lamp will now be readily under- stood. It is only necessary to take advantage of this variation of the current strength, by causing it to bring into play forces which tend to move the carbons to their proper position, and thus to maintain a nearly steady current through the lamp. THE ELECTRIC LIGHT. 189 As already stated, innumerable methods have been devised for doing this. Almost every form of dynamo machine has its accompanying lamp, so as to constitute a complete system. In some of the earlier forms, as in Foucault's first automatic regulator, a train of clock-work was used for producing motion in the carbons, the train or trains (for sometimes one is used to bring together, and another to separate the carbons) being started by the increase or de- crease of the current strength beyond certain limits. This may be accomplished by causing the current, or a part of it, to pass through the coils of an electro-magnet. When the current weakens, the magnet releases its armature, and this action starts the mechanism, which causes the carbons to approach each other: when It becomes stronger, the armature is attracted, and this arrests the motion of the mechanism, and stops the approach of the poles. There are many more successful regulators than Foucault's ; and in most of them the action of the current itself, aided by gravity, is suffi- cient. In many forms two electro-magnets are used, one of which is in circuit with the car- bons, while the other, of much higher resistance than the arc, and therefore taking only a small part of the current, is connected around the car- bons, as a shunt ; so that the current divides, 190 A CENTURY OF ELECTRICITY. the greater part passing through the carbons and one magnet, and the smaller part through the other magnet. These magnets are so ar- ranged as to oppose each other in their ten- dency to change the position of the carbon rods. If the latter are too near together, the resist- ance of the arc is relatively small, and a larger proportion of the current will go through the coil of the magnet in circuit with the carbons. The action of this magnet is to separate the carbons ; but, as this separation continues, the increasing resistance of the arc throws more and more of the current into the coil of the other magnet, which shortly becomes the most effec- tive, the movement is arrested, and, if neces- sary, the carbons are made to approach each other again. It will be observed that this method of connecting and using two magnets possesses the great advantage of making the ad- justment dependent upon variations in resist- ance within the lamp itself, a condition es- sential to the successful use of several lamps on the same circuit. Nearly all lamps in prac- tical use have only one movable carbon. This is the upper one, and its descent is generally caused by its own weight, the mechanism oper- ating to check its downward movement, and also to lift it through small distances. A de- vice is also attached, in virtue of which the THE ELECTRIC LIGHT. 191 breakage of the carbons, or their exhaustion, is not allowed to interfere with the flow of the current through the lamp, an arrange- ment necessary to prevent the failure of one lamp resulting in the stoppage of all others on the circuit. It may be easily attached to the high resistance magnet above described ; so that when, through accident to the carbons, the current through its coils becomes excessive, an armature is drawn up which permanently shifts the main current through the lamp by an inde- pendent route. Mention ought to be made of a novel form of arc light which, a few years ago, attracted much attention. It is known as the " Jabloch- koff candle," and consists of two straight pieces of carbon placed parallel to each other, sepa- rated by a thin layer of some insulating ma- terial. The arc is formed from one carbon to the other across the end of this insulating layer, which is consumed or volatilized as the carbons grow shorter. Thus all mechanism is dispensed with, the carbon rods being at the proper dis- tance throughout their length. This system has* not proved as successful as it promised to be in the beginning. The performance of the arc lamp is being constantly improved : it is extensively in use all over the world, and is an established commer- 192 A CENTURY OF ELECTRICITY. cial success. Its light is extremely brilliant, but, even with the best regulators, still some- what irregular : it therefore finds its place as an illuminator of streets, large public halls, manufacturing establishments, etc., and it can- not yet be said to be suitable for what may be called " domestic " use. Nothing is more natural than that the appear- ance of a platinum wire, heated to a white heat by the passage of a current, should suggest the possibility of a system of electric lighting by incandescence. But, probably owing to the fact that most metals melt before reaching a temperature at which the percentage of lumi- nous radiation is large, and also, doubtless, to the greater brilliancy and novelty of the arc light, progress in this direction was for a time somewhat slow. The first serious attempt to construct an incandescence lamp appears to have been made about the year 1845, by J. W. Starr of Cincinnati, O. Mr. Starr carried his invention to Europe, hoping to receive recogni- tion there. After meeting with some encour- agement and securing some assistance, he started to return home ; but, unfortunately, he died at sea, at the age of twenty-five years. He seems to have anticipated many of the recent investigations in this direction, and his scheme included the production of the necessary electric THE ELECTRIC LIGHT. 193 currents by improvements in induction ma- chines. His lamp consisted of a piece of car- bon, heated to incandescence, in a Torricellian Vacuum. He was followed by numerous in- ventors, who experimented with different sub- stances, as Starr had done before them, but who finally settled upon carbon as the most suitable. At present there are a number of well-known incandescence lamps, differing from each other rather in details of construction, and in the manner of preparing the carbon, than in any essential particular. The first real impetus in this direction was given by the experiments of Mr. Edison. With characteristic industry and enthusiasm, when he first attacked the problem, now nearly ten years ago, he made an exhaustive examination of the adaptability of various materials, during the course of which he discovered several inter- esting and before unrecognized properties of metals. But the results of his investigations led him to adopt a carbon filament, and a par- ticular form of it, which he prepares from a fine quality of bamboo. Strips of this are cut to the proper size and form, and are then " car- bonized " by being exposed to intense heat while confined between two plates of iron or nickel. After this process, the extremities of the filaments are electroplated with copper, so 13 194 A CENTURY OF ELECTRICITY. that a proper junction may be formed with the platinum wires by means of which they are sealed in the well-known pear-shaped glass chamber. These chambers are exhausted by the use of mercury pumps, and the necessary attachments for connecting the lamps in the circuit are added. Another form largely in use is the Maxim lamp, the filament of which is cut out of card- board by means of a die of the proper form, and afterward carbonized. The S wan incandescence lamp is extensively used in this country in con- nection with the Brush system of electric light- ing. Its filament is of cotton thread, which receives a preliminary " parchmentizing " by being immersed in a solution of sulphuric acid and water, after which it is carbonized. Several other forms of incandescence lamps are candidates for public favor, and generally they do not differ in any essential feature from those described. At first the most serious diffi- culty with lamps of this species was their ten- dency to break down through the disintegration and volatilization of the carbon filaments, af- ter long exposure to the high temperatures de- manded. This has been largely overcome, and there is little trouble to-day in obtaining lamps of long life. The incandescence system offers many ad- THE ELECTRIC LIGHT. 195 vantages over arc lighting, the most notable being the ease of distribution of the light, and its almost absolute steadiness. The most per- fect arrangements for its introduction and use have been devised, and its superiority over gas or other illuminants is admitted by all. Indeed, to one familiar with the present condition of the art, the surprise is not that the incandes- cence lamp can be used, but that it is not used much more extensively than it is. If the peo- ple were a little less conservative in the matter, and electric lighting companies less anxious to secure extravagant dividends upon their stock, electricity would shortly take the place to which it is justly entitled, as the most perfect illuminant at present known. CHAPTER VIII. THE TRANSMISSION OF ENERGY BY MEANS OF ELECTRICITY. THE ELECTRIC MOTOR. IN 1821, Faraday, then an assistant to Sir Humphry Davy in the Royal Institution, suc- ceeded in producing continuous rotation of a magnet around a wire through which a cur- rent was passing. This first elec- tro-magnetic rotation was accom- plished by immersing a small steel magnet in mercury, one end being weighted with platinum so that it floated in a position nearly ver- tical ; a current was passed down- ward through a wire which was plunged in the mercury, and about this wire the magnet revolved as long as the Current flowed. Faraday's continuous rotation By a reversal of this, of a magnet about a current that is, by using a fixed magnet and a movable conductor, Barlow devised what is known gen- THE TRANSMISSION OF ENERGY. 197 erally as " Barlow's Wheel," the form of which is almost identical with that of Faraday's first dynamo, already described. Instead of turning this wheel, and thus gener- ating a current of electricity, a current is led into it from any convenient source, entering at the axis, and leaving at the circumference, or vice versa. If the wheel or disk be properly Barlow's Wheel. placed between the poles of a magnet, as in Faraday's experiment, rotation will be set up, the direction of which will depend upon the di- rection of the current and the position of the magnetic poles. The current which causes mo- tion in this simple machine may come from an- other precisely like it, except that in it the disk must be turned by hand or by some other source of mechanical energy. The experiment 198 A CENTURY OF ELECTRICITY. is useful as showing the interchangeability of the dynamo, which generates a current of elec- tricity, and the motor, which converts that cur- rent back into what is generally known as " me- chanical energy," the motion of visible masses of matter. Nearly all forms of telegraph receiving instru- ments are machines in which this conversion is made ; but in few, if any, of them is continuous rotation produced. As this is almost necessary in any machine constructed for the general purpose of " doing work," the name " electro- motor " is usually understood to belong only to such arrangements as produce rotation. Nu- merous ingenious and costly experiments were made in the early days of applied electricity, following Faraday's discovery of electro-mag- netic rotations. In a certain sense, they were all failures, although they were not without their value to the inventor and the student of electricity. In the first place, the methods then utilized for generating powerful currents of electricity were expensive and troublesome ; and, in the second place, ignorance of the real principles involved led to a faulty construction of the motors used. In spite of this, machines capable of doing considerable work were built. In this country Henry and Page did something to further the THE TRANSMISSION OF ENERGY. 199 solution of the problem, the latter, especially, devoting much time and labor to it. After much experiment, he built a motor which did work equivalent to five or six horse power, and with which he ran a circular saw and a lathe. He also applied it with considerable success to the movement of cars on railway tracks. In Europe, Davidson, Froment, and Jacobi worked upon motors, and the last constructed one which attracted much attention at the time, and was possessed of much real merit. It was large enough to drive a boat carrying a dozen people. A large battery of more than one hun- dred cells was required to run the machine, and the power produced was exceedingly ex- pensive. All of these machines are now interesting from an historical stand-point; but, for reasons al- ready given, they were not successful. The im- provement of the dynamo-electric machine, and the recognition of its " reversibility " (by which is meant that a dynamo will run as a motor if a current is supplied), revived interest in the sub- ject, and within the past few years many valua- ble and important advances have been made. Although the construction of motors cannot yet be said to have passed the experimental stage, there are many purposes for which they are es- pecially adapted, and for which they are des- 200 A CENTURY OF ELECTRICITY. tined soon to come into general use. When water-power may be obtained at little cost, a current of electricity may be generated, and conveyed to a neighboring point, and there util- ized for running light machinery so as to be really more economical than a steam-engine. Electricity is not likely to be called in, except where power is to be transmitted some distance ; and it must be remembered, that, even when the dynamo and motor are constructed on prin- ciples theoretically perfect, the latter must al- ways give up somewhat less energy than it con- sumes, so that, in the present state of the art, not a very high percentage of efficiency is to be expected. Considered as a question of cost per horse-power, the movement of the armature of a relay in a telegraph-office is enormously ex- pensive ; but questions of convenience and avail- ability often override that of cost. The ease with which the electric motor can be manipu- lated and controlled, the freedom from danger by fire, and the fact that it can readily be in- troduced wherever a current from a dynamo is at hand, are strong arguments in its favor, and give it an availability far beyond that of the steam-engine. Extensive experiments have been made in this country and in Europe in utilizing the motor for driving street-cars and cars upon short THE TRANSMISSION OF ENERGY. 201 railway lines, and with results that give prom- ise of ultimate success. Several short electric- railway lines are in operation in Europe, and one or two lines for street travel in this coun- try. In short, the motor is now in a stage of its development somewhat similar to that of the electric light ten years ago, and there is reason to believe that it will yet overtake its more brilliant forerunner. CHAPTER IX. THE TELEPHONE. THE name of Dr. Charles Page has already occurred several times in these pages. It was his fortune to be a pioneer in several of the most important developments of electricity ; but none of his discoveries were more novel, or led to more important and interesting results, than a curious observation which he made in 1837. It was that a bar of iron could be made to emit sounds when rapidly magnetized and demagnet- ized, and that the pitch of the sound depended on the rapidity with which the changes were made. This was the first recognition of the possibility of producing musical tones by elec- tricity. Thirty-nine years later there was exhibited, in the Centennial Exhibition at Philadelphia, a small instrument, which the most distinguished electrician of the time pronounced to be the "wonder of wonders in electric telegraphy." By its use, not only was sound produced by means of electricity, but a sound at one end of an electric circuit was reproduced at the other, THE TELEPHONE. 203 and with such fidelity that human speech at one end was faithfully repeated at the other in every respect but in loudness. The instrument was exhibited by Alexander Graham Bell, who had patented the invention only a few months before. As exhibited at Philadelphia, it was far from being satisfactory in its operation, but it proved that the problem of " talking by tele- graph " had been solved. The transmission 1 of musical tones by means of electricity had been previously accomplished, and was an operation well understood. Philip Reiss, in Germany, had, many years before, made use of Page's device as a receiver, and had contrived a transmitter very similar in appear- ance to several in use to-day. Other methods of accomplishing the same result had been con- trived, and doubtless more than one inventor had the transmission of articulate speech in mind. Indeed, another American, Mr. Elisha Gray, was at work in this direction ; and, by a curious coincidence, Mr. Gray deposited his specifications and drawings for a speaking tele- phone in the United States Patent Office, in the form of a caveat, on the 14th of February, 1876 ; and on the same day Mr. Bell filed his applica- 1 The word " transmission " is here and afterward used to avoid circumlocution. In reality, sounds are not transmitted by the electric telephone, but simply reproduced at the other end of the line. 204 A CENTURY OF ELECTRICITY. tion for a patent, the latter being received a few hours earlier than the former. The coinci- dence becomes more interesting when it is re- membered that it was also on the 14th of Feb- ruary (1867) that Wheatstone and Siemens simultaneously presented to the Royal Society their independent discovery of the important fact that dynamo-electric machines could be constructed and operated without the use of permanent magnets. It would seem desirable for discoverers and inventors who have anything of importance to communicate to the public on this particular day of the year, to lose no time in its announcement. Since the successful introduction of the tele- phone, innumerable claimants for priority in its invention have appeared. It is claimed that Reiss transmitted speech with his device, and other inventors claim to have succeeded in ac- complishing the same thing previous to the date of Bell's patent. The question has long been in litigation, and it will perhaps sometime be settled by the highest judicial tribunal. It can- not be denied, however, that Bell's invention was the immediate cause of the development of speech transmission by means of electricity. The Bell telephone, as originally produced, was an extremely interesting combination of a dynamo-electric machine and a motor, the two THE TELEPHONE. 205 Bell's transmitter as exhibited in Philadelphia in 1876. Bell's receiver as exhibited in Philadelphia in 1876. 206 A CENTURY OF ELECTRICITY. being identical in form and construction ; and, as such, its operation will be readily understood. When a sound is produced, energy is expended in its production : this energy cannot be de- stroyed, although it can seldom be recovered. Ordinarily, when a word is spoken, the energy necessary to or consumed in its utterance fjrst appears as a series of waves of compression and rarefaction in the air, where most of it is finally transformed into heat. It is possible, however, to transmute at least a part of this energy into other forms. Solid bodies may be made to vi- brate by the sound of the human voice, and by a suitable contrivance it may be made to do work in running a machine and overcoming other resistances, always, of course, of no great magnitude. In the telephone the sound of the voice is made to do work ; this is converted into the energy of an electric current ; and this, in turn, is reconverted into mechanical energy, re- sulting in sound. The form of the ordinary Bell telephone receiver is so well known as hardly to require description. Internally it consists of a small cylinder of steel which is permanently magnetized, and around one end of which is a coil of fine wire. Just in front of this end of the magnet, but not quite in contact with it, is a thin circular mem- brane or disk of iron supported at its circum- THE TELEPHONE. 207 ference. Originally the transmitter was pre- cisely like this receiver. One end of the fine wire coil of each is joined to the line connecting the two points, and the other end is connected with the earth. Now, if a sound be produced near the thin disk of the transmitting instrument, it will be made to vibrate. Although these vibrations Sectional view of Bell's receiver as now generally used. are exceedingly minute, they are sufficient to produce changes in the magnetic field in which the coil of fine wire lies, as in Page's induc- tion machine ; and, as explained in a previous chapter, any change in the nature of this field will produce induced currents in the wire coil. These currents will be transmitted through the line, and, flowing through the coil surrounding the pole of the receiving magnet, will produce variations in the intensity of its magnetization. Just what goes on at the receiving end has been a subject of considerable dispute, and the opera- 208 A CENTURY OF ELECTRICITY. tion there is unquestionably a complex one. Since sound is produced, there must be vibrations of parts of the receiver ; and these must vary in rapidity and form, along with the variations in rapidity and form of the electric waves gener- ated by the action of the transmitting instru- ment. There is doubtless a vibration of the thin plate of the receiver, due to variations in the strength of the pole of the magnet near which it is placed ; but talking can be heard when the metal disk is absent, so a part of the result must be attributed to what is called " mo- lecular " vibration, as in Page's original device for producing sound. But the wonder of it all is, that the trans- mitting disk takes on, and the receiving appa- ratus reproduces, all the various phases and forms of motion impressed upon the air by the voice, and essential to reproduction of that voice in articulate speech. Nothing like it in simplicity of construction, combined with com- plexity of operation, is to be found in any other human contrivance. The electric currents thus generated and transmitted are necessarily extremely minute. The amplitude of vibration of the disk has been estimated to be only a small fraction of the length of a wave of yellow light, of which there are about forty thousand to the inch. It has THE TELEPHONE. also been determined that the receiver repro- duces not more than one ten-thousandth part of the " volume of sound " received by the trans- mitter. As a " motor," it must be as having extremely low efficiency, although, on the whole, very effective. The telephone as at first used, and as just described, was much less satisfactory in its per- formance than it is at present. Its working has been vastly improved by the use of other forms of transmitting instruments, by means of which variations of current strength of much greater intensity are transmitted over the line, while still retaining the characteristics neces- sary for the reproduction of speech. The transmitters generally in use at present depend upon a curious and important discovery made by Hughes in 1878. It consisted essen- tially in the fact, that, if a piece of carbon be allowed to rest lightly upon another, and an electric current be passed from one to the other in a circuit in which there is a Bell telephone receiver, the latter will respond to the faintest sounds in the vicinity of the carbons. Various other substances (imperfect conductors are gen- erally better) may be used instead of carbon, and the arrangement is called a " microphone," Its operation is due to the fact that imperfect contact exists where the two carbons touch 14 210 A CENTURY OF ELECTRICITY. each other ; and the slightest disturbance is sufficient to alter the extent of that contact, and thus to vary the resistance of the circuit. In accordance with the law of Ohm, the current varies with the resistance ; and this variation of the current acts, as already explained, to pro- duce sound from the receiver. Various forms of transmitters employ the principle of Hughes's microphone. An important modification con- sists in the arrangement of the transmitter in a local circuit rather than in the line. The re- sult of this is that much greater variations of current strength can be produced. The trans- mitter includes a much larger part of the re- sistance of the local circuit than it would of that of the line ; so that, for a given alteration of its own resistance, that of the circuit is altered by a much larger percentage. The local cir- cuit includes the primary wire of a small in- duction coil, the secondary or outer coil of which is connected to the receiving instrument through the line. The operation of the system is somewhat as follows : the sound-waves fall- ing on a membrane or disk similar to that in the receiving instrument, sets it in motion in such a way as to produce variation of pressure at the microphone contact, generally placed in the rear of the disk, and corresponding varia- tions in the strength of the local current re- THE TELEPHONE. 211 suit. Changes in the strength of a current flowing in the vicinity of a coil are equivalent to movements of that current towards or away from the coil ; and, as shown by Faraday, in- duced currents must traverse the coil. These induced currents go into the line, and do their work in the receiver at the distant end, precisely as in the original form of the instrument. The introduction of the microphone transmitter with the local circuit and induction coil has greatly strengthened the telephone, and rendered its use much more easy. Mr. Edison devised a transmitter in which a small disk or button of soft carbon, prepared from lampblack is used as the element of varia- ble resistance, the movements of the membrane modifying the pressure which it normally exerts upon the carbon. Owing to the excessive sen- sitiveness of the resistance of this form of car- bon to variations of pressure, it is admirably adapted to this use. A large number of trans- mitting instruments have been invented ; but not many have come into use, except those which depend for their operation upon the principle of the microphone. A few devices for telephone receivers other than Bell's have been invented, one or two of which are novel and original, especially those of Edison and Dolbear. The latter may be called an " electro- 212 A CENTURY OF ELECTRICITY. static " telephone, as it contains no permanent magnet and no helix of wire. In fact, it de- pends upon the principle of attraction and re- pulsion between two electrified bodies. Prac- tically, it is an extremely satisfactory receiver. Difficulties of a legal character have prevented the introduction of these instruments up to the present time. CHAPTER X. . SECONDARY AND THERMO-ELECTRIC BAT- TERIES. ONLY a few years ago a good deal of commo- tion was created, in both the scientific and the unscientific world, by the appearance of what has been variously called the " storage bat- tery," the " secondary battery," and the " elec- tric accumulator." Some method of econom- ically storing or accumulating energy so as to be easily transportable has long been the hope and aspiration of every intelligent mechanical engineer. For a time the belief that the prob- lem was solved through the use of electricity was wide spread ; and innumerable stock com- panies, representing a fabulous amount of capi- tal, were quickly organized for the purpose of developing this new industry. The expectations of the promoters of these schemes have not been realized, but a good deal of valuable information concerning the be- havior of secondary batteries has been accumu- lated ; at an expense far greater, however, than would have been necessary, had the whole sub- 214 A CENTURY OF ELECTRICITY. ject received in the beginning an exhaustive examination at the hands of a competent com- mission under government authority and at government expense. The vast importance of the questions involved would seem to justify such a course. The first secondary or storage battery ever made has already been referred to. It was con- structed by Ritter in 1803, and its operation has already been explained. The subject was revived in 1843 by Grove, who constructed a gas-battery to illustrate the operation of polari- zation ; and again by Gaston Plante* in 1859, who went to work systematically to see what could be done in the way of storing electricity. He discovered, after trying many metals, that electrodes of lead, immersed in dilute sulphuric acid, were more suitable than anything else for the production of polarization effects. After passing a current for some hours, from a couple 'of cells of Bun sen's battery through a cell com- posed of two large sheets of lead immersed in this liquid, he was able to take from it currents of great strength and considerable duration : in other words, large quantities of electricity could be received back from the cell. His large, cells were prepared b}^ placing one sheet of lead upon another, preventing contact by the use of rubber bands, and then rolling the whole SECONDARY BATTERIES. 215 into a compact cylinder. This, when immersed in a vessel of dilute acid, could be charged by means of a current from a dynamo or from an ordinary battery. The action which took place during the charging was at first simply the lib- eration of the gases oxygen and hydrogen, as in Ritter's battery; but these gases combined with the lead, altering its appearance, causing it to have a spongy texture, and covering one of the electrodes with a film of peroxide of lead. It was found that if the charging was repeated first in one direction and then in the other, for some days, a great improvement in the charac- ter of the cell was brought about, and this op- eration was called " forming " the cell. Plante's work did not command great atten- tion not, indeed, as much as it deserved ; and it was a modification of his method devised by Faure which created the first excitement in financial circles. Faure's improvement con- sisted in coating the lead plates in the beginning with red lead, by which means the operation of forming was avoided, and the capacity of the cells was greatly increased. The introduction of Faure's battery was quickly followed by a great number of proposed secondary or storage batteries, in all or nearly all of which electrodes consisting of lead plates are made use of. They are therefore extremely heavy, and are difficult 216 A CENTURY OF ELECTRICITY. to transport. As a means of conveying energy from one point to another, they present only advantages of availability. Four cells brought to London in 1881 weighed seventy-five pounds, and were said to be charged with one million foot-pounds of energy, equal to the work of a horse for half an hour ; but this is not more than the energy stored in an ounce or two of coal. The energy of the battery is, however, much more easily drawn upon and utilized than that of the coal. It must not be imagined that electricity is actually stored up in one of the cells ; what is stored is the energy of the electric current, which disappears in producing certain chem- ical changes in the cells, a large part of this energy being capable of reproduction as an electric current. A form of battery proposed by Messrs. Thomson & Houston illustrates the principles of their operation. The ordinary gravity-battery contains, as is well known, a plate of copper at the bottom of the cell, and over this is a solution of sulphate of copper which is blue in color. Above this is a clear solution of sulphate of zinc, in which the zinc electrode rests. If the zinc and copper poles be joined by a short, thick wire, a current of electricity passes, metallic copper is deposited on the copper plate, and zinc is dissolved in the SECONDARY BATTERIES. 217 solution. If this operation be allowed to con- tinue, and no fresh crystals of sulphate of cop- per be added, the blue color will finally disap- pear, the copper having all been deposited in a metallic form. Now pass a current from a dy- namo, or from another battery of several cells, in a direction opposite to that of the cell in its original condition, and the " recovery " of the cell will take place ; that is to say, zinc will be deposited on the zinc plate and a solution of copper sulphate will be formed. When this is completed, the cell may be used as an indepen- dent source of electricity, as before. Messrs. Thomson & Houston do not, of course, make use of the cell in precisely this form, but begin with a solution of zinc sulphate in which is immersed a copper plate and also a carbon plate, the latter taking the place of the zinc electrode in the arrangement described above. When a current from a dynamo is passed into a cell or series of cells of this kind, copper is dissolved from the copper plate, and zinc is de- posited on the carbon ; so that at the end of the operation the cell is itself capable of generating a current, and in so doing it falls back to its initial condition. The progress of charging can then be repeated. There are innumerable ways in which storage batteries would be immediately utilized, if they 218 A CENTURY OF ELECTRICITY. were found to be trustworthy. As an adjunct to electric-lighting plants, they would be espe- cially valuable, as the energy consumed at night could be accumulated during the day. Many earnest attempts to perfect them have been made, and it cannot be denied that they have been afforded a fair trial. Although they are now in use to some extent for special purposes, it must be admitted that much remains to be done in the way of improvement, in order to establish a claim upon the confidence of the public. Their want of durability is their most serious fault. For a time, and in skilled hands, they may behave admirably, but they may un- expectedly break down without apparent rea- son. Two principal methods of generating elec- tricity have thus far been considered, that of the battery, due originally to Volta; and that of the induction machine, due originally to Faraday. No account, however brief, of the growth of the science of electricity, or the de- velopment of the art, would be complete with- out reference to a third method, which was dis- covered by Seebeck of Berlin in 1821. One of the earliest contributions growing out of Oer- sted's discovery, it has since played a most im- portant part as affording an instrument of great value for purposes of research, as well as in THERMO-BATTERIES. 219 raising questions of great theoretical interest and importance. Seebeck's discovery was that if a circuit be formed of two wires of different metals, or even of the same metal in different physical conditions ; and, if one of the junc- tions of these two wires be at a higher tempera- ture than the other, a current of electricity will be established in the circuit. By selecting metals best adapted for the purpose, and by combining them after the manner of a battery arranged in "series," currents of considerable strength may be produced in circuits of low re- sistance. Combined with a very sensitive gal- vanometer, a thermopile thus constructed fur- nishes a method for determining the minutest differences of temperature. Numerous attempts have been made in recent years to construct thermo-electric batteries of such dimensions as to produce currents of electricity comparable with those produced from an ordinary battery or dynamo-electric machine. By using as many as six thousand elements, a current sufficient to maintain an electric light was generated in Paris in 1879. The efficiency of these batteries is extremely low, not more than four or five per cent, of the heat applied being reproduced in the form of electric energy. In spite of this, they would be widely used were it not for the fact, that, 220 A CENTURY OF ELECTRICITY. like the storage battery, they are liable to de- teriorate. This difficulty does not seem insur- mountable, and the thermo-battery may yet be extensively employed in work for which it is especially adapted. By its use, heat is trans- formed directly into the energy of electric cur- rents, without the interposition of a steam- engine ; and in spite of its low efficiency, in connection with a fairly perfect system of elec- tric storage, it may occupy a wide field of use- fulness in rescuing vast quantities of waste heat from useless dissipation. CHAPTER XI. CONCLUSION. FEOM the rubbing of a bit of amber, to the telegraph, the telephone, the electric light, and the electric railway, is a long distance in more than one sense. That it has been traversed by the ingenuity of man, and mostly during the last hundred years, goes far to justify the most ex- travagant praise which, even by poetical license, man has bestowed upon himself. The attempt has been made in these pages to give a some- what connected account of this wonderful prog- ress, and especially to bring into prominence the few principal points from which these successful attacks upon the mysteries of nature have been made. Within this hundred years there have been three notable discoveries in electricity, around which all others cluster, and from which they all have grown. These three have immor- talized the names of Galvani and Volta, Oer- sted and Ampere, and Faraday. It has been impossible to consider, indeed it would have been impossible in these pages to 222 A CENTURY OF ELECTRICITY. catalogue, all of the contributions to the science, or the devices of ingenious inventors who have provided for its practical application. It has only been attempted to develop the fundamen- tal principles which underlie these applications, and to illustrate them in the explanation and discussion of some that are distinguished for their importance, or novelty and promise. It might naturally be expected that some- thing in the way of a forecast of the possible future of this department of human knowledge would be undertaken. However tempting that task might appear, the usefulness of its per- formance might seriously be questioned. The history of prophecy in the past would seem to prove that it is at least dangerous to predict what can not be accomplished in the future. It may be well, however, to put a check upon the extravagance of the imagination in matters of this sort. The enthusiastic should not for- get that the greatest generalization of modern science has for its principal object the establish- ment of limitations. The doctrine of the con- servation of energy has done for the philosopher and for intelligent inventors what the doctrine of conservation of matter, as established by the early chemists, did for the seeker after the philosopher's stone. It has shown that no en- ergy can be obtained from a place where it is CONCLUSION. 223 not, and its recognition guides alike the student of science and the so-called "practical " man in their researches and inventions. More than ever before in the history of science and inven- tion, it is safe now to say what is possible and what is impossible. No one would claim for a moment that during the next five hundred years the accumulated stock of knowledge of geogra- phy will increase as it has during the last five hundred. The great continents have been dis- covered and explored ; the great rivers and mountain-ranges have been traced, measured, and mapped ; and, although much still remains unknown, future progress must be mostly in matters of detail. In the same way it may safely be affirmed that in electricity the past hundred years is not likely to be duplicated in the next, at least as to great, original, and far- reaching discoveries, or novel and almost revo- lutionary applications. Nevertheless, in passing over the ground the reader must have observed many avenues opening into the region of the possible which are ^et unexplored. Although more than fifty years have elapsed since Fara- day's discovery of induction, the full benefits to be derived from its development are not yet realized. It can be fairly expected that great advances will be made in the near future, and particularly 224 A CENTURY OF ELECTRICITY. as progress in one department of science is so linked with that in others. While growth may in general be anticipated along the lines al- ready laid out, the discovery of a new element, possessing unusual and remarkable electric properties, or the production of a new alloy with similar characteristics, might at once open the way to a host of practical applications of elec- tricity not now dreamed of. It is also to be ex- pected that many devices at present more or less imperfect and unsatisfactory will be per- fected and completed in the not distant future. The telephone and the electric light are the first applications of electricity to claim admis- sion to every home, the special training neces- sary to the manipulation of the telegraph naturally restricting its use. The telephone, ten years after its first introduction, is more satisfactory in its performance, and far more ex- tensively used, than was the telegraph in the corresponding period of its history. In every house, in cities and towns, there must be a supply cf electricity to be drawn upon, as water is at present. Then, in addition to its use as a source of light, much of the labor of the household will be performed by means of small electric motors. The economical storage of this form of energy must soon be accom- plished, and this will greatly enlarge the field CONCLUSION. of its usefulness. The successful transfer energy of falling water, through metallic ductors to distant points, is one of the results' which can and will be reached. " Seeing " by electricity has been much talked of: some schemes for its accomplishment have been suggested, and the operation is one which can- not be classed with the impossible. These and many other things will doubtless come in time, along with other useful applications of the elec- tric current not now thought of. From an origin remote and uncertain, elec- tricity came down to modern times, and for more than two thousand years almost nothing was known of the laws which control it. In 1786 it appeared to Galvani in the guise of a stranger, and as such received a hearty welcome. Its identity was soon established, but it proved to be much more amenable to treatment in the new character than in the old. During the past hundred years it has pushed its way to the front, and made itself indispensable to the comfort and happiness of man, to a degree little less than marvellous. It now enters upon the second centenary of its new life, during which there is every reason to believe that its useful- ness will be vastly extended, although its growth and development may be less phenom- enal. 15 INDEX. , 27. Alibert, account of Galvani's discov- ery, 32, 33. Amber, 13. Ampere, 75, 76. Arago, production of magnetism by electricity, 79. Arc, electric, formation of, 188. Armature, Siemens', 169. Gramme, 171. Bacon, 15. Barlow's wheel, 197. conclusion that the electric telegraph was impossible, 90. Battery, electrical, 10. or pile of Volta, 37-39. improvement of, 49. action of, 49. electro-motive force of, 51. current produced by, 52. resistance of, 54. ideal, 54, 55. polarization of, 56, 57. Daniell's, 58, 59. Grove's, Bunsen's, Leclanchd, and Gravity, 61, 62. secondary, 57. Bayle, Robert, 16. Beatification by means of electric- ity, 29. Bell, A.G. , multiplex telegraphy, 129. Buffon, 23. Bumpers, electrified, 10. Cable, Atlantic, 141-144. submarine, 138-150. Candle, Jablochkotf, 191. Cavendish, experiments on conduct- ing powers, 27. Cell, Voltaic, contact theory of, 36. Chemical theory of Voltaic cell, Clarke, improvement in dynamo, 156. Oollinson, 21, 22, 25. Color, effect of, on electrical prop- erties, 19. Compagnie de 1' Alliance, 167. Condenser used with submarine ca- bles, 147. Contact theory of the Voltaic cell, 36. Copley Medal bestowed upon Frank- Coulomb, law of attraction and re- pulsion, 27, 28. Cuneus, 21. Current, detection and measurement of, 84-86. strength of, from battery, 52. depends on resistance and electro-motive force, 53. D'Alibard first draws electricity from the clouds, 25. Davy, Sir Humphry, 41-47. Decomposition of water, 40. Delany, multiplex telegraph, 133. De Lor carries out Franklin's sug- gestion in Paris, 25. Dolbear, telephone receiver, 211. Dufay, 18. Duplex telegraphy, 112-124. Dynamo, the first, 158. devised by Pixii, 163. improved by Saxton, 165. Clarke, 165. Page, 165. constructed by Nollet, 166. the Compagnie de 1'Al- liance. 167. Wilde's double, 170. self-exciting, 170. review of its construction and operation, 181. Dynamos of Brush, Weston, Thom- son and Houston, and Edison, 179, 180. Edison, quadruplex system, 126. telephone transmitter, 211. receiver, 211. 228 INDEX. Electric telegraph, 65. spark, first noted, 16. Electric virtue, 17. Electricity, animal, 35. name first used by Gilbert, 16. classified as vitreus and resin- ous, 19. transmitted through long wire, 21. drawn from the clouds, 25. curative powers of, 14, 29, 30. healing power of, 46, 47. light produced by means of ,46. its influence on a magnet dis- covered, 75 used to produce magnetism, 79. generated by heat, 218. production of musical tones by, 202. velocity of, 93-95. Electro-dynamics, 75. Electrolysis, Nicholson and Carlisle. 40. Hisinger and Berzelius, 42. Erman of Berlin, 42. Dr. Priestley, 43. Davy, 42-44. Electro-magnet, constructed by Sturgeon, 79. improved by Henry, 81. Electro-magnetic induction, funda- mental principles of, 159, 160. Electro-metallurgy, 64. Electro-motive force, seat of, in Vol- taic cell, 37. of battery, 51, 52. produced by motion of con- ductor, 175. Electro-motor, 198. advantages of, 200. Electroscope, 16, 39. Electrotyping, 64. Faraday, Michael, 152. discovers induction, 157 explanation of Arago's rota- tions, 157. Faraday's ring, 155. first dynamo, 158. device for continuous rota- tion, 196. Farmer, Moses G-., duplex system, 112. Faure's secondary battery, 215. Field, Cyrus W., 10. Atlantic cable, 141. Field of force, 173, 174. Foucault's regulator, 189. Franklin, 21-24. Franklin, letter to Collinson, 9. original plan for drawing elec- tricity from the clouds, 25. experiment with kite, 27. Galvani, 35, 36. observation upon frogs' legs, 32,33. Galvanism, 35. Galvanometer, 85, 86. Galvanoscope, 84. Gherardi, editor of Galvani's works. 33. Gilbert, William, 15. Gintl, system of duplex telegraphy, Gramme, armature for dynamos, Gray, Elisha, multiplex telegraphy, 129. Gray, Stephen, 17. Gutta percha, 140. Hawksbee, Francis, 17. Heat in any part of a circuit, 187. Henry, Joseph, 81. improvements in construction of electro-magnets, 81-84. Hisinger and Berzelius, observation of, 42. Holmes, dynamo for use in light- houses, 168. Hughes, microphone, 209. Jablochkoff' s candle, 191. Kinnersley, 19. Kleist, 21. Lamp, electric, 63. arc, 184-192. incandescence, 192. of Starr, Edison, and Maxim, 192-194. Lenz, laws of induction, 162. Leyden jar, 20. charged by Voltaic battery, 47. Lightning identified with electricity, 24,25. Lines of force, 172-174. Magnet, 68. mysterious properties of, 69. Magnetic needle, 67. influenced by an electric cur- rent, 75. Magnetism, 70, 71. relationship to electricity, 72. produced by means of elec- tricity, 79. INDEX. 229 Microphone, 209. Morse, Professor S. F. B., 99. Morse code, 98. Motor, electric, 198. Multiplex telegraphy, 125-138. Muschenbroeck, 21. Newton, experiments in electricity, Nicholson and Carlisle, decomposi- tion of water, 40. Nollet, Abbe", investigation of cura- tive powers of electricity, 29, 30. Nollet, construction of first large dynamo, 166. Oersted, Jean-Christian, 72. Oersted's experiment, 74, 75. Ohm's law, 187. Pacinotti, dynamo, 171. Page, Charles G., inprovement in dynamo, 155. production of musical tones by electricity, 202. Pixii, dynamo, 163. Plante", secondary battery, 214. Polarization of battery, 56, 57. how prevented, 59-61. Potassium discovered by Davy, 44. Regulator for arc lamps, 189-191. Resistance in a circuit, 53. of a battery, 54. Ring armature, 176. Saxton, improvement in dynamo, 165. Schweigger, galvanic multiplier, 81, 83. Secondary battery, Ritter's, 57. batteries, 213-218. Seebeck, thermo-electricity, 218. Seeing by electricity, 225. Siemens, Dr. Werner, armature in- vented by, 168. Signals, Henry's arrangement for receiving, 97. Silk, insulation by, observed by Gray, 18. Siphon recorder, 148. Skeleton Gramme armature, 177. Sound, reading by, 106. Starr, J. W., incandescence lamp, 192. Stearns, improvements in duplex telegraphy, 113. Storage battery, 213-218. Sturgeon's electro-magnet, 79. Telegraph, electric, 65. early history of, 88, 89. not exclusively an American invention, 90. needle, 91. of Schilling, 91. first commercial establish- ment of, 92. of Gauss and Weber, 92. of Steinheil, 92. contributions of Sir Charles Wheatstone, 95. first line on the Great West- ern Railway, 96. American, or Morse system, 99. development and essen- tial features of, 103- 106. chemical, 107-109. duplex, 112-124. multiplex, 125-137. harmonic, 135. Telephone, Bell's, 203. Reiss's, 203. Gray's caveat, 203. operation of, 206-211. transmitter, microphone, 210. Edison's, 211. receiver, Dolbear's, 211. Thermo-battery, 218, 219. Thermo-electricity, 218, 219. Thomson, Sir William, on the Atlan- tic cable, 141. siphon recorder, 148. Thomson and Houston, secondary battery, 216. dynamo, 179, 180. Tube, electrical, 22. Tyndall, 156. Velocity of electricity^ 93-95. Volta, 35, 36. Voltaic cell, chemical theory of, 37. Von Guericke, Otto, 16. Water, decomposition of, 40, 41. Watson, Sir William, 21, 23. Wheatstone, Sir Charles, 93. experiments on velocity of electricity, 94. contributions to the tele- graph, 95, 96. Wollaston, 78. A STANDARD WORK. A Book for all Persons who Make, Buy, Sell, Use, or Want to Know anything about Tools or Machines, or are Interested in the History and Development of Mechanic Art. KNIGHT'S AMERICAN MECHANICAL DICTIONARY. A descriptive Word-Book of Tools, Instruments, Ma- chines, Chemical and Mechanical Processes; Civil, Mechanical, Railway, Hydraulic, and Military Engi- neering ; a History of Inventions ; General Techno- logical Vocabulary ; and Digest of Mechanical Appli- ances in Science and the Industrial and Fine Arts. BY EDWARD H. KNIGHT, A. M., LL. D. Civil and Mechanical Engineer. From the Scientific American. 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