ELEMENTARY ELECTRO-TECHNICAL SERIES 'ALTERNATING ELECTRIC CURRENTS EDWIN J. HOUSTON, Ph.D., (Princeton) AND A. E. KENNELLY, So. D. . NEW YOKE THE W. J. JOHNSTON COMPANY 253 Broadway 1895 rK> yVS COPYEIGHT, 1895, BY THE W. J. JOHNSTON COMPANY PREFACE IN preparing this little volume on Alter- nating Electric Currents, as one of a series entitled the Elementary Electro -Technical Series, the authoi^ ^believe that they are meeting a demand, that exists on the part of the general public, for reliable information respecting such matters in electrical knowledge as can be readily un- derstood by those not specially trained in electro -technics. The subject of alternating- electric cur- rents is, to-day, perhaps, the most promi- nent in the electrical engineering field. Although when profoundly treated, the subject is so extremely technical as not only to necessitate the use of advanced mathematics but also to require, on the part of the student, considerable knowl- edge of electricity, yet the authors feel tBEFACE confident that a considerable portion of the subject can readily be understood by the general public. They therefore offer this volume, with the belief that since the commercial applications of alterna- ting currents are rapidly becoming so im- portant, it is no longer a question of willingness, but of necessity, on the part of the general public, to become familiar with the outlines of this branch of elec- tro-technics. CONTENTS PAGE I. INTRODUCTORY 5 II. ALTERNATING ELECTROMOTIVE FORCES AND CURRENTS . 21 III. UNIPHASE ALTERNATORS . . 53 IV. POWER 81 V. TRANSFORMERS . . . . ' 99 VI. ELECTRIC LAMPS ..... 133 VII. ELECTRIC MOTORS .... 153 VIII. MULTIPHASED CURRENTS . . 167 IX. MULTIPHASE MOTORS . . . 183 INDEX 205 /ALTERNATING ELECTEIC CURRENTS. CHAPTER I. INTRODUCTORY. O IN a river, far enough above its mouth to lie beyond the reach of tidal influences, the water constantly flows in one direc- tion; namely, down stream, or from the source toward the mouth. Farther down the river, within the tidal limits, the di- rection of flow alternates, or is reversed four .times in about twenty-four hours: the water flowing alternately up stream 6 ALTERNATING ELECTRIC! CURRENTS. for about six hours, and down stream for about six hours. In continuous electric currents, the elec- tric flow is unidirectional; i.e., takes place continuously in one direction through the conducting channel, like a river above the tideway. In alternating -electric currents the direction of flow in the conducting circuit, or electric channel, is alternately reversed, like a river within the limits of tidal influence. In a river, the current, or flow of water, changes direction but four times in every 24 hours; that is, during this time there are four alternations or changes of direction. In an alternating- electric circuit, the al- ternating-electric current, or flow of elec- tricity, changes direction, or is reversed, many times per second. The number of INTKODtJCTOfcY. 7 alternations per second is commonly called the frequency of alternation. In practice, the frequency of alternation is from 50 to 270; or, in other words, in prac- tical alternating -current circuits, the elec- tric current makes from 50 to 270 alterna- tions per second, according to the system of machinery employed. But the fre- quencies of alternating currents may, under certain circumstances, greatly ex- ceed 270 alternations per second. In the case of telephonic circuits, over which articulate speech is transmitted, al- ternating-electric currents are employed, the frequency of which may be 1000 or more alternations per second. In the experiments of Tesla, in which special ef- fects called Tesla effects are produced, extraordinarily high frequencies are em- ployed, reaching sometimes millions of alternations in each second of time. 8 ALTEBNATING ELECTBIC Kecent investigations have shown that light is, in all probability, an effect pro- duced in space by alternating -electric currents of frequencies reaching as high as 800 trillions per second. In the case of a tidal stream, the time required for the flow of water to return to the condition it had at any moment, may be called the period of the stream. Thus, suppose a river at high water is just be- ginning to ebb; then a period will include the time required to again reach high water, and will embrace the time of one full ebb and one full flood; in this case, about 12 hours. During one period the flow of water in the river will have com- pleted one cycle, and will have undergone two alternations, or reversals of direction. Every complete cycle, therefore, consists of two alternations. In the case of the INTRODUCTORY. 9 river, the duration of ebb and flood are un- equal. In the case of all practical alter- nating currents, the duration of each re- versal or alternation is the same. The period of an alternating -electric current is the time required to complete two alternations, or, in other words, to effect one complete cycle. The number of cycles per second is called the frequency. The time occupied in each reversal is sometimes called a semi-period. Conse- quently, an electric current, making 100 reversals or alternations per second, would have a frequency of 100 alterna- tions, or 50 complete cycles, per second. In the case of most tidal streams, the water rises or falls at a comparatively uni- form rate; that is, if the range of the tide is six feet, and the difference of level pro- 10 ALTERNATING ELECTRIC CURRENTS. duced during ebb or flood is rigorously one foot per hour, then the level of the water in the river, at any time, might be graphically represented as in Fig. 1, where we assume that at noon, each day, high water occurs three feet above the mean level; at 3 P. M. the mean sea level is reached; at 6 P. M., low water; at 9 p. M., HIGH WATER _ LOW WATER FIG. 1. TIDAL FLOW OF RIVER. mean level, and at midnight, high . water, completing the cycle in a period of 12 hours. In this ideal case, the water is flowing from noon to 6 p. M. and from midnight to 6 A. M. out of the river, at a steady rate, of say 500,000 gallons per hour, and is flowing, at the same rate, from 6 P. M. to midnight, and from 6 A. M. to INTRODUCTORY. 11 noon, steadily back into the river. If, therefore, it be required to represent the rate -of -flow of the river, that is, the quan- tity of water passing per hour, or per sec- ond, it will be necessary to employ a new diagram, such as that shown in Fig. 2. Here distances above the line 0, corre- . 100,00 O FLOW 100,00 200,00 soo,oo FULL FLOW UPSTREAM FULL FLOW UP8TRB '"I 4 *""" (0 K i . i FULL FLOW DOWNSTREAM FULL FLOW DOWNSTREAM FIG. 2. CURVE OF TIDAL FLOW. spond to flood tide, or flow up stream, and distances below the line, correspond similarly to ebb tide, or flow down stream. Thus, between noon and 6 p. M., 500,000 gallons per hour, or nearly 140 gallons per second, flow steadily down stream toward the mouth, while from 6 P.M. to 12 midnight, there is the same flow up stream. 12 ALTERNATING ELECTRIC CURRENTS. If the above diagrams represented the actual condition of affairs, high water and low water could only exist for an infinites! - mally small interval of time, whereas, we know that slack water has an appreciable duration, and that the rate of rising or falling is not uniform, but is greatest about FIG. 3. TIDAL LEVEL OF RIVER. mean tide. This is represented for the ideal case of a 12 -hour period and a uni- form tide, in Fig. 3, and the flow diagram in Fig. 4, corresponding to Fig. 3, shows that the rate -of -flow, instead of changing direction abruptly, does so gradually, so that instead of the rectangular wave of Fig. 2, we have a smooth wave. INTEODUCTOEY. 13 Figs. 2 and 4 may also be taken to rep- resent alternating -electric current flow as well as alternating tidal flow, except that a period would then correspond to but a fraction of a second, instead of approxi- mately 12 hours, and the rate-of-flow FIG. 4. CURVE OF TIDAL FLOW. would be measured or marked off, not in #a//ons-per-hour, but in units of electrical flow called Fig. 5 is a reproduction of Fig. 2, except that the period is 1 -100th of a second, corresponding to an electrical frequency of 14 ALTEBNATING ELECTBtC CIJBBENTS. 100 cycles, or 200 alternations per second; while the flow is alternately, say 50 cou- lombs of electricity per second in one direction, and then 50 coulombs -per- sec- ond in the opposite direction. Ill m 5*VJ ? 1 I I *~ 10 8 * < * "<> I ( A* >' i Hi > " i FIG. 5. CURVE OF ALTERNATING-CURRENT FLOW. A coulomb-per-second, considered as a rate of flow, is called an ampere. Instead, therefore, of using the phrase coulomb- per-second, we may use the word ampere. INTRODUCTORY* 15 The current strength, or flow, represent- ed by Fig. 5, is alternately 50 amperes in one direction and 50 amperes in the opposite direction throughout all parts of the conducting circuit. In an alternating-current circuit, that is, in a complete conducting path through which alternating -electric currents may flow, the current strength, at any instant, as expressed in amperes, is the same at all parts of the circuit, so that if the current strength be 50 amperes in one direction, it will, as a rule, at that moment, be 50 amperes in that direction throughout the circuit, and, when the reversal takes place, it will practically do so coincideni.- ly throughout the. ckciiiL.and the current 1 "" "" ' strength becomes, as is seen in Fig. 5, 50 amperes in the opposite direction in all parts of the circuit. 16 ALTERNATING ELECTRIC CURRENTS. Fig. 6 is practically a reproduction of Fig. 4, and represents an alternating cur- rent with a frequency of 50 cycles, or 100 FIG. 6. CURVE OF ALTERNATING-CURRENT FLOW. alternations per second, and a maximum strength of 20 amperes in each alternation. The condition of things represented in Fig. 6, is a much closer approximation to the actual state of most commercial alterna- ting-current circuits than that represented in Fig. 4, since, in fact, the electric cur- INTRODUCTORY. 17 rent can never change instantaneously from a full positive to a full negative strength, or vice-versa, but usually fol- lows some smooth curve. For convenience, we have compared the flow of water through a river channel with the flow of electricity through a conduct- ing channel or circuit. We should, how- ever, carefully avoid falling into the error of carrying this analogy too far, since electricity is not a fluid, although many of the laws of its passage and flow bear close resemblance to the laws of liquid flow. Although, at the present time, the exact nature of electricity is far from being known, yet electricity is generally believed to be an effect produced by an active con- dition in an all-pervading medium called the ether. The ether is believed to fill in- 18 ALTERNATING ELECTRIC CURRENTS. terstellar space and to permeate all bod- ies, even copper wires, and other equally dense forms of matter. Just what may be the nature of that particular ether ac- tivity which constitutes electricity, is not known. It may or may not resemble the particular form of activity in the atmos- phere called whirlwind. The difficulty of obtaining a clear con- ception of the true nature of electricity arises from our inability to recognize even the existence of the ether by our senses, and our still greater inability to recognize the conditions of its activity. In the case of the atmosphere, we can readily appreci- ate the phenomena produced by the wind, since the effects are produced on a scale commensurate with the capabilities of our senses. But, were we situated on a dis- tant planet, and had no experience what- INTEODUCTOEY. 19 ever of an atmosphere, even though we could perceive, through sufficiently pow- erful glasses, the effects of storms on the earth, we would, probably, have as great difficulty in understanding the nature of phenomena produced by wind power, as we now have in understanding the nature of electrical phenomena, as possible ef- fects of ether disturbance. The researches of the eighteenth cent- ury gave rise to the belief that electricity was a subtle fluid to which the name of electric fluid was given. The researches of the nineteenth century have promoted the belief that this fluid is no other than the all-pervading ether which serves to con- vey over apparently empty spaces heat, light, gravitational force, and magnetism. Certain characters of disturbance in this medium produce phenomena which we recognize as electrical, while other dig- 20 ALTERNATING ELECTEIC CUEEENTS. turbances of a distinct but interconnected character with the preceding, give rise to phenomena which we recognize as magnetic. CHAPTER II. ALTERNATING ELECTEOMOTIVE FORCES AND CURRENTS. IN all commercial applications of elec- tricity the following combinations of parts are needed; namely, (1) A device called a source, where the electric current originates. (2) Devices called translating or recep- tive devices. (3) Conducting paths connecting the translating devices with the electric source. In all cases, after an electric current has left its source and produced some peculiar effect in a receptive device, placed in its path or circuit, means must be provided 22 ALTERNATING ELECTRIC CURRENTS. whereby the current may flow back again to the source. In other words, the elec- tricity invariably leaves the source, passes through various conducting paths, pro- duces effects in the translating devices, and flows back to the source from which it came. For this reason, the conducting path is usually called a circuit, although of course it is not necessary that the path through which the electricity flows should be a circular path. Electric sources do not primarily pro- duce electricity, but a particular variety of force called electromotive force, (general- ly abbreviated E. M, F. ). This force, in its turn, tends to produce electric current, In point of fact, an electric source, al- though it will always produce electro- motive force in a conducting circuit con- nected to it, yet will not produce an elec- ELECTEOMOTIVE FOKCES. 23 trie current in such circuit, unless the circuit be closed or completed. Electromotive forces are either contin- uous or alternating. A continuous elec- tromotive force is unidirectional; i. e., has continuously the same direction, and pro- duces, when it acts upon a closed circuit, what is called a continuous electric current. An alternating electromotive force is one which alternates in direction, and, when applied to an electric circuit, produces an alternating electric current; that is, an electric .current, the direction of periodically changes with the ch the direction of the E. M. F. A voltaic cell is an example of an elec- tric source which produces a continuous electromotive force. A common and con- venient form of voltaic cell, much em- ployed on telegraph lines, is called the 24 ALTERNATING ELECTEIC CURRENTS. Daniell Gravity Cell. Such a cell is shown in Fig. 7. It consists of a plate of copper C 9 and a plate of zinc Zn, immersed re- spectively in aqueous solutions of copper JL FIG. 7. GRAVITY CELL. sulphate and zinc sulphate. A solution of zinc sulphate will float on a solution of copper sulphate, being lighter than it, and since this fact is utilized to keep the liq- uids separated, the form of cell in which the solutions are thus separated, is called the gravity cell. ELECTROMOTIVE FORCES. 25 The current produced is conventionally assumed to leave the cell at its positive or copper pole, and to return to it, after hav- ing passed through the conducting circuit, and its receptive device, at its negative or zinc pole. When the terminals of the cell FIG. 8. ILLUSTRATING REVERSAL IN DIRECTION OF CURRENT THROUGH AN ELECTRIC CIRCUIT ON THE REVERSAL, OF ITS ELECTROMOTIVE FORCE. are connected to a circuit, a current will flow through the external circuit from the copper pole to the zinc pole, as shown in Fig. 8. But if the terminals of the cell be reversed, the direction of the flow through the conductor will be reversed, 26 ALTEKNATING ELECTBIC CURRENTS. and, if these reversals are made five times per second, then there will be five alter- nations of electromotive force and current in the circuit per second. The alternating currents employed in practice, are not, however, obtained in this way, but from special machines called alternators. In its action on an electric circuit, a continuous electromotive force resembles the action of a watermotive force, or pres- sure in a reservoir, which forces a steady stream of water through an outflow pipe. An alternating electromotive force resem- bles in its action the action of an alterna- ting water motive force, or pump, alternately pumping water into and out of -a reservoir through a pipe. Water engines, operated by water pressure alternately exerted on opposite sides of a piston, after the general manner of the action of a steam ELECTROMOTIVE FORCES. 27 engine, afford an instance of such an al- ternating watermotive force. When a continuous electromotive force is applied to a conducting circuit, such, for example, as a mile of insulated cop- per wire, the current which passes through the circuit will be twice as great as it would be, if the same E. M. F. were ap- plied to a circuit of the same length of such wire, but of only half the weight or area of cross -section; for, the thicker wire conducts electricity twice as well as the thinner wire; or, mother words, offers but one-half the resistance. Electrical resistance is usually ex- pressed in units called ohms. The ohm is the resistance offered by a given length of conductor of definite cross -section. When the resistance of any circuit is 28 ALTERNATING ELECTBIC CUBBENTS. known in ohms, the current, produced by applying to this circuit a known E. M. F., can be calculated in amperes, by a rule called Ohiris law, from the name of its discoverer, Dr. Ohm, of Berlin. Ohm's law is usually expressed as fol- lows: The current in any conducting circuit, ex- pressed in amperes, is equal to the total elec- tromotive force in the circuit, expressed in volts, divided by the resistance of the circuit, expressed in ohms. In other words, the amperes in any circuit are equal to the volts divided by the ohms. Thus, the electromotive force usually supplied to incandescent electric lamps is about 110 volts, and since the resistance of the carbon filament in a sixteen-candle power lamp, when lighted, is, say 220 ohms, the current strength, ELECTKOMOTIVE FORCES. 29 which will pass through such a lamp, is 110 volts -4- 220 ohms = 1-2 ampere, If the electric resistance of any insu- lated wire be measured in ohms, the value will be found to be the same, whether the wire be straight or bent; i.e., whether the wire be stretched in a straight line, or be wrapped in a close coil; for, when a continuous current is once established in a wire or conductor, bends or turns in the direction of the conductor do not offer any additional resistance to the flow of the current. When, however, an alternating electromotive force is ap- plied to a wire, the strength of the current established in the circuit is considerably influenced by the disposition of the wire, that is, whether it forms a single loop, or whether it forms a coil of many turns. In the latter case, the current which 30 ALTERNATING ELECTEIC CUEEENTS. flows is much smaller than that obtained by dividing the E. M. F. in volts, by the resistance of the coil in ohms. In other words, a different law appears to govern the current strength in an alternating-cur- rent circuit than that which governs it in a continuous -current circuit. A circuit containing coils of wire, acts toward an alternating E. M. F. as if it possessed a higher resistance than when traversed by a steady current. In other words, the passage of an alternating current through a coil of wire is opposed by an influence which tends to choke or diminish the current. This influence is called the re- actance of the coil. The nature of react- ance will be understood from a consider- ation of the following principles: When an electric current is sent through a con- ductor, the conductor thereby acquires all the properties of a magnet, as was first ELECTROMOTIVE FO&CES. 31 shown by Oersted, in 1819. Could we see the actual state of things which exists in the neighborhood of an active conductor, it is believed - that we would be able to see around the conductor, a streaming motion in concentric circu- lar paths, of the highly tenuous, all -per- vading medium, called the ether. The ether streaming motion is called magnetism. It is most energetic in the immediate neighborhood of the conductor, gradually becoming weaker at greater dis- tances from it. Moreover, the direction of the streaming depends upon the direction of the current in the conductor. For ex- ample, if, as in Fig. 9, the current passes downward"~through the plane of the pa- per, that is, from the observer, the direc- tion of the streamings will be the same as the direction of the hands of a watch. 32 ALTERNATING ELECTRIC CURRENTS. These ether streamings occur in the space around every magnet, as well as in the space around an active conductor, and constitute what is called a magnetic field. If the conductor be given the form of a ' From Observer Toward Observer FIG. 9. DIAGRAMS OF FLUX PATHS ROUND A WIRE CARRY- ING A CURRENT FROM AND TOWARD OBSERVER. loop and the ends of the loop be connect- ed with an electric source, so that an elec- tric current flows through the circuit so formed, then the ether streamings, or the magnetic flux surrounding the wire, will be so directed that all the flux will enter ELECTROMOTIVE FORCES. 66 the loop at one side and leave it at the opposite side. The only effect produced by changing the direction of the current, will be to change the direction in which the flux passes through, or threads the loop. If, for example, with one direction of current flowing through the conducting loop, the magnetic flux enters the loop from above and passes out below, then reversing by the direction of the electric current, the flux would enter the loop from below and pass out from above. The effect of impressing any E. M. F. on a conducting loop is, therefore, to cause magnetic flux to thread or pass through the loop. Conversely, the effect of causing magnetic flux to pass through a loop is to produce an E. M. F. in the loop. This E. M . F . continues only while the flux passing through the loop is changing in 34 ALTERNATING ELECTRIC CURRENTS. amount; or, in other words, while it is in- creasing or decreasing. An E. M. F. set up in this manner in a conducting loop is called an induced E. M. F. The direction of the induced E. M. F. is opposite to the direction of the E. M. F. which was re- quired to produce the flux that caused it. In order to distinguish the E. M. F. pro- ducing the flux, from the E. M. F. pro- duced by the flux, the former is called the impressed E. M . F . In other words, the passage of magnetic flux through a con- ducting loop, consequent upon the appli- cation of an E. M. F. to such loop, will tend to set up in the loop an E. M. F. oppo- sitely directed to that of the impressed E. M. F. The induced E. M. F. is, conse- quently, called a counter electromotive force; and, since it is produced by induc- tion, it is sometimes called the counter electromotive force of self-induction. ELECTROMOTIVE FORCES. 35 The intensity of the counter E. M. F. so set up, depends upon the rate of change in| the amount of flux passing through the j loop at any moment, and not on the total amount of flux. Consequently, when the direction of current is reversed, as in an alternating -current circuit, the direction of the flux is reversed, and a rapid change occurs in the rate at which the flux is passing through the loop. The effect, therefore, of applying an al- ternating E. M. F. to a coil of wire is to produce, by induction, a resistance to cur- rent flow greater than the resistance to steady currents. This total apparent re- sistance, which is generally called imped- ance, arises from the fact that the rapid filling and emptying of the coils with mag- netic flux, set up an E. M. F. counter or opposed to the E. M, F. driving the flux 36 ALTEKNATING ELECTRIC CURRENTS. through the coils, and, therefore, impedes the flow of current through the coils. The effect of the impedance is to prevent the immediate application of Ohm's law to an alternating-current circuit. The resistance of 100 feet of insulated copper wire of the size represented in Fig. 10, and which is known commercially as FIG. 10. No. 13, A. W. Gr. WIRE, FULL SIZE. No. 13, American Wire Gauge, cotitracte'd A.W.G. is approximately l-5th of an ohm. If a continuous E. M. F. of one volt be maintained between the ends of this wire, the current strength through the wire, whether straight or wound into a coil, would, by Ohm's law, be five amperes (1 volt -s- l-5th ohm = 5 amperes). But if an alternating E. M. F. of one volt, reversing ELECTROMOTIVE FORCES. 37 250 times a second, and, therefore, having a frequency of 250 reversals, or 125 cycles per second, be connected to the ends of the wire, the current strength through the wire, if the wire be wound into a coil of many turns, will be considerably reduced, say to 2 amperes, and the impedance, or apparent resistance of the wire, will be 1-2 ohm, instead of l-5th ohm. f ?> The impedance increases both with frequency and with the number of turns in the coil. But, as we have already seen, a counter E. M. F. is produced in a coil by a change of flux passing through the coil. The effect of introducing iron into the path of the magnetic flux, is to increase the amount of flux which passes, owing to the fact that iron conducts mag- netic flux much better than air. If, then, a coil of wire be wound on a suitable core 38 ALTERNATING ELECTRIC CURRENTS. of iron, the flux passing through the coil, at each reversal of current, will be great- ly increased, and, consequently, the reac- tance of the coil will be increased, or the coil will possess a greater impedance and a more marked choking effect, when the core is present, than when it is absent. It might be supposed that alternating - electric currents possess a marked dis- advantage over continuous currents from the fact that the introduction of coils of wire into their circuit necessarily tends to impede or choke the current flow; for, as is well known, nearly all electric appara- tus contain coils of wire, as, for example, electromagnets, But this very fact, so far from being an unmitigated detriment, is often employed to great advantage, where the amount of current which can flow through a circuit is automatically choked ELECTROMOTIVE FORCES. 39 or throttled by the impedance of coils of insulated wire. In fact the capability of introducing reactance, practically without resistance, into an alternating current cir- cuit, is one of the principal advantages of alternating currents. It is true that an electric current, wheth- er continuous or alternating, can be read- ily diminished in strength by the intro- duction into the circuit of mere resist-' ance, called ohmic resistance, because its resistance depends only on the nature of the wire, its length and area of cross-sec- tion, and is independant of the disposi- tion of the wire, or its coiling. But, in the case of an alternating current, the counter E, M. F. prevents a portion of the electromotive force from acting and, therefore, decreases the amount of elec- trical work done, or energy usefully ex- 40 ALTERNATING ELECTRIC CUEEENTS. pended, while with the continuous cur- rent, although the current is reduced, yet the entire E. M. F. is acting and, con- sequently, there is a greater expendi- ture of power. An application of the methods of vary- ing, in certain cases, the strength of cur- rent flowing through any circuit, is seen in the solution of a problem, which is often met in practice; namely, to turn down or decrease the brightness of an electric lamp. If this be done, as has fre- quently been attempted, by introducing into the circuit of the lamp, a mere ohmic resistance; namely, a conductor with but a few turns, then, although the strength of current passing through the lamp is de- creased, and power saved in this respect, yet the same current is now passing through the resistance and producing use- ELECTROMOTIVE FORCES. 41 less heat in it. On the contrary, when a reactance, i. e., a coil of many turns, is employed with an alternating current, not only is the current passing through the lamp decreased, but practically no energy is lost in the reactance. Fig. 11 represents a form of device for turning down lights, called a theatre dim- mer. Here a portion of the circuit con- taining the lamps is wrapped in the form of a coil C\ around a laminated ring of soft iron K; that is, a ring consisting of plates of soft sheet iron, laid side by side. On the opposite side of the soft iron ring /C a copper shield H, is placed, capable of being slid over the core K, to the right or the left about the axis Z>, by the mo- tion of the hand wheel. With the rela- tive positions occupied by the shield H, and the coil C 9 shown in the figure, the 42 ALTEKNATING ELECTRIC CURRENTS. effect of the coil is to throttle, or choke, the current, by its reactance, and thus di- minish the intensity of the light given by the lamps. If it be desired to increase the amount of light, that is, to turn the ^ FROM DYNAMO FIG. 11. THEATRE DIMMER, REACTIVE COIL. lights up, the metal shield H, is moved by the hand wheel toward the reactive ELECTROMOTIVE FORCES. 43 coil C, thereby diminishing the reactance of the coil, and thus permitting more cur- FIG. 12. THEATRE DIMMER. rent to flow through the circuit. A mo- tion, therefore, of the metal shield H, toward C, increases the intensity of the light, while a motion from C, diminishes the intensity. A perspective view of the apparatus is shown inFig.12. Fig.13 shows other forms of theatre dimmer, which operate by the choking effect of react- 44 ALTERNATING ELECTRIC CURRENTS. ive coils furnished with a movable core consisting of a bundle of soft iron wires. ^ FIG. 13 ALTERNATING CURRENT THEATRE DIMMERS. Both continuous and alternating cur- rents are capable, when passed through ELECTROMOTIVE FORCES. 45 coils of insulated wire provided with iron cores, of producing electromagnets as shown in Fig. 14. Continuous -electric currents are generally employed for this purpose, since the magnetizing coils do not then act to throttle the current. When alternating- electric currents are passed through the coils of an electromag- FIG. 14. FORM OF ELECTROMAGNET. net, although such a magnet does not pos- sess as powerful attraction for its arma- ture, as when excited by continuous cur- rents, yet it often possesses the advantage of exerting a more nearly uniform puj.1 over a greater distance. Of course, in alternating -current electromagnets, the 46 ALTEENATING ELECTEIC CUEEENTS. magnetism is constantly reversing in di- rection, with each reversal of the current, each pole becoming alternately of north and south polarity. In electroplating, deposits of gold, sil- ver and other metals are thrown down by the action of an electric current on the conducting surfaces of articles placed in suitable vats. The surfaces which are to receive these deposits, if not already con- ducting, are made so by various processes, and immersed in solutions of the metals with which they are to be coated. The current employed for this purpose is invar- iably a continuous current. It is a well- known fact, that an article, which has been placed in a plating bath and has re- ceived a coating of deposited metal by the electric current passing through the bath in a certain direction, will have all this ELECTROMOTIVE FORCES. 47 metallic coating gradually dissolved if the Current be sent through the bath in the opposite direction; for, in all cases of electro -plating, the metal is only deposit- ed on one of the conducting surfaces con- nected with the poles; i.e., on the nega- tive, and is dissolved from a plate of metal connected with the opposite or positive pole. Since, in an alternating- current circuit, both the article to be plated and the plating metal become alternately pos- itive and negative, it might be supposed that it would be inrpossible to produce any permanent plating whatever by such a current, and, although this is true to the extent of preventing plating from being carried out practically by such methods, nevertheless, permanent electro -plating effects can be produced by alternating currents, when certain relations exist between the size of the article to be 48 ALTERNATING ELECTRIC CURRENTS. plated and the strength of the current passing. ^ So far as the heating effects of the elec- tric current are concerned, alternating currents produce the same amount of heat that continuous currents do. For ex- ample, if an incandescent lamp be con- nected to a continuous -current circuit of 110 volts pressure, and, subsequently, to an alternating -current circuit of 110 volts pressure, the amount of light and heat, which the lamp will give off, will be the same in both cases. A marked difference exists between the physiological effects of an alternating and a continuous current. When a continuous current is sent through the human body, chemical and physiological effects are pro- duced, entirely distinct from those which ELECTKOMOTIVE FORCES. 49 attend the passage of an alternating cur- rent under similar circumstances. When passing through the vital organs of the body, any electric current, whether con- tinuous or alternating, may, if sufficiently powerful, cause death. Alternating cur- rents, however, at commercial frequencies and pressures, are much more apt to pro- duce fatal effects on the human body than continuous currents. In New York State, alternating electric currents are used for the execution of criminals, and, when properly employed, produce absolute, instantaneous, and painless death. ^ The experiments of Tesla and others have shown that at frequencies and pres- sures far higher than those employed for ordinary commercial purposes, the physi- ological effects of alternating currents be- come less severe, and that at extraordina- 50 ALTEENATING ELECTEIC CUEEENTS. rily high frequencies, enormous pressures may be handled with impunity. It should be remembered, however, that the physiological effects produced by a current depend largely on the resistance offered to its passage through the body by the skin. For example, when an alter- nating current is sent through the human body, by immersing the hands in saline solutions connected with an alternating- current circuit, a pressure even as low as five volts will usually produce very pain- ful sensations. Care, therefore, should al- ways be taken in handling the wires from any high-pressure electric source particularly if that source be one supply- ing alternating currents. In an alternating -current circuit, both the strength and the direction of the E. M. F. and current are periodically varying, ELECTKOMOTIVE FORCES. 51 being at certain times at greatest strength and at others entirely absent. It is evi- dent that it would not be correct to estimate the value of an E. M. F. or a current at either its greatest or its least value ; nor is it usual to take the average value. In- stead of this a certain value, both of the E.M.F. and the current, called respective- ly the effective E. M. F. and the effective current strength, s\xe taken as estimated from their equivalent heating effects. Thus, an alternating- current pressure of 100 volts is one which, as already mentioned, will produce in an incandescent lamp the same heating and, 'therefore, the same degree of illumination as 100 volts of continuous- current pressure. In the same way an al- ternating-electric current, whose values at different successive instants in any cycle would be considerably above or below one ampere, would be regarded as having an 52 ALTERNATING ELECTRIC CURRENTS. effective current strength of one ampere, if it produced the same heating effect in a coil of wire as a continuous electric cur- rent of one ampere. This method of estimating the values of alternating E. M.F.'s and currents is uni- versally employed, and entirely dispenses with the necessity for a determination of the shapes of the alternating -current waves, just as any method of measuring tides, which depended upon a measure- ment of the total quantity of water moved up stream during each tide, would dis- pense with the necessity for determining the exact shape of the tidal wave. CHAPTER III. UNIPHASE ALTEKNATORS. DURING the last few decades there has been witnessed a marvelous development in the commercial applications of electric- ity. Perhaps the most striking feature in this development is to be found in the strength of the electrical currents em- ployed to day, as compared with the strength of those which were commercial- ly possible only a few years ago. Elec- tricity has commercially entered fields, which, but a comparatively short time ago, .would have been closed to it by reason of the expense attending its production. This development has not been ren- dered possible so much by improvements 54 ALTEKNATING ELECTKIC CUKRENTS. in the apparatus operated by electricity, as it has been in the improved methods for producing electricity more cheaply. For example, to take the field of electric light- ing, in which the most marked develop- ments were first manifested; although the arc lights of to-day are, in their way, marvels of mechanical and electrical inge- nuity, yet, in point of fact, they do not dif- fer radically, in their general construction, from those produced fifty years ago. Why then did riot these early arc lamps enter into more general use ? Surely not on account of any lack of appreciation on the part of the general public, of the ad- vantages possessed by the voltaic arc as an artificial illuminant, but because, in those early days, the only practical means for producing electrical currents was an expensive and inconvenient source of electric supply; namely, the primary, or UNIPHASE ALTERNATORS. 55 voltaic battery. That which rendered electric lighting, as well as most of the many other commercial developments of electricity which followed in its wake, commercially possible was the production of a means for cheaply producing elec- tricity, on a large scale; viz the invention of the generator known as the dynamo - electric machine. It is a well -recognized principle, in the physical world, that in order to perform work of any kind, whether mechanic- al, chemical or electrical, energy must be expended. Consequently, the production of a definite amount of electrical energy requires the expenditure of a definite amount of work. A machine is a device for transforming one kind of work into another. Thus a steam engine and boiler form a machine 56 ALTEENATING ELECTRIC CUEEENTS. for transforming, into mechanical work, the work of heat, liberated by the burn- ing of coal. Despite the- fact that the steam engine has been repeatedly im- proved, since the early days of Watt, in 1765, yet in the best forms of triple -ex- pansion engines, as produced to-day, the work delivered by the engine amounts to but about sixteen per cent, of the work delivered by the coal; so that, although the steam engine can transform the work of heat into mechanical motion, it throws away, during the process of transforma- tion, five parts out of every six. Contrast- ing with this the modern dynamo machine, the latter will be found a far more effi- cient agent for the transformation of ener- gy ; for, even in small sizes, of say one H.P. , it is capable of delivering, as electrical work, 75 per cent, or about three parts out of every four, of the mechanical work ex- UNIPHASE ALTERS ATO&S. 5? pended in driving it, while in large sizes, of, say thousands of H.P. it is capable of delivering as electrical energy 98 per cent, of the mechanical energy it receives. Although in practice dynamo -electric machines are generally driven by steam engines, yet their economy over other electrical sources is so great as to war- rant this use, despite the low efficiency of the steam engines. Since the expense of maintaining steam power decreases markedly with the size of the steam plant, and since, as we have seen, the capability of the dynamo increases with its size, it is generally found more expedient, in practice, to generate electrical currents in large quantities at a few points called central stations, distributing the electrical power to consumers by means of suitable distribution circuits, than it is to have 58 ALTERNATING ELECTRIC CURRENT^. as many individual plants as there are consumers of the electric current. This is especially the case where dynamos are driven by cheap water power. A visit to any central station, where electricity is being generated on a com- mercial scale, will, on even a casual obser- vation, enable one to readily divide the machinery into two distinct classes; name- ly, the driving machinery and the driven machinery. The driving machinery will consist either of steam engines or of water wheels. The driven machinery will consist of various forms of dyna- mos. The driving and driven machinery are connected together, either by means of belting or ropes, or are rigidly coupled together on the same shaft. At first sight it may seem that different, types of dynamo machines differ radically UNIPHASE AKTEBtfAtfORS. 59 in their detailed construction. A closer inspection, however, will show that such differences are apparent rather than real; for it will then be seen that all have cer* tain parts in common; namely, the part called the armature, in which the electric current is generated, and the part called the field magnet in which the magnetic field of the machine is generated. Attention has already been called, in the second chapter, to the fact that when loops of wire are filled and emptied with mag- netic flux, electromotive forces are gener- ated in the wire. The dynamo -electric machines that we see operating in a central station, are devices for filling and empty- ing, with magnetic flux, conducting loops that are placed on the armature of the machine. In order to do this, either the armature or the field is rotated. Usually 60 ALTEKNATING ELECTKIC CURRENTS. it is the armature that is rotated, since the armature is generally the lighter part. The E. M. F. generated in such conduct- ing loops, reverses its direction twice dur- ing each rotation of the armature in a bi- polar field; i. e., a field having one north and one south pole. All dynamo -elec- tric machines are capable of ready division into two sharply marked classes; namely, those in which alternating E. M. F.'s are delivered to the consumption circuits, that is, the circuits external to the machine, producing in them alternating -electric cur- rents, and those in which such E. M. F.' s are commuted, or given the same direction, by means of devices called commutators. In other words, all dynamo -electric ma- chines can be divided into alternating- current dynamos or alternators, and con- tinuous-current dynamos. TOIPHASE ALTERNATORS. 61 /We have, therefore, a general principle py means of which we can determine /whether or not a given machine, which we are examining in a central station, is an alternator, or a continuous -current dynamo, since, in the case of the al- ternator, the conducting loops of wire on the armature are connected direct- ly to the external circuit, generally by means of brushes resting on simple col- lector rings, while in continuous -current dynamos, the brushes, instead of resting on collector rings, rest on a commutator, which differs from the rings in the fact that it consists of a number of separate con- ducting bars, insulated from one another. \ The preceding principle, however, needs some modification, since the require- ments of electrical engineering, some- times, render it advisable to construct dy- 62 ALTERNATING ELECTBIC CTTBBENTS. namos so as to render them capable of giv- ing simultaneously both alternating and continuous currents. Various methods are adopted to obtain this result. For ex- ample, in some cases a portion of the con- ducting loops on the armature have the alternating E. M. F.'s generated in them so commuted as to produce a continuous current, while the remaining loops are connected directly to the collector rings, from which the alternating currents are carried off to the consumption circuits, by means of brushes resting on the rings. In such cases, the continuous currents are employed for various purposes, gener- ally for the excitation of the magnetic field through which the armature revolves, which excitation must always be provided by continuous currents. In all alternators, therefore, continuous UNIPHASE ALTERNATORS. 63 currents must be provided to flow through the field coils. Such continuous currents are either supplied by the machine itself, by commuting a portion of the conducting loops on the armature, or are supplied from a separate source. In other words, all alternators can be divided into two sharply marked classes; namely, those that are self excitefL that is, supply their own field magnets with continuous cur- rents, and, therefore, must be supplied with a commutator in addition to the col- lector rings; and those which are separate- ly excited, or which derive the continuous currents for the excitation of their field magnet coils from some external source. Let us now examine some of the dyna- mos that are commonly met with in cen- tral stations in the United States. Take, for example, the dynamo shown in Fig. 15. 64 ALTERNATING ELECTRIC CURRENTS, FIG. 15. BIPOLAR CONTINUOUS-CURRENT GENERATOR. UNIPHASE ALTERNATORS. 65 This is a bipolar dynamo; that is to say, its field magnets M, M, excited by large coils of wire as shown, produce two poles, S, between which the armature A, FlG. 16. QUADRIPOLAR CONTINUOUS-CURRENT GENERATOR. is revolved. An inspection of this ma- chine will show that it must belong to the continuous -current type, since the brushes 66 ALTERNATING ELECTEIC CURRENTS. rest on a commutator (7, composed of nu- merous insulated copper bars. Fig. 16 shows a type of quadripolar dy- namo; or a dynamo whose field magnet coils, A, B, C, D, produce four poles be- tween which the armature revolves. Here again this machine evidently belongs to the continuous -current type, since its brushes, in this case four sets, evidently rest on a commutator, M. Fig. 17 shows a type of separately -excit- ed alternator. Here a small continuous current dynamo D 1 , provided with a com- mutator at (7, supplies a continuous cur- rent through the brushes B, to the con- ductors 1 and 2, to the 12 field magnets M, M, etc., of the alternator D. In any bipolar generator, whether con- UNIPHASE ALTEENATOES. 67 FIG. 17- SEPARATELY-EXCITED ALTERNATOR. tinuous or alternating, the two poles are respectively North and South. In a quad- ripolar machine, guch as represented in 68 ALTERNATING ELECTRIC CURRENTS. Fig. 16, the poles are alternately North and South; and, in general, in generators containing any number of poles, the polar- ity is alternately North and South, as are the 12 poles in Fig. 17. A moment's thought will show that a multipolar gen- erator must, therefore, necessarily contain an even number of poles, since any odd number of poles would bring two poles of the same polarity in juxtaposition. In the alternator shown in this figure, the currents produced by the armature are carried to the external circuit, as alternat- ing currents, by means of brushes resting on the collector rings R, R, which, ac- cording to the principles already explained in Chapter II, become alternately posi- tive and negative during the rotation of the armature past each pole. Fig. 18 shows another form of separately- UNIPHASE ALTEKNATOKS. 69 excited alternator. Here the continuous - current generator, instead of being sepa- rate from the machine, and connected with it by a belt, as in Fig. 1 7, is mounted on the FIG. 18. SEPARATELY-EXCITED ALTERNATOR. same shaft as the alternator at D l , and a continuous current, taken from the com- mutator and brushes B, is led to the field magnets M, M, of the alternator D. The alternating currents produced in this gen- 70 ALTERNATING ELECTKIC CURRENTS. erator are carried to the external circuit by means of brushes resting on the collect- or rings R, R. The main driving pulley of the machine is shown at P. Heretofore, all the generators we have examined have had but a single circuit of wire on their field magnet coils. Some- times, however, it is necessary to provide two separate circuits in the exciting coils on the field magnets. Such machines are called compound- wound, or composite ma- chines. The object of double-winding on the field magnets is to maintain automatically the same pressure at the terminals or brushes of the alternator, whether it is supplying a strong or a weak current in its circuit; or, as it is sometimes termed, to regulate automatically the pressure under all loads. Fig. 19 represents such a self -regulating compound-wound alterna- UNIPHASE ALTEKNATORS. 71 tor. Here one of the circuits on the field magnets M, M, is separately excited by the continuous - current generator D lf The other circuit on the field magnets is ex- FIG. 19 COMPOUND -WOUND, SEPARATELY - EXCITED ALTERNATOR. cited by a portion of the alternating cur- rent supplied by the machine, and which is commuted by a commutator C, The 72 ALTERNATING ELECTRIC CURRENTS. alternating current is carried to the ex- ternal circuit by the rings R, R. The electrical connections of such a compound-wound machine are shown in Fig. 20. Here the exciter D 19 sends from FIG. 30. DIAGRAM OF CONNECTIONS IN A PARTICULAR COM- POUND-WOUND, SEPARATELY- EXCITED ALTERNATOR. its brushes a continuous current through an adjustable resistance, or regulating de- vice called a rheostat, and through a fine wire circuit to the field coils M, M, which are connected in series. The coils C\ C, C v etc. , mounted on the revolving armature A l generate alternating currents, which are UNIPHASE ALTEBNATOftS. 73 connected to the collecting rings R, R, and to the commutator C, as shown; namely, one end is connected directly to the collecting ring R, and the other end to the ring R l , through the commutator C. Under these circumstances a certain portion of the current passes around the commutator through the path marked G. S. shunt, of German silver wire, passing on as alternating currents to the collect- or ring, and by means of the brushes to the external circuit or line, as alternating currents, while the remainder, or com- muted portion, is fed through the brushes B, B, to the coarse wire circuit of the field magnets. The effect of this arrange- ment is, that as the strength of the cur- rent supplied to the external circuit in- creases, the portion of this current sup- plied to the coarse wire circuit of the field magnets increases, and the field magnets 74 ALTEBNATIHG ELECTBIC CURRENTS. are thereby strengthened, thus increasing the E. M. F. of the machine by increasing the magnetic flux passing through the coils on the armature. A self -excited alternator supplies from its own armature, through a commu- tator, all the current required for the exci- tation of its field magnets . All alternators may, therefore, be divided into three gen- eral classes; namely, (1) Separately -excited machines, in which the currents required for the excitation of the field magnets are obtained from a con- tinuous-current dynamo. Such alterna- tors employ no commutators but only a pair of collector rings. (2) Self -excited machines, which supply all the current required for the excitation of their field magnets, after such current has been rendered continuous by the ac- UKIMASE ALTEBtfAToHS. 75 tion of a commutator. Such machines, therefore, employ a commutator in addi- tion ;bo collector rings. (3) Compound- wound alternators, which consist practically of a combination of the two preceding types. In other words, the principal excitation of the field magnets is obtained from a separate dynamo, while the additional excitation, needed to main- tain a constant pressure at the collector rings under all conditions of load, is ob- tained from their own armature current through the action of a commutator. Fig. 21 represents a self-excited alter- nator with a commutator at C, and col- lector rings at R, R, for the delivery of alternating currents to the circuit. In order to familiarize the reader with the varieties of alternators in common use in the United States, two additional ex- 76 ALTERNATING ELECTBIC CURRENTS. amples of alternating -current machines are given in Figs. 22 and 23. An examina- FIG. 'Jl. SELF-EXCITING ALTERNATOR. tion of these figures will show that the ma- chines represented belong to the same gen- eral type as those already described, the UNIPHASE ALTERNATORS. 77 differences being either in mechanical construction or in the relative arrange- ment of the parts. For example, Fig. 22 FIG. 22. 2000-LiGHT ALTERNATOR. shows a separately -excited alternator of 10 poles with collector rings at R, R, sup- plying alternating currents, through the leads 1 and 2, to the external circuit. The 78 ALTERNATING ELECTRIC CURRENTS. separate exciter D l , supplies commuted or continuous currents to one winding of the field magnets, M, M, and part of the ar- mature current from the alternator D, is supplied through the commutator (7, to the other winding of the field coils. This machine is, therefore, a compound- wound, separately -excited alternator, and agrees in all essential electrical features with the machine shown in Fig J9. Fig. 23 shows a form of alternator in which only a pair of collector rings is employed. Here the separate exciter, necessary for supplying continuous cur- rents to the field magnets, is not shown, and, as there is no commutator on the machine, it is clearly not compound - wound. This alternator corresponds elec- trically to the type of machine shown in Fig.17. Beside the forms of alternators above UNIPHASE r ALTERNATORS. 79 described, there are many others. All, however, possess the same fundamental features although these features may dif- FIG. 23. 1000-LiGHT ALTERNATOR. fer markedly in their construction details. For example, in some alternators the ar- mature is fixed and the field rotates. In others, both armature and field are fixed, 80 ALTERNATING ELECTKIC CURRENTS. but a rotating frame is so placed in rela- tion to both as to generate E. M. F. 's in the conducting loops or coils on the ar- mature. Such alternators are called in- ductor alternators. CHAPTER IV. P O W E 11. VISITING an electric central station at the time of full load, that is, when the station is generating its full electric power, it is evident that a great deal of energy is being expended or work being done. The fires under the boilers are working at full draft; the steam engines are working at full steam pressure and speed, and the dynamos, if belt-driven, are receiving practically all the energy liberated by the engines through their tightened connecting belts. Evidently, therefore, the driving machinery is trans- mitting an enormous amount of power to the driven machinery. Indeed, not infre- quently several thousand horse-power are 5Z ALTEKNATING ELECTRIC CURRENTS. thus delivered in large central stations, from the steam engines to the dynamos. But there is no immediate evidence to the eye, as to what becomes of all this power. Our everyday experience would lead us to expect some more evident effect pro- duced by the expenditure of so much power. Were the engines suitably mounted on wheels and placed on a rail- road track, the same amount of power ap- plied to driving wheels would be sufficient to carry the entire plant along the road at a considerable speed. In the central sta- tion, however, the energy is transformed into electrical energy which is being si- lently carried away by the conductors. These silent conductors, however, are capable of delivering up the energy given to them at various points along their cir- cuit, and if all this energy were employed to drive electric motors, the total work POWER. 83 which could be performed by such mo- tors, provided no loss occurred in trans- mission, would of course be equal to that developed by the steam engines. It is evident, then, that a circuit con- veying an electric current, may, in its turn, be regarded as a source of driving power by which the motors are driven. But in the case of the steam engine, there is an evident connection between the driving and the driven dynamo ; namely, the belting or shafting. There must also be some connection between the dynamo as a driving and the motor as the driven machine. Here the connection, though far less evident, consists in the conducting circuit connecting the dynamo and motor; or, in other words, the conducting circuit, and its electric activity, take the place of the driving belt. 84 ALTERNATING ELECTRIC CURRENTS. Suppose a water motor is operated in a city by the flow of water through a pipe, connected with a reservoir on an adjoining hill. Here, clearly, the source of energy received by the motor is the moving or falling water. This energy, in its turn, was received from a pump which raised the water into the reservoir, from, possibly, a river or lake at a lower level. Moreover, the amount of energy received by the motor is perfectly definite, since each pound of water, falling from a height of one foot, conveys an amount of work called a foot-pound, so that, if the reservoir contains a million pounds of wa- ter, and the difference of level between the reservoir and the motor is 100 feet, then the total source of work upon which the motor can draw, is 100x1,000,000 or 100,000,000 foot-pounds. This stock of power in the reservoir PO>VEK. 85 might be expended by the water-motor in a day, or in an hour, according to the rate at which the motor works, and, therefore, permits the water to flow from the reser- voir. In other words, the ability of the unreplenished reservoir to keep the motor running for a given time, depends upon what is called the activity of the motor, or the rate at which it is doing work. This activity is usually expressed in. foot-pounds per second, or in foot pounds per minute. The commercial unit of activity is the horse-power, or 550 foot-pounds per sec- ond. If, then, the motor be a one horse- power motor, and, for simplicity of cal- culation, be supposed to be a perfect ma- chine; i.e., to waste no power in friction, then the flow of water through the pipe will be 5 1-2 pounds per second, and this quantity of water falling one hundred feet 86 ALTERNATING ELECTRIC CURRENTS. in one second will produce an activity of 5 1-2 x 100=550 foot-pounds per second. Although electricity is not a liquid like water, yet, since many of the laws which control the flow of water are also applic- able to the flow of electricity, it is con- venient, in considering the manner in which an electric current is able to transmit power to a motor, to regard electricity as though it were a fluid in actual motion. As in the case of water in motion, the amount of activity trans- mitted can be expressed by the pounds of water flowing per second, multiplied by the difference of level in feet through which it flows, so in the case of an elec- tric current, the activity transmitted can be expressed by the rate-of-flow of elec- tricity in coulombs-per-second s multiplied by the difference of electric pressure u POWER. 87 through which it flows, expressed in volts. Moreover, as the activity in the current of moving water is expressed in foot-pounds per second, of which 550 make a horse- power, so the activity in the current of electricity is expressed in volt- coulombs per second, or in icatts, of which 746 make a horse -power. If, therefore, we multiply the rate of electric flow in a circuit, ex- pressed in amperes, by the difference of electrical level or pressure, expressed in volts, the product will be the activity in the electrical circuit expressed in watts, 746 watts being equal to one horse-power. The activity, or the rate of delivering power from a water reservoir, can be in- creased either by increasing the difference of level, or by increasing the rate-of-flow; so in an electric current, the activity, or the rate of delivering electric power, can 88 ALTERNATING ELECTRIC CURRENTS. be increased either by increasing the dif- ference of electrical level in volts, or by in- creasing the rate of electric flow in amperes. Steam engines are generally rated in horse-power (contracted H.P.); that is to say, a one-horse-power steam engine is capable of doing an amount of work equal to 550 foot-pounds per second. A one- horse-power steam engine, therefore, is capable of lifting a pound weight 550 feet high, or 100 pounds 5 1-2 feet high, in each second of time. Electric generators are usually rated in watts; but since a watt is so small a unit of activity, being only 1 -746th of a horse-power, the kilowatt or 1000 watts is the unit generally adopted. Thus, a 1000- watt generator, or a 1 KW. generator, might supply one ampere in its circuit at POWEE. 89 a pressure of 1000 volts, between its brushes; or, it might supply 50 amperes at a pressure of twenty volts, or 1000 am- peres at a pressure of one volt. The following examples of electrical activities, required for the operation of apparatus in common use, may prove of interest: An ordinary incandescent lamp, of 10- candle-power, requires about 50 watts, so that at this rate one electrical horse -power will supply nearly fifteen lamps. The pressures at which such lamps are com- monly operated are either about 100 volts or 50 volts. A 100-volt IG-candle-power lamp, will, therefore, usually take a cur- rent of approximately 1-2 ampere, since 100 volts x 1-2 ampere = 50 watts; while if the lamp be intended for a fifty-volt cir- cuit, it will require a current of one am- 90 ALTERNATING ELECTRIC CURRENTS, pere, An incandescent lamp* therefore, requires about l-15th of a H.P, or about 37 foot-pounds per second to be supplied to it at its terminals in electrical energy, An arc lamp, of the ordinary 2000 can- dle-power rating, usually requires some 450 watts for the production of the arc at a pressure of 45 volts. This represents a current strength of 10 amperes, since 45 volts x 10 amperes = 450 watts. An arc lamp, therefore, requires to be supplied with an activity of about 3-5ths of an elec- trical horse -power; or, in other words, for every arc lamp supplied to a circuit, the engine driving the arc light generator must supply 3-5ths of a horse-power, and something over for losses in transmission. An electric current of 5000 amperes, supplied from a central station to a net- work of trolley conductors, in a street railway system, under a pressure of 550 volts at the dynamo brushes, will repre- sent a total activity of 550x5000=2,750,- 000 watts, or 2750 KW. or 3686 H. P. in electrical energy supplied to conductors. Although, as we have seen, the rate of work or activity, in a continuous current, is equal to the number of amperes multi- plied by the number of volts, yet when we come to apply the same rule to the case of alternating currents, we find that it is only true under certain circum- stances. This is for the reason that, in the con- tinuous-current circuit, the pressure is al\\ ays acting to drive the current in the direction in which it is already moving, while in an alternating -cur rent circuit it may be at times aiding the current for- 92 ALTERNATING ELECTRIC CURRENTS. ward, and at times opposing it. Fig. 24 represents the current strength in an al- ternating-current circuit, and also the E. M. F. of the generator by which that cur- b.0 O 01 FIG. 24. WAVES OF ALTERNATING E. M. F. AND CURRENT IN STEP. SI m \ FIG. 25. WAVES OF ALTERNATING E. M. F. AND CURRENT IN A CIRCUIT, OUT OF STEP. poWEit. 93 rent strength is produced. The two sets of waves are seen to be in step, the crests of the E. M. F. waves coinciding with the crest of the current waves. In such a case the product of the effective volts and the effective amperes gives the elec- tric activity, just as in the case of a con- tinuous-current circuit. Fig. 25, however, represents the more usual case in whiclk the pressure or E. M. F. is in advance of the current. It will be observed that at the moment when the pressure has its greatest value, or rises to the crest of its wave, the current strength will not have reached the crest of its wave, the result will be that the pressure will have dropped below the zero line 00, or will have become negative, while the current is still above the zero line, or in the positive direction. In other words, the pressure or E. M. F., instead of aiding the current at this in- 94 ALTERNATING ELECTRIC CURRENTS. stant, is opposing it. Under these circum- stances if we multiply the effective, num- ber of volts by the effective number of am- peres, we shall obtain an activity which is greater than that actually produced in the circuit. In other words, the appar- ent activity in watts will be greater than the actual activity in watts, and the dis- crepancy will depend upon the distances between the crests of the pressure and current waves; i. e. 9 upon the amount of time, in each period, during which the pressure is opposing, instead of driving. | The apparent activity, has, therefore, to ; be multiplied by a quantity called the power factor, in order to obtain the real activity. The value of the power factor depends upon the difference of phase. The waves of current and pressure are said to be in phase, or in step, when their crests and troughs occur simultaneously ; 95 and when the waves of pressure become separated from the waves of current, the two waves are said to differ in phase. Even in an alternating -current circuit, under certain circumstances, if we take for both of these quantities their effective values, as we have heretofore pointed out, the activity is correctly represented by the product of the E. M. F. by the current. This would be the case in a circuit of in- candescent lamps where the circuit is prac- tically free from loops, since, in such a cir- cuit, induction is practically absent. Such a circuit is sometimes called an induction- less circuit; but when, as is the case in most practical alternating -current circuits, con- ducting loops, in the shape of coils of wire, are present, then, as we have already pointed out, the successive filling and emptying of these loops with magnetic flux, 96 ALTERNATING ELECTKIC CUEKENTS. on the rapid periodical increase and de- crease in current strength, will set up E. M. F.'s in the wire, counter or opposed to the E. M.F.'s producing such flux, so that the combined effect of the impressed and the counter E. M. F.'s produces what is called a resultant^. M. F. which is shift- ed in position, or differs in amount and phase from the impressed E. M. F. But with this resultant E. M. F. the current is always in step. This resultant E. M. F., multiplied by the current in step with it, gives the true activity of the current. Since, however, the circumstances pro- ducing the displacement of the current, in phase, are often complex, it is well to multi- ply the impressed E. M. F. by the current and introduce a power factor rather than to determine what the resultant E. M. F. in the circuit may be. For example, an in- candescent lamp, supplied direct from POWER." 97 mains at an alternating pressure of 100 volts, may take, say half an ampere of cur- rent. The activity in the lamp will be 100x1-2=50 watts, and the power factor is (ne, or 100 per cent. This is for the reason that there is no reactance in the lamp, and the current waves through its filament are almost exactly in step with the waves of pressure at its terminals. Consequently, the activity of the lamp, and the light it emits, will be the same, whether it.be connected to 100 volts alter- nating or continuous pressure. If, however, the same alternating- cur- rent mains be connected with a coil of many turns, the resistance of which is the same as that of the lamp filament, while the continuous current will be half an ampere as before, the alternating cur- rent will be much less, perhaps, only l-10th 98 ALTERNATING ELECTRIC CURRENTS. of an ampere, this being due to the reactance of the coil, as already explained. The activity in the continuous current will be 50 watts, but in the alternating cur- rent it will not be 100 x l-10th or 10 watts, but considerably less, because the waves of pressure and current, owing to the re- actance of the coil, are out of phase, and the power factor of the coil will be less than say 30 percent., making an elec- trical activity in the coil only 10x30- 100ths=:3 watts. CHAPTER V. TRANSFORMERS. IF we leave the central station and fol- low an alternating- current circuit, erected upon poles, up to the first point where the current is utilized, we will probably see apparatus of the general type repre- sented in Fig. 26, either placed upon a pole, as shown in the figure, or in some convenient location on the side of a build- ing. Such an apparatus is called a trans- former, and is only employed on alternat- ing-current circuits. It remains now to examine the general construction of alter- nating-current transformers, and the part they take in the economical distribution of electric currents over extended areas. If an alternator, at a central station, is 100 ALTERNATING ELECTRIC CURRENTS. supplying 100 volts at its collector rings, a 100 -volt lamp connected at the brushes of such a machine will burn at full incandes - FIG. 26. ALTERNATING-CURRENT TRANSFORMER WITH DIRECT SERVICE WIRES. cence or brilliancy. Suppose, now, that this alternator be connected to a pair of wires five miles in length. If the lamps were TRANSFORMERS. 101 connected to the lines as shown in Fig. 27, at distances of 1, 2, 3, 4 and 5 miles re- spectively, we should find that the bril- liancy of the lamps diminished as the dis- tance from the alternator increased; the reason being that the pressure, or voltage, between the lines at the lamp terminals, would decrease as we receded from the (3 FIG. 27. DIAGRAM ILLUSTRATING THE FALL OF ELECTRIC PRESSURE OR VOLTAGE ALONG A CIRCUIT. alternator. This decrease in pressure of electricity flowing from an alternator, through a long conductor, finds its ana- logue in the decrease of the pressure of water flowing from a reservoir through a long pipe as shown in Fig. 28. If the reservoir supply water through a pipe, and pressure gauges be connected at dif- ferent distances, say 1, 2, 3, 4 and 5 miles, 102 ALTERNATING ELECTRIC CURRENTS. as shown, then, when the flow is entirely shut off at the distant end, assuming no leakage through the pipe, the gauges will all show the same pressure; but when the flow is fully established through the pipe, the gauge at the outflow, where the wa- El sH 5 FIG. 28. DIAGRAM ILLUSTRATING THE FALL OF HYDRAULIC PRESSURE ALONG AN OUTFLOW PIPE. ter escapes, will, owing to the loss of head, or drop of pressure, arising from the fric- ti^^^^^wat^J^jffie^^^gijS^ the lowest pressure. The pressure at inter- mediate distances between the reservoir and the outflow, will be intermediate be- tween the pressure at the reservoir and TRANSFORMERS. 103 the pressure at the outflow. Similarly, in an electric circuit, the resistance offered by the conductors to an electric flow produces a drop of pressure, so that under the conditions shown, the most dis- tant lamps will only receive say 70 volts, while the intermediate lamps will receive pressures intermediate between 110 and 70 volts. The fall of pressure depends on the size of the wire and the strength of the current required for each lamp. An ordinary incandescent lamp of 16- candle -power requires to be supplied, as already stated, with an activity of about 50 watts. Since, in the preceding case, the pressure is assumed to be 100 volts, each lamp would take approximately 1-2 ampere of current (100 volts x l-2ampere = 50 watts). If the lamp could be construct- ed so that it would properly operate when 104 ALTERNATING ELECTRIC CURRENTS. supplied with say l-20th of an ampere, or 10 times less current, the current supplied by an alternator to such lamps, under simi- lar conditions, would be 10 times less, and the drop of pressure in the mains would, therefore, be 10 times less, since the drop of pressure in any conductor, expressed in volts, is always equal to the current which it carries in amperes multiplied by its resistance in ohms. But such a 50- watt lamp, taking only l-20th ampere, would have to be designed for a pressure of 1000 volts (1000 volts x l-20th ampere = 50 watts). Such lamps can not be con- veniently made at the present time, and even if they could be made, 1000 volts is an unsafe alternating- electric pressure to introduce into a building. The only way in which this troublesome drop of pressure can be avoided, without the use of special apparatus, when the best arrangement of TRANSFOKMEKS. 105 wires has been adopted for the distribu- tion of light, is to decrease one of the fac- tors on which the value of the drop de- pends; namely, to decrease the resistance of the wires, by increasing their size and weight. In other words, we can always decrease the drop indefinitely, by increas- ing the size of the conductors indefinitely. But heavy conductors of copper are ex- pensive, and a point is soon reached when the distance, to which electric supply can be carried from a central sta- tion to lamps, is commercially impossible. Happily, however, the use of trans- formers with alternating currents renders it possible to obtain all the advantages of high-pressure transmission and yet read- ily to reduce such pressure to 50 or 100 volts within the building it is desired to supply. The corresponding conditions of 106 ALTERNATING ELECTRIC CURRENTS. hydraulic transmission are represented in Fig. 29 where a long pipe, PP, of small cross -section, carries water from a reser- voir R, at a high pressure and enters the FIG. 29. DIAGRAM REPRESENTING LONG DISTANCE WATER POWER TRANSMISSION THROUGH SMALL PIPE AT HIGH PRESSURE, WITH TRANSFORMATION TO LARGE PIPE, Low PRESSURE, LOCAL SYSTEM. high-pressure cylinder of a pump M, con- nected with a large, low-pressure cylinder of the pump M, which drives forward a large quantity of water from a local reser- TKANSFOEMEBS. 107 voir at a reduced pressure, through a large pipe p p, to the water motor in its vicinity. By such an arrangement, therefore, it is possible to transmit water power to a great distance by a small pipe, and yet deliver a large volume of water to a motor which is designed to be operated at a low pres- sure. In the same way, by the use of alternating currents in connection with transformers, it is possible to obtain all the advantages to be derived from the transmission of high pressure electric cur- rents over small wires, and yet so trans- form or change the pressure at the point of consumption as to permit the use of incandescent lamps that will only oper- ate economically under low pressures. It has already been pointed out that the value of the electrical activity transmitted by any circuit when the power factor is 108 ALTEBNATItfG ELECTBIC CUKKENTS. 100 per cent, or unity, is equal to the prod- uct of the amperes multiplied by the volts, and it is clear that a small electric current, carried at a high pressure, say 10 amperes at 1000 volts, would give the same amount of activity, namely, 10 KW., as would a current of 100 amperes at 100 volts, but would require a much smaller wire. An alternating -current transformer is a device for enabling electric energy to be economically transmitted at high pressure and low current strength, to the point of delivery, and then reducing or transform- ing this supply to a large current at a cor- respondingly lower pressure. Let us inquire into the means where- by a transformer is capable of performing this important function. To do this we will first examine its construction. The TRANSFORMED. 109 alternating- current transformer consists essentially of two coils of wire, one usually coarse and the other fine, the fine wire coil being of much greater length and hav- ing a greater number of turns than the coarse wire. Fig. 30 shows one of the sim- FIG. 30. SIMPLE FORM OF ALTERNATING- CURRENT TRANSFORMER. t ' plest forms of transformers. It consists, as shown, of two coils of wire P and S, wound on a core of iron wire. When an alternating current is sent through coil P, called the primary coil, it will, by induc- tion, produce an alternating E. M. F., of the same frequency, in the coil S, which 110 ALTERNATING ELECTRIC CURRENTS. is called the secondary coil, and this second- ary E. M. F. is employed to send an alter- nating current through the lamps or other apparatus which are to be operated. In the case supposed, the high-pressure cur- rent would be sent through the- primary coil P, whose terminals are connected to the line, and the low-pressure current would be induced in the secondary coil S, whose terminals are connected as shown with the apparatus to be operated. The alternating- current transformer operates as follows: On the passage, of the alternating current through the pri- mary coil P, the coil become alternately magnetized in opposite directions; that is to say, its loops become successively filled and emptied with an oscillating magnetic flux. The coil thereby has a counter E. M. F. set up in it, or, in other TRANSFORMERS. Ill words, acts as a choking coil. At the same time, the flux through the iron core successively fills and empties the second- ary coil S, and thereby induces in it an E. M. F. which will alternate at the same frequency as that in the primary. If the circuit of the secondary coil is open; i. e., disconnected from its apparatus, the pres- ence of this secondary E. M. F. will not affect the reactance or choking effect of the primary coil; but if, on the contrary, the secondary circuit be closed through its load of lamps, motors, or other appa- ratus, the current in the secondary coil will tend to magnetize the core in the op- posite direction to that of the primary coil, and so diminish the reactance of the primary winding. The choking effect of the primary coil will, thereby, be reduced as the secondary current and the load in- crease. In other words, the transformer 112 ALTERNATING ELECTRIC becomes self -regulating, the choking ef- fects of the primary coil automatically varying so as to permit the right amount of current and power to be received from the high pressure mains, in order ade- quately to supply the secondary or low pressure consumption circuit. Let us now examine the pressures which exist in the primary and secondary circuits. If each coil P and S 9 has the same number of turns, the E. M.. F. in- duced in the secondary will be practical- ly the same as that supplied or impressed upon the terminals of the primary, so that there would be no transformation or change as regards pressure and current. If, however, the secondary coil is made up of but half the number of turns in the primary coil, the flux passing through the iron core only links with half the number TKANSFORMEKS. 113 of secondary turns that it links with in the primary coil, and the E. M. F. in- duced in the secondary will be but half .as great as that in the primary. If the primary impressed E. M. F. were 1000 volts effective, that in the secondary cir- cuit would be about 500 volts. Again, if the secondary coil contain say one tenth of the number of turns existing in the primary coil, then its E. M. F. would be correspondingly reduced and would become approximately 100 volts. If in this case the wire forming the secondary coil were maintained of the same diameter as that in the primary coil, the small sec- ondary coil would, for the same electrical activity in each circuit, have to carry ten times the current strength which is sup- plied to the primary. It would be neces- sary, therefore, to increase the cross- section of the secondary coil, say ten 114 ALTERNATING ELECTRIC CURRENTS. times, so that the bulk of the two will, in practice, be approximately the same. It is evident that if the coil S, assumed in the last condition to contain one tenth of the number of turns in the coil P, could be connected to the high-pressure termi- nals, or, in other words, be employed as the primary, that the coil P, would have an E. M. F. induced in it, whose value would be ten times as great as that in the mains. Transformers may, therefore, be divided into two sharply-marked classes; namely, step-down transformers, where the pressure in the secondary is less than the primary pressure, and step -up transform- ers, where the pressure in the secondary is greater than the primary pressure. In actual practice, transformers are not built in the exact manner shown in the last figure. The primary and secondary coils TEANSFOEMEES. 115 may be variously disposed as regards each other, but in all cases they are brought as close together as possible, and are so surrounded by laminated iron as to cause the flux produced by the primary to pass through or become linked with all the turns of the secondary. Since the coils may assume various positions, it is evident that different types of transform- ers may differ radically in their appear- ance. They will, however, all possess the same essential features; namely, pri- mary and secondary coils, and a laminated iron core common to both. Fig. 31 represents a laminated iron core C, of sheet iron stampings, having a form resembling that shown in Fig. 32, within the hollow spaces of which are inserted the two coils P and S 9 one being the primary coil of say 1000 turns of fine wire, and the 116 ALTERNATING ELE CTKIC CURRENTS. other the secondary coil (for convenience divided into halves) with a total of say 100 turns of coarser wire. Since the primary coil may be connected to mains at say 1000 volts pressure, and is in close juxta- position to the secondaiT coil, from which wires are carried into the building to be FIG. 31. TRANSFORMER SHOWING INTERNAL CONSTRUCTION. supplied by the current, it is evident that the insulation of the two coils from each other must be carefully preserved, since, otherwise, the pressure of 1000 volts might be led into the building. In order to en- sure a high degree of insulation, the coils are sometimes immersed in an insulating TRANSFORMERS. 117 oil. TJie transformer coils shown in Fig. 31 at A, are placed in the iron vessel shown at B, which is then filled with oil. Another form of oil-insulated, step-down transformer is shown in Fig. 33. Here the FIG. 32. SHEET IRON STAMPING. For Transformer Shown in Fig. 31. primary coil has its ends brought out at p, p, and its secondaries at s, s, divided, as before, into two halves for convenience. This transformer is enclosed as shown in Fig. 34, in a box filled with oil, the pri- 118 ALTERNATING ELECTRIC CURRENTS. mary terminals being brought out through the fuse-box at P and P, and the second- ary terminals at S and S. In order to prevent the current gener- FIG. 33. 100-LiGHT TRANSFORMER WITHOUT Box. ated in the secondary circuit from becom- ing dangerously great, should an accident- al short-circuit occur in the wires of the TRANSFORMERS. 119 building supplied, a device called & fuse- block is employed with transformers. This device consists of an iron box con- taining lead fuse wires which are inserted FIG. 34. 100-LiGHT STANDARD TRANSFORMER. + in the primary circuit, so that the cur- rent from the high-pressure mains, in or- der to reach the primary coil, has to pass through these fuse wires. The fuse wires 120 ALTERNATING ELECTKIC CURRENTS. are composed of a lead alloy of such size that they carry safely the normal working current of the transformer, but, on an undue excess of current, become so heated as to melt, and open the circuit, thus automatically disconnecting the transformer from the mains. The porce- FIG. 35. FUSE-BOX AND FUSES. lain or earthenware fuse -block is shown in Fig. 35 at B, with a fuse wire WW, laid across it, having its ends clamped under connection screws. The box is TRANSFORMERS. 121 provided with the lid L, so that when the fuses have been melted or ''blown'" new wires can be readily inserted. Another form of transformer fuse-box is FIG. 36. DETAILS OP TRANSFORMER FUSE-BOX. shown in Fig. 36, detached from its trans- former case. Here, two unglazed porce- lain handles H, H, are inserted by hand into two separate porcelain apartments in an iron box. Within these compartments 122 ALTERNATING ELECTRIC CURRENTS. are brass contact pieces, only one of which S 19 is visible in the figure, so arranged that when the handle H 9 H, is pressed home into the compartment, connection is maintained between them through the fuse wires, W 9 W 9 clamped between bind- ing posts T, T, and connected with flexible plugs S 9 S 9 which fit into the receptacles S l . The lid L, is provided for closing the box. The advantage of this particular form is that when the handles are pushed in, thus connecting the transformer with the high-pressure mains P, P, the sudden or explosive fusing of the wire cannot injure the operator, whose hand is pro- tected by the back of the handle H. Fuse wires are also inserted in the sec- ondary circuits of the transformer, some- times in the transformer itself as at S 9 in Fig. 26, and, sometimes, in separate fuse- boxes within the building. 123 We have seen in Figs. 31, 33 and 34, that the secondary coils are divided into two separate halves. The advantage of this method lies in the fact that some houses have their lamp circuits wired for 50 volts pressure, and others for 100 volts pressure. If now, the coils of each of the two separate circuits of such a trans- former, having a pressure of 50 volts, are so arranged that the current passes from one secondary coil through the next in succession, so that the two coils are con- nected as though they formed an unbro- ken winding, then their E. M. F. 's will be added, making a total of 100 volts. On the contrary, if it be .desired to use a pressure of but 50 volts, then the two coils are employed side by side, or so con- nected to the house wires that each of the coils supplies half the current delivered. Such connections are shown in Fig. 37. 124 ALTERNATING ELECTRIC CURRENTS. At A, 100 volts are obtained for the sec- ondary circuit by connecting the two coils in series, as it is called, so that the ar- rows represent the direction of the cur- rent at some particular instant. At J5, FIG. 37. METHOD OF CHANGING SECONDARY CONNECTIONS. 50 volts are obtained by the parallel con- nection of two coils, or, as it is sometimes called, by their connection in multiple. If at A, the transformer is delivering 10 am- peres at 100 volts pressure, or 1000 watts, at B, it will be delivering 10 amperes in TRANSFORMERS. 125 each coil, or 20 amperes in all, at 50 volts pressure, and, therefore, also 1000 watts* In Fig. 31. the capacity of the transform* FIG. 38. OUTDOOR TYPE OF TRANSFORMER. er represented is 600 watts, or such as is capable of furnishing current for opera- ting 12 fifty-watt lamps. That in Figs. 33 and 34, 5000 watts, or 100 such lamps. 126 ALTERNATING ELECTRIC CURRENTS. A still larger transformer of 7500 watts capacity, or capable of operating 150 fifty- watt lamps, is represented in Fig. 38. This particular transformer is not insu- lated with oil, but depends upon the in- sulating covering of its coils for protec- tion, the free space within the cover or iron shield being filled with air. The current required to supply a trans- former at full load may readily be ascer- tained when the primary pressure is known. For example, in the case of a 7500 -watt transformer, if the primary pres- sure is 1000 volts, the primary current must be 7 1-2 amperes (1000 volts x 7 1-2 amperes = 7500 watts), if we assume that the primary power factor is 100 per cent, and that no loss occurs in the transform- er. Strictly speaking, the power factor, even at full load, is not quite 100 per TRANSFORMERS. 127 cent., and a little loss of energy occurs in the transformer; i.e., the transformer be- comes warm in doing its work, so that the current strength supplied from the primary circuit at full load must be somewhat in excess of 7 1-2 amperes. PIG. 39. 500-LiGHT TRANSFORMER, INDOOR TYPE. A form of 25,000-watt transformer (25 KW. or about 33 H. P. ) intended for 500 fifty-watt, 16-candle-power incandescent lamps, is represented in Fig. 39. This transformer is intended to be located in a cellar, or other suitable place within doors. 128 ALTERNATING ELECTRIC CURRENTS. It will be seen that as the capacity of the transformer increases ; i. e. , as the transformer has to supply more and more power, its dimensions increase, but not in the same proportion as the increase in capacity; so that if a 1 KW. or 20-light transformer weighs, in its case complete, 140 pounds, or gives 7 watts per pound of total weight, a 25 KW. transformer will, probably, weigh only 2000 pounds, or give 12 1-2 watts per pound, while a 200 KW. transformer will, perhaps, give 25 watts per pound, and a 1200 KW. trans- former 100 watts per pound. It is much cheaper, per kilowatt of output, to con- struct transformers in large sizes. When a step -down transformer is em- ployed to reduce the pressure in a build- ing from 1000 to 100 volts, it is clear that at the central station supplying the mains TRANSFORMERS. 129 leading to the building a generator must be employed of 1000 volts E. M. F. or more. This is commonly the case, and alterna- ting-current generators in the U. S. gen- erally produce either about 1000 or 2000 volts effective at their terminals. When, however, the current has to be trans- mitted over lines of great length, and it is necessary, for purposes of economy in conductors, to employ much higher pres- sures, say 10,000 volts, it is desirable, both on the score of safety and economy, to employ a step-up transformer, supplied by the generator at a lower pressure, rather than to endeavor to construct a generator to directly develop such pres- sure. In such cases, of course, these step -up transformers would be connected directly to the alternator terminals. Large step -down transformers, intended 130 ALTERNATING ELECTRIC CURRENTS. to supply an extended system of mains, are frequently installed in a small sub-sta- tion, for which reason they are sometimes FIG. 40. SUB-STATION TRANSFORMER. called sub -station transformers. Since such transformers may take the entire load of a large alternator, they necessarily re- TKANSFOEMEKS. 131 quire to be of considerable dimensions. A form of such transformer is shown in Fig. 40, its length being about 6 feet. In de- signing such transformers care is taken to provide for the dissipation of the heat generated in their iron core and conduct- ors when in action. Here the laminated core, consisting of large, thin sheets of iron, forming the frame or body of the ap- paratus, CC, is closely linked with the coils, c, c, c, c. The whole apparatus is carefully ventilated to permit of the free access of air and the insulation of the coils carefully preserved by means of sheets of mica. Another advantage secured by the use of a few large transformers in place of a number of smaller ones is a greater effi- ciency. A large transformer in the course of its daily duty will probably supply, to its secondary circuit, 96 per cent, of the energy it receives at its primary terminal, 132 ALTERNATING ELECTRIC CURRENTS. only 4 per cent, being lost in the trans- formers. The same amount of power being distributed by a number of small transformers might perhaps result on the average to a delivery of 80 per cent, and a loss of 20 per cent. In other words a small transformer wastes proportionate- ly more energy than a large one. CHAPTER VI. ELECTKIC LAMPS. HAVING examined in the previous chap- ters the method of generating alternating currents, the means employed for their distribution, and the apparatus by which their strength can be varied, it remains to discuss some of the different types of electric apparatus to which such currents are supplied. These are of a variety of forms, but the most important, at the present time, are lamps and motors. When ah electric current is sent through a conductor of high-resistance and small cross-section, so that a considerable amount of electric energy is expended in a smal] mass of material, the conductor is 134 ALTERNATING ELECTRIC CURRENTS. heated, perhaps, to the temperature of luminosity, when it will emit light and heat. This is the principle on which an incandescent electric lamp is operated; a short thin filament or thread of carbon forming the high-resistance conductor. The carbon filament acquires its high temperature in a fraction of a second after the current has been sent through it, as can be determined by observing the time which elapses from the closing of the cir- cuit by turning the key or switch of an incandescent lamp until the lamp gains its full incandescence. In the same man- ner, on the interruption of the current by the opening of the circuit, an equally short time is required for the lamp to lose its brilliancy especially in slender filaments. In an alternating -current circuit, the current not only changes its strength but ELECTRIC LAMPS. 135 also changes its direction, during the dif- erent parts of an alternation. Conse- quently, twice in each cycle, at the mo- ment when the change of direction occurs, there can be no current in the circuit, as will be evident from an inspection of Fig. 6. When alternating currents are supplied to incandescent lamps at a fre- quency of 100 cycles per second, it is evi- dent that 200 times in each second there is no current passing through the lamp. It might, therefore, be supposed that the lamp would go out and be relighted 200 times a second. In reality an incandes- cent lamp tends to do this, and would do it were it not for the fact that the intervals of cessation of current are so brief that the lamp has not sufficient time in which to appreciably cool down, so that such changes of temperature as do* occur, are not visible to the eye, and the lamp does 136 ALTERNATING ELECTRIC CURRENTS. not visibly flicker. In order, however, to obtain this absence of flickering a certain frequency of alternation is necessary; for, it is evident that if the frequency becomes very low, sufficient time will elapse, be- tween the current waves, to permit the carbon to sensibly decrease in brightness, thus permitting the retina of the eye to retain the impression of flickering. It has been found, in practice, that flickering in an incandescent lamp does not occur when the frequency of the alternation exceeds 30 to 35 cycles per second. In practice, in the United States, alternators for incandescent lighting are usually de- signed to produce a frequency much high- er, say from 125 to 135 cycles per second. When the energy from an electric cur- rent is utilized in an incandescent lamp, by far the greater part is uselessly ex- ELECTKIC LAMPS. 137 pended in producing heat, or non-luminous radiation. It has been found that a com- paratively slight increase in temperature will cause a marked increase in the amount of light emitted by a glowing filament. Consequently, the commercial efficiency of a lamp that is its ability to convert electrical energy into light en- ergy,will be greatly increased, by any cir- cumstance which will safely permit of an increase of temperature of its filament. This can readily be shown by applying suc- cessively increasing pressure or voltage to the terminals of a lamp, and so causing greater current to flow through it, the in- crease in the current being followed by a marked increase in the amount of light given off. Were it possible to double the ordinary working temperature of the fila- ment of an incandescent lamp, without destroying it, we would very markedly in- 138 ALTERNATING ELECTRIC CURRENTS. cease its light-giving power. In point of fact even a slight increase above the or- dinary temperature produces a great in- crease in the brilliancy of the lamp. But while an improvement is thus ob- tained in the light-giving power of a lamp, the life of the lamp, or the number of hours during which it will continue to give out this light, is greatly diminished. The problem for increasing the efficiency of an incandescent lamp has, therefore, been to obtain a conducting substance which would continuously stand a high temperature. Carbon is the only sub- stance which has, thus far, been found available for commercial use. There is a certain temperature at which it is found most economical to operate carbon fila- ments, both in regard to their amount of light and duration of life. Below this tern- ELECTRIC LAMPS. 139 perature, while the life greatly increases, the candle-power rapidly falls off. An in- candescent lamp, burning at dull red tem- perature, will have an indefinitely long life- time, while a similar lamp, operated at the ordinary temperatures commercially em- ployed, will burn from 600 to 1800 hours. i Since the filaments of incandescent lamps are made of various lengths and cross -sections, or, in other words, since their filaments have varying electrical re- sistances, the pressures required to pro- duce in them the requisite temperature will necessarily vary. In practice, lamps are constructed which require pressures varying from 2 volts to 250 volts. High- pressure lamps, of any given candle-power, have long, thin filaments, while low-pres- sure lamps, of the same candle-power, have short thick filaments. 140 ALTERNATING ELECTRIC CURRENTS. Various forms are given to incandescent lamps, but all consist essentially of the same parts; namely, an incandescing fila- ment of carbon placed in an exhausted glass chamber and connected with the cir- FIG. "1. 16-C.P. INCANDESCENT LAMP. cuit by a socket, the wires leading the cur- rent into the lamp being automatically connected with the circuit by the act of inserting the lamp in its socket. Some forms of incandescent lamps are shown in Figs. 41, 42, 43 and 44. Fig. 41 is a form of 16 -candle -power lamp in exten- sive use, consisting of a filament bent in a ELECTRIC LAMPS. 141 single loop. The lamp base is provided with a screw thread for insertion in the FIG. 43. INCANDESCENT LAMPS AND SOCKET. 142 ALTERNATING ELECTRIC CURRENTS. screw socket. Fig. 42 represents another form of lamp, in which the screw thread is FIG. 43. INCANDESCENT LAMPS. in the interior of the base, instead of on the external surface. This figure ELECTKIC LAMPS. 143 also shows a lamp inserted in the socket which is provided with a key K. Figs. 43 and 44 show another form of incandescent FIG. 44. INCANDESCENT LAMPS. 144 ALTERNATING ELECTEIC CURRENTS. lamp furnished with different bases ; a is intended to give 10 candle-power, b, 16 candle-power, c, 20 and d t 32. All the incandescent lamps here shown are equally applicable for use on contin- uous or alternating- current circuits. In practice, where the area of distribution to consumers is not great, the continuous current is usually employed, but where the area of distribution is large, and the lighting scattered, it is usually more economical to use aj^rna_ting currents in connection with step -down transformers. Incandescent .lamps as supplied from step-down transformers are always con- nected in parallel, that is, the lamp's termi- nals are connected across the mains as shown in Fig. 45, which represents a two- wire system of distribution. Sometimes, ELECTKIC LAMPS. 145 however, the lamps are connected as shown in Fig. 46, where the 20 lamps shown are connected between three wires of the three-wire system of distribution represented. x FIG. 45. Two- WIRE SYSTEM OF MULTIPLE CONNECTED LAMPS. In cases, however, where incandescent lamps are required for street lighting over an extended area, where the lights are, therefore, scattered, the systems of dis- 146 ALTERNATING ELECTRIC CURRENTS. tribution shown in Figs. 45 and 46, are too expensive, and it is also too expensive to employ a special or separate trans- former for each lamp post. In this case the method of distribution sometimes FIG. 46. THREE-WIRE SYSTEM OF MULTIPLE CONNECTED LAMPS. employed is that represented in Fig. 47, where the lamps are connected in series, the current passing successively through ELECTEIC LAMPS. 147 each lamp. In the method of distribu- tion shown in Figs. 45 and 46, the failure of any one of the lamps to operate, as, for example, by the breaking of its filament, v FIG. 47. SERIES DISTRIBUTION OF INCANDESCENT LAMPS WITH ALTERNATING CURRENTS. does not affect the supply of current to the other lamps. When, however, the lamps are connected in series, the discon- tinuity of one lamp would open the entire circuit were it not for the small choking 148 ALTERNATING ELECTEIC CUEEENTS. coil which is placed as a shunt or by-path to each lamp. While the circuit is main- tained through the lamps, very little cur- rent passes through the choking coil, so that the waste of current and energy through the latter is very small. If, how- ever, the lamp breaks its circuit, the choking coil carries the current without appreciably affecting the supply to the rest of the lamps in the circuit. These choking coils are represented in Fig. 47 as being connected around the terminals of each lamp. Fig. 48 represents such a street lamp with its choking coil. Here the lamp is provided with an external shade and globe to protect it from the weather. Fig. 49 gives a more complete view of the choking coil. Alternating currents are also employed for arc lighting. As in the case of incan- ELECTRIC LAMPS. 149 descent lighting, in order to prevent the variations in the current strength from producing marked flickering in the light, a certain frequency is necessary. It has been found in practice that the arc lamps FIG. 48. COMBINED FIXTURE AND REACTIVE COIL. will show no disagreeable flickering if the frequency exceeds 45 cycles per second. Alternating -current arc lamps do not differ in general construction from continu- ous-current arc lamps, save in the details 150 ALTERNATING ELECTKIC CURRENTS. of their regulating mechanism. Since, however, the upper and lower carbons be- FIG. 49. STREET LAMP REACTIVE COIL. come alternately positive and negative, the rate of consumption of each carbon is sensibly the same. A form of arc lamp, ELECTRIC LAMPS. 151 FIG. 50. ALTER- NATING-CURRENT ARC LAMP. 152 ALTERNATING ELECTEIC CURRENTS. suitable for use for an alternating-incan- descent circuit of either 50 or 100 volts pressure, is shown in Fig. 50. In circuit FIG. 51. REACTIVE COIL OR COMPENSATOR FOR ARC LAMPS ON ALTERNATING-CURRENT CIRCUIT. with the lamp or lamps is connected a choking coil or compensator, as shown in Fig. 49, whose object is to regulate au- tomatically the amount of current passing through the lamp. CHAPTER ELECTRIC MOTORS. IT is a well-known fact that when a con- tinuous electric current passes through a continuous -current generator at rest, the generator will be set in motion. The ear- ly history of this discovery still remains in some doubt. It is claimed that the first observation of this power of a dynamo to act as a motor, or, in other words, this reversibility of the dynamo, was the result of an accident, which occurred during the Vienna Exhibition of 1873, when the cur- rent of one generator was accidentally led through the circuit of a second generator. According to, perhaps, more credible ac- counts, this property was the direct re- sult of research in 1867. However this 154 ALTERNATING ELECTRIC CURRENTS. may be, the first dynamo that was ever publicly exhibited running as a motor, from the current supplied by a similar dynamo, was at the opening of the 1873 Vienna Exhibition. It is a well-recognized scientific princi- ple that work is never lost or. in other words, that the total amount of energy ex- isting in the universe is constant. Work may be made to assume different forms, but can never be annihilated. When, for example, mechanical work is expended in driving a dynamo, apart from certain ex- penditures, all this work is transformed in- to electrical work. When this electrical work is properly applied to the armature of another generator standing at rest, the electrical work is transformed into me- chanical work, as is evidenced by the abil- ity of the motor to drive machinery. We ELECTBIC MOTORS. 155 have seen that a horse -power is equal to an activity of 746 watts. Consequently, if the electric motor were a perfect ma- chine; i. e., wasted no power, it would take 746 watts, from the circuit supply- ing it, for every horse -power it exerted in its work; and, if operated at a pressure of 100 volts at the mains, would, therefore, receive 746-100=7.46 amperes, per horse- power delivered. Owing to the necessary losses of energy in the motor, a greater current strength than this will in practice be needed, perhaps, 10 amperes, depend- ing, however, upon the size of the motors. Large electric motors frequently possess a very high efficiency; i. e. 9 their output in mechanical work is very nearly equal to their intake in electrical work. Since, as we have seen, a motor can readily be driven at a long distance from the genera- tor supplying it, is evident that the elec- 156 ALTEENATINO ELECTEIC CUEEENTS. trical transmission of power possesses marked advantages. An example of a continuous -current motor is shown at Fig. 52. PlG. 52. CONTINUOUS-CUKKJSJNT STATIONARY MOTOR If two continuous -current generators, similar in all respects, be electrically con- nected by a circuit say one mile in length, one being driven by a steam en- gine as a generator, while the other is ELECTKIC MOTORS. 157 running at the same speed as a motor, then, as we have already seen, the current is alternating in the armature of each ma- chine, but, owing to the action of the commutator, is continuous in the line between them. Assuming the two ma- chines to be running at the same speed, if the commutators are suddenly removed from each, the two machines will continue running, though the current on the line, as well as the current through the arma- tures, will now be alternating. The two machines, which must now be regarded as alternating- current machines, will still be acting as generator and motor, or a the driving and the driven machine. In this respect, therefore, continuous and alternating -current dynamos are alike; since, in either case, one acting as the generator can drive the other as the 158 ALTERNATING ELECTRIC CtJRRENTS* motor. They differ, however, in this re* spect, that, whereas, in the case of the continuous ^current circuit, the motor will start from a state of rest, and can be driv- en either at the same speed as the gener- ator or at different speeds; in the case of the alternating -current circuit, the motor will not start from a state of rest and can not be operated until it has been brought up to the same speed as the generator; or, as it is usually termed, until it has been brought into step with it. Once the motor has been brought up to the speed of the generator, it can, if well designed, be made to take its full load mechanically and electrically, without falling out of step. Since such an alternating- current motor will not operate unless it is run- ning at the same speed as the driving alternator, it is called a synchronous motor. ELECTRIC MOTORS. 159 When synchronous motors are em- ployed, it is, therefore, necessary to de- vise some means whereby they can be brought up to their normal speed before they are connected with the circuit sup- FIQ. 53. 250-H.P. ALTERNATING - CURRENT SYNCHRONOUS MOTOR plying them. Various devices have been proposed for this purpose. The one in most general use is that shown in Fig. 53. Here the synchronous alternating- current 160 ALTEBNATING ELECTBIC CUEBENTS. motor S, of 250 H. P., is intended to drive machinery by the pulley P, through the clutch C. In order to start the motor, the clutch is opened, and a small motor M, called a diphase motor, which will be de- scribed in a subsequent chapter, is operat- ed, and drives the large motor armature through the friction pulleys Q and R. As soon as the armature A, has, in this way, been brought up to speed, the small mo- tor M, is disconnected, and the armature A, is connected with its circuit, when it takes alternating currents, and is ready to receive its load as a synchronous motor. The clutch C 9 is then thrown in, rigidly connecting the motor shaft with the driv- ing pulley P. Finally, the small driving motor M, is moved back by the handle H, so that its pulley Q, is out of contact with the pulley R. The armature A, re- ceives its current through the contact ELECTKIC MOTORS. 161 rings 6r, 6r, at the end of its shaft, and, by means of the commutator K, s applies the continuous currents required for the ex- citation of its own field magnets, in the same manner as though it were a self- excited generator. FIG 54. ALTERNATOR WITH SYNCHRONOUS MOTORS. Fig. 54 represents a 3000 -volt alterna- tor, suppling two synchronous motors di- rectly from the same pair of mains, the starting motors 'not being shown. The pressure at the brushes of these motors is marked as being 3000 volts effective, 162 ALTERNATING ELECTRIC CURRENTS. representing about 4200 volts at the peak of each alternation of pressure. II WtMHUff I I I I I I I I 11 I FIG. 55. ALTERNATOR WITH TRANSFORMER AND ITS SEC- ONDARY CIRCUIT. Fig. 55 represents an alternator A, sup- plying a pair of high-pressure mains M, M, and a primary coil P, of a transformer T, ELECTKIC MOTOKS. 163 whose secondary coil is connected to the arc lamp L, incandescent lamps /, /, and a synchronous motor SM, all operated in parallel. It is evident that since a synchronous motor has only one speed of rotation and requires some appreciable time to start from rest by auxiliary means, that it is unsuited to machinery which requires to be operated at varying speeds and for intermittent periods. For all purposes, however, where the power is required continuously, or for many hours a day at a steady rate, as, for example, in pumping or driving large counter shafts in a ma- chine shop, the synchronous motor is a very useful machine, Up to the present time no single-phase alternating- current motor, of say more 164 ALTEENATING ELECTKIC CUKKENTS. FIG. 56. ONE-EIGHTH H.P. ALTERNATING- CURRENT FAN MOTOR. ELECTRIC MOTORS. 165 than half a horse-power in capacity, has yet been produced in the United States, which is capable of starting at fall load, from FIG. 57 ALTERNATING -CURRENT FAN MOTOR. rest, on ordinary alternating circuits, and which will run with a reasonable amount of economy. There are, however, a num- 166 ALTERNATING ELECTRIC CURRENTS. ber of small alternating- current motors, some of which operate with the aid of a commutator, as, for example, the fan mo- tor, shown in Fig. 56. Here the current through the fields is reversed at every al- ternation of the alternating current, but by means of the commutator, the effect of this reversal of magnetism is reversed upon the armature current, and a contin- uous magnetic pull produced. Unfortu- nately the efficiency of such machines is comparatively small, so that they are only capable of being employed in small sizes, where economy is not of much impor- tance. Another form of alternating- cur- rent motor of this type is seen in the fan motor shown in Fig, 57. CHAPTER VIII. MULTIPHASED CURRENTS. THE difficulty pointed out in the last chapter, as regards the starting of syn- chronous motors, has led to a special de- velopment in alternating-current appara- tus called multiphase apparatus. The synchronous motor is supplied by a single alternating current. The multiphase mo- tor is supplied by more than a single cur- rent. In practice either two or three cur- rents are employed for driving multiphase motors, thus giving rise to diphase motors, which are supplied by two separate alter- nating currents, and triphase motors, which are supplied by three separate currents. Multiphase motors, therefore, require spe- cial generators for the production of the 168 ALTERNATING ELECTEIC CURRENTS. currents they employ. We shall now pro- ceed to discuss the construction and op- eration of diphase and triphase generators. It must first be remarked that in a di- FIG. 58 RELATION" BETWEEN Two DIPHASE ALTERNATING CURRENTS. phase motor, for example, it is not suffi- cient to simply supply to the motor any two, separate, alternating currents. The MULTIPHASED CURRENTS. 169 proper operation of the motor requires that the two separate currents shall pos- sess a certain relationship to each other; namely, that one shall be a quarter of a cycle in advance of the other, as shown in Fig. 58. A diphase generator, therefore, must be constructed not only so as to produce two equal separate alternating currents, but these alternating currents must also have a quarter of a cycle of phase difference between them. Such a condition will enable the motor to start, as well as to preserve a uniform pull or torque upon its driving shaft. A diphase motor is driven by two sepa- rate series of electrical impulses one quar- ter cycle apart. This condition finds an analogue in the ordinary steam locomo- tive, which, as is well known, is driven by two separate -steam cylinders placed on 170 ALTERNATING ELECTRIC CURRENTS. opposite sides of the driving engine. In the early history of the steam locomotive, when but a single cylinder was used, it was found, at times, that the engine could not be started from a state of rest, since it had stopped on a dead centre, and re- quired, like the synchronous motor, to be started before it could by driven. This difficulty, as is well known, is now obviated by the use of two pistons, set at a quarter of a cycle, or 90 apart. In order to obtain two separate alterna- ting E. M. F. 's, a quarter cycle apart, in two separate circuits, either two separate windings are employed on a single arma- ture, or two separate armatures are rigid- ly connected and driven on the same shaft. The latter method is represented in Fig. 59, where a 750 KW. or 1000 H. P. diphase generator is shown. This genera- Mtri/riPHASED CUBBENTS. 171 tor consists of two complete uniphase gen- erators A and B; I. e., generators of the ordinary single alternating -current type, FIG. 59. 750-KiLOWATT COLUMBIAN EXPOSITION, DIPHASE ALTERNATOR. rigidly connected together in such a man- ner that the armature of one machine is just far enough ahead to produce its alter- 172 ALTERNATING ELECTRIC CURRENTS. nating E. M. F. a quarter of a cycle in ad- vance of that of the other. This machine is compound-wound, supplying its field magnets partly from the commutator (7, and has three collector rings R, R, R, one of the outside rings for each current and the middle ring, as a common con- F IG . go DIAGRAM SHOWING THE Two METHODS OF CONNECT- ING DIPHASE ARMATURE WINDINGS THROUGH COLLECTIVE RINGS WITH EXTERNAL CIRCUITS. nection for both, as shown in Fig. 60. The belt tightening handle is shown at H. Another form of diphase generator is shown in Fig. 61. Here a single armature has two windings, the E. M. F. in one of which is developed a quarter of a cycle MULTIPHASE!) CURRENTS. 173 before the other. The three conductors A , B, C, carry off the two diphase cur- rents, while the conductors F, F, supply FIG. 61. TOO- KILO WATT MULTIPHASE GENERATOR. the field with a continuous current. The commutator C, supplies current to the field magnets. Another form of diphase generator is 174 ALTERNATING ELECTRIC CURRENTS. represented in Fig. 62. Here two sepa- rate external armatures A and B, do not revolve, while within them revolves the field magnet driven by a pulley P. The FIG. -DiPHASE ALTERNATING-CURRENT GENERATOR. E. M. F. in one armature, say A, is de- veloped a quarter of a cycle, or half an alternation, ahead of that in B. MULTIPHASED CUEEENTS. 175 The circuits of such a diphase generator require, as shown in Fig. 60, either three or four wires. If four wires are em- ployed, the two separate circuits are en- tirely distinct, while if three wires are employed, one of the conductors is com- mon to both circuits. TWO PHASE ALTERNATING CURRENT GENERATOR FlG. 63. DlPHASER AN ITS CIRCUIT. In Fig. 63, a diphase generator or di- phaseris represented at A. The two sep- arate currents, generated in this machine, are led to the transformers T, , T t , T 3 , T 4 , through the three wires of the circuit. The pressure at the generator brushes is 2000 volts effective, beween C and D, or between D and E. The transformers 71 , 176 ALTERNATING ELECTHlC CtJBRENTg, T z and T 4 are connected between a single pair of wires; namely, Z!, , between C and Z>, T 3 between D and E, and jT 4 be- tween D and ,/, so that only one current is supplied to each of these transformers. In all cases, where diphase currents are not to be used simultaneously in a motor, they are separately used as uniphase cur- rents either in lamps or in synchronous motors. T 4 is a transformer on one of the circuits supplying arc lamps L,L, at a pressure of, perhaps, 50 volts. The trans- former JJ , which is really a double trans- former, half between the wires C and D, and half between the wires D and E, sup- plies in its secondary circuits G and //, diphase currents to the diphase motor M. A triphase generator or triphaser is a gen- erator which produces three separate al- ternating E. M. F.'s separated from each MULTIPHASED CURRENTS. 177 other by one third of a cycle, as repre- sented in Fig. 64. Such a machine is shown in Fig. 65. Here the armature has three separate windings upon it, so ar- ranged that the E. M. F. 's generated in FIG. 64. DIAGRAM REPRESENTING PHASE RELATION OF TRIPHASE WAVES OF E. M. F. AND CURRENT. them succeed each other by one third of a cycle. Three collector rings R 1 , R 2 , R 3 , on the right hand armature on the shaft, carry off the current as shown in Fig. 66, to three wires, AA\ BB\ CC\ each of 178 ALTERNATING ELECTRIC CURRENTS. which serves as a return circuit for the other two. The motor windings, transformers, or other devices are connected between the FIG. 65. 500-iLOWATT TRIPHASE GENERATOR. wires as at A 1 B\ B 1 C\ or C 1 A\ Triphasers possess electrical features which have gained for them considerable favor. A triphaser only requires three wires for its MULTIPHASED CURRENTS. 179 three currents. A diphaser requires four wires but can be operated with three. Beside the diphase and triphase gener- ators another system has come into recent FIG. 66. DIAGRAMS REPRESENTING CONNECTIONS OF TRIPHASE WINDINGS WITH THEIR EXTERNAL CIRCUITS. favor, called the monocyclic system. The monocyclic generator, or monocycler, is primarily a uniphase generator, and is in- tended principally for the delivery of or- dinary alternating or uniphase currents, 180 ALTERNATING ELECTRIC CURRENTS. over a system of electric lighting mains. In order, however, to supply starting al- ternating-current motors wherever they may be installed in the system, a special series of coils, of smaller size and cross - section, is placed on the armature so as to produce a small E. M. F. a quarter cycle out of step with the main uniphase E. M. F. This smaller E. M. F. is connected to a third collector ring on a special circuit wire, called the power wire, which has a smaller cross -section than the main uni- phase wires, and is led only to where the motors are to be used. By the use of two transformers, connected with the power wi e and the main wires, triphase E. M. F.'s are produced in a secondary circuit for the operation of triphase motors, while between the main wires in all other parts of the system, ordinary uni- phase E. M. F/s are maintained. MULTIPHASED CUKKENTS. 181 A form of belt-driven 150 KW. monocy- clic generator is represented in Fig. 67. Here the three collector rings are shown FIG. 67. 150-KiLOWATT MONOCYCLIC GENERATOR. at R, R, R, and the commutator C, is for the compounding of the field magnets. Fig. 68 represents the armature of such a 182 ALTERNATING ELECTRIC CURRENTS. machine, with its three collector rings and its commutator. It is often found difficult to determine, from the appearance of such a machine, whether it is of the monocyc- FIG. 68. MONOCYCLIC ARMATURE. lie, diphase, or triphase type, but a close inspection of the armature will usually indicate that the main coils ZZ, AA, BB, are larger than the intermediate coils or lesser coils T, T, T, T, T, T. CHAPTER IX. MULTIPHASE MOTORS. PRIOR to the introduction of the multi- phase machinery there were but two methods whereby electric power could be commercially transmitted over a consider- able distance; namely, either by the use of continuous -current motors, or by the use of synchronous alternating -current motors. As we have already pointed out, in order to obtain the advantages of the electrical transmission of power it is nec- essary to employ a high pressure on the conducting line so as to save copper in the conductor. While this is possible by the use of continuous -current motors, and, in point of fact, has been employed, yet the presence of commutators, which 184 ALTEKNATING ELECTRIC CURRENTS. such a system necessitates, both on the generator and motor, has been found, in practice, to give rise to no little risk and trouble, since the total pressure between the lines, being thus brought directly to the opposite sides of the commutator, should an arc discharge occur over the commu- tator, there would be a danger of its de- struction. In order to lessen these difficulties, the plan has been tried of distributing the line pressure to a number of motors all rigidly connected to the same shaft, and traversed successively by the driving cr.Tent. If, under a line pressure of say 2503 volts, five motors were so coupled together, then each motor would receive a pressure of one fifth of the total, or 500 volts. Al- though this device reduces the pressure across each commutator,. yet the insulation of each machine has to be carefully main- MULTIPHASE MOTORS. 185 tained, since, otherwise, a discharge might take place through the commutators lathe shjyt, under the whole pressure of the line, thus disabling the plant. Consequently, early i i the history of alternating currents, appreciating the advantage in practice, arising from the absence of a commutator, the uniphase generator and motor were connected, by means of conducting lines, for power transmission. To a certain ex- tent this combination was successful; for, as has already been pointed out, beside the advantage of collecting rings instead of commutators, the system possessed a marked advantage from the ease with which the pressure could be varied by the aid of suitable transformers. When the line pressure is too high to employ safely at the brushes of generator and motor, these latter can be constructed for lower pressures and larger currents, and then, 186 ALTERNATING ELECTRIC OtJRRENTS. by the use of step -up transformers at the generator K and step -down transformers at the motor, all the advantages of high pres- sure in the line, and low pressure at the machinery, can be secured, without great additional risk or cost. Such a system of transmission, however, necessitates the employment of the uniphase synchronous motor, and was, therefore, totally unfitted to cases where the motor had to be fre- quently stopped and started. Happily these practical difficulties in the commercial transmission of power have been removed by the introduction of multi- phase alternating- current apparatus, and while it is true that the use of such ap- paratus necessitates the employment of at least one additional conductor, yet the advantages possessed by the multiphase system are so considerable, that even al- MULTIPHASE MOTORS. 187 though this conductor involved extra cost in the copper, yet the advantages obtained would render its adoption economical. In point of fact, however, the amount of cop- per actually required for the three -wire multiphase system is one fourth less than that for the same amount of power by the uniphase system employing the same pressure in the line. As at present employed multiphase cur- rents are readily divisible into diphase, triphase, and monocyclic. Consequently, it will be convenient to treat motors under the same general heads. In point of fact,however, the difference between these forms of motors is comparatively trivial. A diphase motor differs from a triphase motor mainly in the fact that it has two circuits in its fields instead of three. 188 ALTERNATING ELECTRIC CURRENTS. In order to understand the operation of any multiphase motor, we will consider the effect produced on a suitable field-wind- ing when multiphase currents are supplied to it. It is necessary to remember that two separate alternating currents, flowing through two separate circuits, do not form a diphase system, unless the two currents differ in phase by a quarter cycle, or are 90 apart. When such diphase currents are sent through properly wound field frames, they tend to produce in them a magnetic field of a curious character ; name . ly, the poles produced do not only alter, nate in direction with changes in the direction of the current, but act as though the field rotated. For example, if in Fig 69, we consider the pair of coils 1, 3, on the opposite sides of the field frame, and suppose that a single uniphase current is supplied to them, it is evident, that if dur- MULTIPHASE MOTOBS. 189 ing any wave of current the pole 1 is a north pole and 3, a south pole, then during the next wave of reversed current* these poles will be reversed or 1 will be FIG. 69. DIAGRAMS ILLUSTRATING EFFECTIVE ROTATION OF A DIPHASE MAGNETIC FIELD. south, and 3, north. The same conditions will be maintained in the adjacent poles 2 and 4, which are alternately north and south, and south and north. But if the 190 ALTEBNATIKG ELECTftlC CUBBENTS* waves of current through C and D, come half an alternation later than the waves in A and B, we obtain a series of conditions represented; namely, (A) 1 is north, 3 is south, while 4 and 2 are in transition, there being no current in them at that instant. (B) In the next quarter cycle, 4 and 2 are now active, while 3 and 1 are in tran- sition. (C) At the next quarter cycle 3 and 1 have again come into action in the opposite direction, while 2 and 4 are in transition, and finally: (D) In the fourth quarter of the cycle, 1 and 3 are in transition, while 4 and 2 are active. If, now, we examine these figures we shall see that the N. and S. poles have steadily progressed around the field frame in the direction of the hands of a clock, so that, although alternating currents have MULTIPHASE MOTOES. 191 been employed, yet by reason of their proper phase difference in the two separate circuits, their effect has been to cause the magnetic field to rotate. If a compass needle were introduced into the middle of the field frame, it would, if left free to spin around the axis, rotate about that axis at the rotary speed of the field; namely, one revolution per cycle. Such a rotating com- pass needle may be considered as a small armature capable of acting as a motor. A piece of soft iron pivoted upon an axis at the centre will revolve in the same way. In practice it is usual to construct a lamin- ated armature core, like that of a contin- uous-current motor, wound with closed coils or closed loops, so as to induce power- ful currents in these coils by the rotation of the magnetic flux through them, and thus develop a powerful magnetic attrac- tion between the revolving magnetic field 192 ALTERNATING ELECTRIC CURRENTS. and these currents . Such mo tors are there - fore sometimes called induction motors. In order to reverse the direction of a polyphase motor it is only necessary to re- verse the direction of one of the windings on the motor, so as to reverse one of the pairs of poles, when the field will rotate in the opposite direction. With the appa- ratus actually employed a switch is arranged, so that, by its motion, one of the field windings is reversed. A triphase motor differs from a diphase motor only in that its field windings con- tain either six coils, or some multiple of three, instead of four coils or some mul- tiple of four. The effect of the current waves succeeding each other in the differ- ent windings, by one third of a cycle, pro- duces a continuously rotating field. MULTIPHASE MOTOES. 193 Fig. 70 represents a 15 H. P. diphase motor. FF is the field frame of laminated iron with suitable windings inside to pro- FIG. 70. FIFTEEN- HORSE-POWER BIPHASE MOTOR. duce the revolving field, within which the armature rotates driving the pulley P. 194 ALTERNATING ELECTRIC CUEEENTS. It is important to observe that in syn- chronous motors, the field frame need not be laminated, since the field poles do not change polarity, being excited by a con- tinuous current, but in multiphase mo- tors, since the field magnets are excited by alternating currents, it is important that the iron be laminated, in the frame as well as in the armature, since, other- wise, loss of power and injurious heating would occur. Fig. 71 shows a form of triphase motor for 71-2 horse-power. The three con- ducting wires are led through the winding of the field to the terminals A, B, C, and the armature shaft has a series of con- tacts (7, which is not a commutator, al- though somewhat resembling one in ap- pearance. When the handle H, is in the position shown, certain resistances MULTIPHASE MOTORS. 195 are included in the circuit of the armature windings, so as to enable the motor to start from rest. It is found, that if the full pressure be supplied to the field of FIG. 71. -TRIPHASE INDUCTION MOTOR, 7% H.P. the motor with the armature in its ordi- nary short -circuited condition, such pow- erful currents are induced in the armature 196 ALTEENATING ELECTBIC CUEEENTS. as to weaken its starting power. By the insertion of extra resistance, however, these currents can be reduced to the proper strength in the armature circuits to obtain a powerful starting power or torque, and, when the machine has attained full speed, the handle is pushed in toward the field frame, thereby sliding the con- tact ring C, into the strong clips of C 1 , short-circuiting the extra resistance, and cutting it out of circuit. The size of this motor is indicated by a foot-rule RR, shown at its base. Fig. 72 represents a similar triphase motor for 125 H. P. The three terminals of the field winding are shown at the top of the frame F, F, F, F; within revolves the armature A, A, A. As in the last case, the handle H, when the motor has been brought up to speed, throws forward a MULTIPHASE MOTORS. 197 collar K, into a receptacle, thus cutting the starting resistance out of the circuit of FIG. 72. 125-HoRSE-PowEB INDUCTION MOTOR. the armature coils. It will be seen that these triphase motors are very simple in appearance, have self-oiling bearings, and, 198 ALTERNATING ELECTRIC CURRENTS. having no commutator, require the mini- mum of att ention. Another form of small induction motor is represented in Fig. 73. This is a tri- FIG. 73. MONOCYCLIC MOTOR. phase motor frequently operated on a monocyclic circuit. MULTIPHASE MOTORS. 199 Figs. 74 and 75 show a form of diphase motor, with front and rear view. The three collector rings R\ R\ R , are em- p IG> 74. BIPHASE MOTOR. ployed for the purpose of inserting resist- ance in the armature circuits under the control of the handle H, which is only 200 ALTEENATING ELECTRIC CUEEENTS. employed in starting the motor. As soon as full speed is reached, the additional re- sistance is entirely cut out of circuit. The interior of the field frame for this FIG. 75 BIPHASE MOTOR. motor is represented in Fig. 76. It will be seen that there are two separate field frames placed side by side, but differing MULTIPHASE MOTORS. 201 in relative position. One of the two di- phase currents supplies the series A, B, C, and the other diphase current, the series A l , B l , C 1 . Under these conditions, al- FIG. 76. MOTOR FIELD. though no rotating magnetic field is pro- duced, yet by the effect of these alterna- ting magnetic poles upon the armature, a rotating magnetic field is developed upon 202 ALTERNATING ELECTRIC CURRENTS. it. The armature is represented in Fig. 77. At A, the core is shown, consisting of two separate halves ZTand H\, each revolving under one series of field mag- nets in the field frame. The appearance FIG. 77. MOTOR ARMATURES. of the armature after winding is shown at B, where the wire occupies the grooves between the iron teeth on the armature surface. The winding is carried com- pletely across the double armature, so MULTIPHASE MOTORS, 203 that the currents produced in the winding by one series of field poles react upon the neighboring series. This motor is de- signed for a frequency of about 130 cycles per second. Triphase and diphase mo- tors, while they can be designed for other frequencies, are more commonly em- ployed at a frequency of 60 or 30 cycles per second. The practical trend at the present time is toward the introduction of multiphase systems for the transmission of electric power. This tendency has resulted from the great flexibility possessed by multi- phase systems. Such, in brief, is a description of the more important commercial applications of alternating -current apparatus. When we consider that the developments in this latest field of electrical improve- 204 ALTERNATING ELECTKIC CURKENTS. ment have occurred practically within less than a decade, we cannot but believe that the next decade will witness even still greater improvements in this rapidly- advancing art. THE END. INDEX. A Action of Biphase Armature Windings, 172. Active Conductor, Magnetic Properties of, 30, 31. Activities, Electrical, Examples of, 89-91. Activity, Apparent, in Alternating-Current Circuit, 94. , Influence of Eeactance on, 96-98. of Motor, Definition of, 85. of Source, Methods of Increasing, 87, 88. Adjustable Resistance, 72. Advantages Possessed by Multiphase Motors, 183-186. Air Insulation of Transformer, 126. Alternating-Current Arc Lamp, 149-151. - Circuit, 15. - Circuit, Power Factor of, 94, 95. Circuits, Advantages of in Long-Distance Transmission, 107, 108. - Cycle, 9. Dynamo-Electric Machine, 60. , Electrical Activity in, 91, 92, 93. Electromagnet, 45. Fan Motors, 164-166. Flow, Curve of, 16. 206 ALTERNATING ELECTRIC CURRENTS. Alternating -Current, Period of, 9. - Transformer, Insulation Between Primary and Secondary Circuits, Necessity for, 116, 117. Transformer, Operation of, 110, 111. Transformer, Self- Regulating Properties Pos- sessed by, 111, 112. Transformer, Simple Form of, 109. Transformers, 99-132. Transformers, Primary and Secondary Con- nections of, 100. Transformers, Ratio of Primary and Second- ary Pressures in, 112, 113. Alternating Currents, Dangers of, to Life, 50 - E. M. F., 23, 26. Electric Current, 6, 23. Electric Currents, Dangers Possessed by, 38, 39. Electromotive Forces and Currents, 21-25. Tidal Currents, 6. - Watermotive Force, 26. Alternation, Frequency of, 7. Alternations, Semi-period of, 9. Alternator, Compound-Wound, Separately-Excited, Illustration of, 78, 79. , Separately-Excited, 67, 69. 207 Alternators, 26, 60. , Self-Excited, 63. , Self-Excited, an Example of, 76. , Self-Regulating Compound- Wound, 70, 71. , Separately-Excited, 63. Ampere, 14. Apparatus, Multiphase, 167. Apparent Activity in Alternating-Current Circuit, 94. Resistance, 37. Arc Lamp, Electrical Activity in, 90. Arc Lamps, Alternating-Current, 149-151. Armature, Monocyclic, 182. of Dynamo-Electric Machine, 58, 59. Automatic Choking Effects of Alternating Currents, 38, 39. Regulation of Dynamos, 70. B Bipolar Continuous- Current Generator, 64. Dynamo, Definition of, 65. Field, 60. Block, Fuse, 119. Blowing of Fuse Wires, 121. Brilliancy of Lamp Filament, 135, 136. 208 ALTERNATING ELECTKIC CURRENTS. C C. E. M. F. of Self-induction, 33, 34. Cell, Terminals of, 25. , Voltaic, 23. Central Electric Station, 57, 58. During Full Load, Peculiarities Presented by, 81-83. Choking Coil, Use of, in Series Alternating-Current Circuit for Incandescent Lamps, 147, 148. , Effect of Coil, 37, 38. , Effects of Alternating Currents, 38, 39. Circuit, Alternating-Current, 15. , Closed, 23. , Completed, 23. , Connections of Biphase Generator, 175, 176. , Definition of, 22. , Impedance of, 35, 36. , Inductionless, 95, 96. Circumstances Affecting Value of Impedance, 37, 38. Closed Circuit, 23. Coil, Choking Effect of, 37, 38. , Primary, of Alternating-Current Transform- er, 109. , Reactive, 42. INDEX. 209 Coil, Secondary, of Alternating-Current Trans- former, 110. Collector Kings of Alternators, 61. Commercial Efficiency of Incandescent Lamp, 137. Commutator of Dynamo-Electric Machine, 60. Compensator or Choking Coil for Alternating-Cur- rent Arc Lamp, 152. Completed Circuit, 23. Composite Dynamo-Electric Machine, 70. Compound-Wound Dynamo-Electric Machine, 70. Machines, 75. Conducting Paths, 21. Conjoined Eelations of Flux and E. M. F., 33, 34. Continuous E. M. F., 23. Current Dynamos, 60. Electric Currents, 6. Convention as to Assumed Direction of Magnetic Flux in a Circuit, 32. Core, Laminated, of Transformer, 108. Coulomb-per-second, 14. Counter E. M. F., 33, 34. Current, Continuous, 23. Currents, Alternating-Electric, 6. , Alternating-Electric, Dangers Possessed by, 38, 39. , Alternating-Tidal, 6, 210 ALTERNATING ELECTRIC CURRENTS. Currents, Multiphase, 167-182. Curve of Alternating-Current Flow, 16. Cycle of Alternating-Current, 9. - of Eiver Flow, 8. D Daniell Gravity Voltaic Cell, 24. Definition of Alternating-Current Transformer, 108. of Bipolar Dynamo, 65. -of C.E. M. F., 33, 34. of C. E. M. F. of Self-Induction, 33, 34. of Circuit, 22. of Machine, 55, 56. of Phase Difference, 94, 95. : of Torque of Motor, 169. - of Watt, 87. Devices, Translating or Eeceptive, 21. Dimmer, Theatre, 41, 42. Diphase Alternating Currents, Relations Between, 168, 169. Alternator, 170, 171. Armature Windings, Action of, 172. Generator, 169. Generator, Circuit Connections of, 175, 176. Magnetic Field, Diagram Illustrating Rota- tion of, 189. INDEX. 211 Diphase Motor, Illustration of, 193, 194. Motors, 160, 167. Motors, Illustrations of, 199, 200. Diphaser, Definition of, 175. Double Winding on Field Magnets, 70. Driven Machinery, 58. Driving Machinery, .58. Drop of Hydraulic Pressure, 102. Drop of Pressure, Effect of Length of Circuit on, 104, 105. of Pressure in Circuits, 101-103. of Pressure in Electric Circuits, Illustration of, 101-103. - of Voltage, 101-103. Dynamo, Quadripolar, Definition of, 66. , Keversibility of, 153. Dynamo-Electric Machine, Armature of, 58, 59. Commutator, 60. , Compound- Wound, 70. , Field Magnets of, 59, Dynamos, Continuous-Current, 60. E E. M. F., Alternating, 23. and Flux, Conjoined Relations of, 33, 34, 212 ALTERNATING ELECTRIC CURRENTS. E. M. F., Continuous, 23. , Effective, Definition of, 51. , Impressed, Definition of, 33, 34. , Meaning of, 22. , Kesultant, 96. , Unidirectional, 23. E. M. F. 's, Induced, 33, 34. Effective Current Strength, Definition of, 51. - E. M. F., Definition of, 51. Effects, Electroplating, Produced by Alternating Currents, 47. , Electroplating, Produced by Continuous Cur- rents, 56. , Heating, produced by Continuous and Alter- nating Currents, 58. , Physiological, 48, 49. , Physiological, of Electrical Currents, 50. , Tesla, 7. Efficiency of Electric Motor, 155. of Incandescent Lamp, Influence of Tempera ture on, 138, 139. Electric Current, Alternating, 23. Currents, Continuous, 6. Lamps, 133-152. Motors, 153-166, INDEX. 213 Electric Motors, Intake of, 155. Resistance, 27. Resistance, Unit of, 27. Electrical Activities of Incandescent Lamp, 89, 90. Activity of Alternating Current Dependent on Phase Relations, 92-94. Activity in Arc Lamp, 90. Activity of Railway Generator, 90, 91. Transmission of Power, 155-157. Transmission of Power, Use of Continuous Currents and Alternating Currents for, 156-158. Electricity and Ether, Action Between, 18-20. , General Nature of, 17, 18. Electrocution, 49. Electromagnet, 45. , Alternating-Current, 45. Electromotive Force, Abbreviation for, 22. Electroplating, 46. , Effects Produced by Alternating Currents, 47. , Effects Produced by Continuous Currents, 46. Ether, 18. Streamings, 31. Examples of Electrical Activity, 89-91, 214 ALTERNATING ELECTRIC CURRENTS. F Fan Motor, Alternating-Current, 164-166. Filament, Incandescence of, 134. of Incandescent Lamp, 133, 134. Field, Bipolar, 60. , Magnetic, Definition of, 32. Magnets of Dynamo-Electric Machine, 59. - Winding of Triphase Motor, 192, 193. Flux and E. M. F., Conjoined Kelations of, 33, 34. , Magnetic, Kate-of-Change of, 35. Foot- Pound, Definition of, 84. - Per-Second, 85. Force, Alternating E. M. F., 26. , Watermotive, 26. Forces, Magnetomotive, 59. Frequency in Alternating-Current Circuit, Influence of, on Steadiness of Lamps, 135, 136. , Influence of, on Impedance, 37, 38. of Alternatiug-Eleclric Currents in Light, 8. of Alternation, 7, 9. of Telephonic Circuits, 7. Fuse-Block, 119. Fuse-Block of Transformer, 120, 121. Fuse Wires, Blowing of, 121, INDEX, 215 G General Nature of Electricity, 17, 18. Generator, Bipolar Continuous-Current, 64. General Construction of Incandescent Electric Lamp, 133, 134. Generator, Di phase, 169. , Monocyclic, 179,181. Generator, Quadripolar Continuous-Current, 65. , Triphase, 176, 177. Generators, Multipolar, 68. H Heating Effects by Continuous and Alternating Currents, 48. High- Frequency Currents, Tesla's experiments on, 49, 50. High-Pressure Incandescent Electric Lamps, 138,139. Horse-power and Kilowatt, Kelation Between, 88, 89. , Definition of, 85. Hydraulic Pressure, Illustration of Drop of, 102. I Illustration of Diphase Motor, 193, 194. of Triphase Induction Motor, 195,196. 216 ALTERNATING ELECTRIC CURRENTS. ' Impedance, Circumstances Affecting Value of ,37,38. , How Affected by Frequency, 37, 38. of Circuit, Influence of Iron upon Value of, 37, 38. of Circuits, 35, 36. Impressed E. M. F., Definition of, 33, 31. Incandescence of Lamp Filament. 134. Incandescent Electric Lamp, Varieties of, 141-143. Incandescent Filament, Brilliancy of, 135, 136. Lamp Base, Varieties of, 141-143. Lamp, Electrical Activities of, 89, 90. Lamp, Filament of, 133, 134. Lamp on Alternating-Current Circuit, Flick- ering in, 135, 136. - Lamp, Parallel Connection of, 144-146. Lamp, Socket of, 140. Lamp, Temperature of, 140 Lamp, Three-Wire System of Distribution of, 146. Lamps High-Pressure, 138, 139. Lamps, Low-Pressure, 139. Lamps, Series Distribution of, 146, 147. Lamps, Two- Wire System of Distribution, 141, 145. Indoor Type of Transformer, 127, 128. INDEX. 217 Induced E. M. F.'s, 33, 34. Induction Motor, Starting [Resistance of, 197, 198. - Motors, 191, 192. Inductionless Circuit, 95, 96. Intake of Electric Motors, 155. Intensity of C. E. M. F., Circumstances Affecting Value of, 35. Iron, Effect of, on Impedance of Circuit, 37, 38. - Influence of, on Keactance of Circuit, 37, 38. K Kilowatt, Definition of, 88. Lamp, Incandescent, Commercial Efficiency of, 137. , Incandescent Electric, General Construction of,. 133, 134. , Incandescent, Life of, 138. Lamps, Electric, 133-152. Length of Circuit, Effect of, on Drop in Pressure, 104, 105. Life of Incandescent Lamp, 138. Light, Frequency of Alternating- Electric Current in, 8. 218 ALTERNATING ELECTRIC CURRENTS. Long-Distance Transmission, Advantages of Alternating Current on, 107, 108. Low-Pressure Incandescent Lamps, 138, 139. M M. M. F.'s, 59. Machine, Definition of, 55, 56. Machinery, Driven, 58. , Driving, 58. Magnet, Electro, 45. Magnetic Flux, Convention as to Assumed Direction of, in a Circuit, 32. - Properties of Active Conductor, 30, 31. Magnetism, Definition of, 31. Magnetomotive Forces, 59. Method of Connecting Transformer Secondary Coils for 50 or 100 volts, 123, 124. Monocycler, 179. Monocyclic Generator, 179, 181. Motor, Illustration of, 198. System, 179. Motor, Definition of Torque of, 169. , Multiphase, 167. Motors, Diphase, 160. , Electric Efficiency of, 155. , Electric Output of, 155. INDEX. 219 Motors, Induction, 191, 192. , Multiphase, 183-203. , Multiphase, Advantages Possessed by, 183-186. Synchronous Electric, 158, 159. Triphase, 167. Multiphase Apparatus, 167. Current, 157-182. Generator, 173. - Motor, 167. Multiphase Motors, 183-203. Motors, Advantages Possessed by, 183-186. Motors, Eotary Properties of Field in, 188-191. Multipolar Generator, 68. Generator, Necessity for Even Number of Poles in, 68. N Negative Pole of Voltaic Cell, 25. Non-Luminous Radiation of Lamp, 137. Number of Turns in a Circuit, Effect of, on Impe- dance, 373, 38. 220 ALTEEtfAf IJfG ELECTBIC CUKKENTS. Oersted's Discovery, 31. Ohm, Definition of, 27. Ohmic Kesistance, 39. Ohm's Law, 28. Oil-Insulated Transformer, 117. Operation of Alternating Current Transformer, 110, 111. Out-door Type of Transformer, 125. Output of Electric Motors, 155. Period of Alternating Current, 9. of Eiver, 8. Phase Difference, Definition of, 94, 95. Difference of E. M. F. and Current Effect of, on Electrical Activity, 94. Physiological Effects, 48, 49. Pole, Negative, of Voltaic Cell, 25. , Positive, of Voltaic Cell, 25. Positive Pole of Voltaic Cell, 25. Power, Electric, 81-98. , Electric, Transmission of, 155, 156. Factor of Alternating-Current Circuit, 94. INDEX. 221 Power Wire of Monocyclic System, 180. Pressure, Drop of, in Circuits, 101-103. Primary and Secondary Circuits of Transformer, Ratio of Pressure in, 112, 113. Primary Coil of Alternating-Current Transformer, 109. Primary Power-Factor of Transformer, 126, 127. Q Quadripolar Continuous-Current Generator, 65. Quadripolar Dynamo, Definition of, 66. E Kail way Generator, Electrical Activity of, 90, 91. Kate-of-Change of Flux, 3. Eeactance, 30. , Influence of, on Activity, 96-98. of Circuit, Influence of Iron on, 37, 38. Reactive Coil., 42. Coil for Series Alternating-Current Incandes- cent Lamp, 150. Coil or Compensator for Alternating-Current Arc Lamps, 152. Receptive or Translating Devices, 21. 222 ALTEB.H ATTNG ELECTEIC CUEEENTS. Regulation, Automatic, of Dynamos, 70. Relations Between Diphase Alternating-Currents, 168, 169. Resistance, Adjustable, 72. , Apparent, 37. , Electric, 27. - of Alternating Currents, 29, 30. of Continuous-Current Circuits, Circumstances Influencing Value of, 29, 30. , Ohmic, 39. ResultantE. M. F., 96. Reversibility of Dynamo, 153. Rheostats, 72. Rings, Collector, of Alternators, 61. River Flow, Cycle of, 8. , Period of, 8. Rotation of Diphase Field, Diagram of, 189. s Secondary Coil of Alternating-Current Transformer, 110. Self- Excited Alternators, 63. Machines, Class of, 74, 75. Self- Regulating Compound- Wound Alternator, 70, 71. Self- Regulation of Alternating- Current Transformer, 111, 112. INDEX. 223 Semi-Period of Alternations, 9. Separately-Excited Alternators, 63, 67, 69. Compound-Wound Alternator, 71, 72. Machines, Class of, 74. Series Distribution of Incandescent Lamps, 146, 147. Socket of Incandescent Lamp, 140. Source, Electric, Definition of, 21. Starting Eesistauce of Induction Motor, 197, 198. Step-down Transformers, 114. Step-up Transformers, 114. Streaming of the Ether, 31. Sub-Station Transformer, 130-132. Synchronous Electric Motors, 158, 159. System, Monocyclic, 179. Telephonic Alternating Currents, 6. , Circuits, Frequency of, 7. Temperature Influence of an Incandescent Lamp, 138, 139. Terminals of Voltaic Cell, 25. Tesla Effects, 7 . , Experiments of, on High-Frequency Cur- rents, 49, 50. The Temperature of Incandescent Lamps, 140. Theatre Dimmer, 41, 42. 224 ALTERNATING ELECTRIC CURRENTS. Theatre Dimmers, Varieties of, 42-44. Three-Wire System of Distribution of Incandescent Lamps, 146 Tidal Flow of River, 11, Level of River, 12. Torque of Motor, Definition of, 169. Transformer, Air Insulation of, 126. , Alternating-Current, Definition of, 108. , Fuse-box of, 120, 121. , Indoor Type of, 127, 128. , Oil-Insulated, 117. , Out-door Type of, 125. , Primary Power Factor of, 126, 127. Transformer, Sub-Station Type of, 130-132. Transformers, Alternating-Current, 09-132. , Laminated Iron Core of, 115, 116. , Step-up, 114. , Use of, in Electric Transmission of Power, 159-163. Translating or Receptive Devices, 21 . Triphase Currents, Phase Relations Between, 177. Generator, 176, 177. Generator, Illustration of, 178. - Induction Motor, Illustration of, 195, 196. - Motor, Field Windings in, 192, 193. Motors, 167. INDEX. 225 Work Winding, Diagram Representing Connections of, 179. Triphaser, Definition of, 176, 177. Two-Wire System of Distribution of Incandescent Lamps, 144, 145. u "Unidirectional E. M. F., 23. Uniphase Alternators, 53-80. Unit of Activity, 85. of Electrical Flow, 13. of Electric Resistance, 27. of Work, 84. Varieties of Incandescent Lamps, 141-143. Volt-Coulomb-per-second, or Watt, Definition of, 87. Voltage, Drop of, 101-103. Voltaic Cell, 23. Watt, Definition of, 87. Winding, Triphase, Diagram Connections of, 179. Wire, Power, of Monocyclic System, 180, Work, Unit of, 84. Elementary Electro - Technical Series. BY EDWIN J, HOUSTON, Ph.D, and A, E, KENNELLY, D.Sc, Alternating Electric Currents, Electric Incandescent Light- Electric Heating, ing, Electromagnetism, Electric Motors, Electricity in Electro-Thera- Electric Street Railways, peutics, Electric Telephony, Electric Arc Lighting, Electric Telegraphy. Cloth, profusely illustrated. Price, $1.OO per volume. The above volumes have been prepared to satisfy a demand which exists on the part of the general public for reliable in- formation relating to the various branches of electro-technics. In them will be found concise and authoritative information con- cerning the several departments of electrical science treated, and the reputation of the authors, and their recognized ability as writers, are a sufficient guarantee as to the accuracy and reliability of the statements. The entire issue, although pub- lished in a series of ten volumes, is, nevertheless so prepared that each volume is complete in itself, and can be understood inde- pendently of the others. The books are well printed on paper of special quality, profusely illustrated, and handsomely bound in covers of a special design. x 0> THE W. J. JOHNSTON COMPANY, Publishers, 253 BROADWAY, NEW YORK. THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL. 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