The Library of Work and Play: Electricity and Its Everyday Uses

One day Harold expressed a desire to see the dynamos, five miles away, which furnish the electric light in our apartment. So I told him to invite his best friend to accompany us and we would go.

When we were some distance from the station the boys noticed the very tall chimneys and inquired why tall chimneys were needed for dynamos. I explained that the dynamos were run by steam-engines, and steam-engines required the burning of coal. "Oh!" said Ernest, Harold's friend, "I read in the paper that electricity is the rival of steam and is going to drive out the steam-engine." I suggested that we were about to see some steam-engines driving electricity out of that power station. But more seriously, I explained that steam-engines were used for many years as locomotives to draw the trains on the elevated railroads of New York City, and when at last they were displaced by electric trains some people thought that it was a case of electricity driving out steam, whereas what had really happened was that the steam power for running those trains had been concentrated at a central station, and its power was merely transmitted to the trains by means of electricity. The trains were, therefore, run by steam power quite as much as ever. In like manner, the surface cars of New York a few years ago were run by a cable, which was merely a very long belt used to transmit to the cars the power of steam-engines located at a central station. When they were changed to electric cars, electricity became the successful rival of nothing else than a twisted wire cable. The cars still run by steam power as before, but that power is transmitted by electricity instead of the discarded cable. Steam has driven out the horse as a power for drawing street cars, and electricity has enabled us to gather all the steam engines into central stations, where now they are furnishing the power for moving surface, elevated, and subway cars for street traffic, as also trains for suburban travel. Central station steam-engines are producing a vast amount of power, distributed all over the city by means of electricity, for doing a great variety of work and for furnishing electric light and heat, all of which we shall presently study. "Just before we go into this central station, can you tell me how the elevator is run in our apartment house?" "It is an electric elevator," said Harold. "And where does the electricity come from?" I inquired. "Well, I know that it comes from the street mains, but do they come from this power station?" "Yes," said I, "and we will now go in and see the steam-engines which lift you up stairs many times each day by sending electricity to run that elevator. If you choose to do so, you may claim for purposes of discussion that your elevator is run by steam."

As we entered the building we came first to the dynamo room and both boys noticed that the tone which met their ears was that which I had produced for them in the telephone the night before. "I shall try to show you before we get through," I said, "that these dynamos are doing something which makes iron pulsate sixty times a second and that that is the cause of the pitch of this tone. But let us begin with the coal which is the source of all this power.

"This particular station at the present time is burning forty tons of coal an hour. That is as much as Mr. -- uses to heat his twelve-room house for a whole year. One pound of coal is capable of liberating enough energy to supply 5? horse-power for an hour. (Written for short 5? H.P.H.) One ton of coal is capable of furnishing (2,000 × 5?) 11,500 H.P.H. Forty tons would yield 460,000 H.P.H. But the best furnaces, boilers, and steam-engines are terribly wasteful of energy. About nine tenths of all this energy is wasted and only one tenth, or about 46,000 horse-power per hour, is delivered by the steam-engines to the dynamos.

"Coal is already scarce in the world and the supply is rapidly being exhausted. Meanwhile we are growing more dependent upon coal. A century ago we used scarcely any power except that of men, horses, and oxen, and what little heat men then used came chiefly from wood. They lived in cold houses, attended cold churches and schools, did not ride in steam or electric cars, and did not have power plants. Our wood is nearly all gone, our coal is going, and we are very rapidly growing more dependent upon heat and power, our chief source of which is coal. Wind power is too uncertain to depend upon, and we turned our backs upon water-power when we began to crowd into cities. What little water-power there is, however, is nearly all in use.

"There is great need both that we learn how to save the major part of the energy of the coal which we now waste, and that we find a substitute for the coal to use when that is gone.

"A part of the heat from the forty tons of coal which is being burned in this particular power plant goes into the water in the boilers. It converts this water into steam. The steam, if free to expand into the air, would occupy about one thousand seven hundred times the volume of the water. We compel it to expand through the cylinders of the steam-engine, using its force of expansion to make wheels go around-to make the dynamo revolve. These dynamos are not devices for producing power but merely for transmitting the power of these steam-engines to far away places where it may be used, as, for instance, in our apartment house, where we are unwilling to walk upstairs and want some power to carry us.

"Our own apartment is fifty feet above the street. I weigh one hundred and sixty-five pounds. If I walk up stairs from the street to our apartment in one minute, which is the rate of a rather slow elevator, I work at the rate of one quarter of a horse-power. One hundred and sixty-five pounds raised two hundred feet in one minute requires one horse-power. You boys each weigh about half as much as I do, and if one of you walks up the same stairs in one minute you exert half the power that I do, or if you run up the stairs in half a minute you exert the same power, that is, one quarter of a horse-power. When we three walk up together in one minute we exert one half horse-power. If we all three run up the stairs in half a minute we expend one horse-power. Now, the speed of elevators for apartment houses is about one hundred feet a minute. We are unwilling to walk up stairs, not because we are lazy but because we have the New York haste, and so we employ elevators which run at the rate of about one hundred feet a minute.

Photograph by Helen W. Cooke

Testing a Generator

"These dynamos enable us to employ the power of this central station to run the elevator in our apartment house. Here is a dynamo rolling over now in the act of sending out power, some of which goes to that elevator; and standing beside it is another waiting to be used when necessary. Examining these dynamos, we find that they are composed of nothing else than iron and copper. About all that we can say of these mysterious machines is that the moving iron generates the electricity and the copper leads it away.

Fig. 1

"Each one of these dynamos has many hundred tons of iron in it. A huge wheel of iron, thirty-two feet in diameter, one hundred feet in circumference, portions of which are surrounded by insulated copper conductors, forms the centre-piece of the machine. This movable part weighs four hundred tons. Around about this is a fixed ring of iron, portions of which are surrounded by insulated copper conductors. Ordinarily the ring which is stationary is called 'the field,' and the wheel, which rotates, is called 'the armature,' although these terms are sometimes reversed for certain reasons. The movable part in these machines rotates about once a second, that is, its circumference moves a little faster than a mile a minute. The iron moving at this high rate of speed creates ether streams or electric currents, which are led off by the copper conductors. The generation of electricity on a large scale requires large masses of iron and high velocity."

I noticed that the boys stood before this machine in a state of utter bewilderment, bewildered as a man who is told that what he had considered north is really south, bewildered as a man who, having wandered through a maze of city streets, looks up at length and unexpectedly finds the building he has been seeking towering before him. The questions they asked were entirely without thought. "What is inside of it?" "Simply more iron and copper, such as you see on the surface," I replied. "But what makes it go?" "The steam engines, of course, four of which you see, are coupled directly to each dynamo." "But where does it get its electricity?" "Don't forget that you are looking at a generator of electricity. Big mass of iron-rapid motion! That is the whole truth. But it cannot satisfy you as an answer until you have become used to it. We have seen all that we ought to see here to-day. Let us drop the whole matter now, but return to my laboratory to-morrow, and I will give you the next step which will help you."

The boys did no talking upon their return journey. Whether one may say they were thinking or not I cannot tell, but certainly their ideas were incubating.

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When we had gathered at my laboratory the next day I took down a spool of one pound No. 24 cotton-covered copper wire (Fig. 2 A), which had its centre filled with wire nails. The boys had seen it before and remembered it. With flexible wires I connected the two ends of the wire on this spool to a sensitive ammeter, B, which had its zero in the middle of the scale, and I laid down upon the table a bar magnet, C.

Fig. 2

"Here," I said, "is a dynamo complete." The bar magnet furnishes the 'field' and this spool of copper wire, A, which I will move back and forth immediately over the magnet from end to end, is 'the armature.' D and e are the line wires and the circuit is completed through the ammeter to show whether we are generating electricity. And now as I move this armature along the field you see the needle of the ammeter move to the right from zero to ten. When the armature is moved in the opposite direction along the field the needle moves in the opposite direction past zero and on to ten at the left. The moving of the needle in the ammeter shows that we are generating electricity. The swinging to and fro of the needle shows that we are generating an alternating current of electricity. It is a mere matter of detail whether we move the armature or the field, as I will show you by letting the spool A rest quietly upon the table and moving the magnet to and fro lengthwise across the end of the spool. Or I may accomplish the same results by moving them both in opposite directions. It is simply necessary that they move with reference to each other. Some dynamos are made with stationary fields and rotating armatures, some with stationary armature and rotating fields, and some with both parts designed to rotate in opposite directions.

"Magnetism is not confined to the magnet. It extends more or less widely into the region about it. It is this region affected by the magnet that we designate its magnetic field. By bringing this sensitive compass needle into the region of this bar magnet from all directions, I show you that it has a slight power to change the direction of the needle when about a foot away. This power grows rapidly greater as the distance grows less. Of course its field extends rather indefinitely, but we may say that this particular magnet has an appreciable field extending about one foot in all directions from it. We find upon examination that some magnets have bigger and stronger fields than others, that all have their strongest fields when first magnetized and lose their strength gradually, but never entirely. We find that hardened iron and steel hold magnetism longer than soft iron, but all iron is magnetized somewhat at all times. Iron that is feebly magnetized can be made into a strong magnet by bringing it into a strong magnetic field. The earth is a feeble magnet, and that is why it gives direction to the compass needle. That is also probably the reason why every piece of iron upon the earth is a magnet, or, to put the cause back another step, we may say that whatever causes the earth to be a magnet also causes every piece of iron upon the earth to be likewise a magnet.

"But thanks to Oersted in Denmark in 1819 and Faraday in England in 1821 and Joseph Henry in Albany, N. Y., in 1827, we have learned to make exceedingly powerful magnets by sending a current of electricity in a whirl around the iron. This is the meaning of the coils of copper wire around iron cores in the dynamo, in electric bells, in telegraph sounders, in motors, etc., etc. To prevent the electric current from taking the shortest route, through the iron core or through the successive layers of copper wire, the iron core and the wire must be covered with something like wood or paper or cotton or silk or rubber-such things as electricity does not readily pass through-that is, insulating material.

"Joseph Henry, while teaching in the Albany Academy, was the first to make electro-magnets. There was no such thing as wire covered with an insulating material then in the market, and he wound all his wire with silk ribbon. But in the year 1834 he made magnets which lifted thirty-five hundred pounds, to the astonishment of every one. A pair of such electro-magnets as I have here (Fig. 3), each consisting of one pound of No. 24 cotton covered copper wire, eight hundred feet long, wound in one thousand turns about an iron core two inches in diameter, will lift several hundred pounds: much more than we three can lift, as I shall now show you."

Fig. 3

The cores of the two magnets were bolted fast to an iron beam, and a large bar of iron with a ring in it was laid across the other free ends of the magnet cores. I made connections with the electric lighting circuit (that in my laboratory is what is called a direct current), and sent a current of electricity around the coils. The two boys and I tugged at the ring in the iron bar to no avail. We were unable to pull the iron bar away from the magnet. But when I opened the switch and cut off the electric current, one boy with one finger in the ring lifted the bar with perfect ease.

"Electro-magnets are now made with a magnetic intensity 90,700 times that of the earth's magnetism. Electro-magnets are used for hoisting iron castings weighing many tons. Here is a picture of an electro-magnet lifting a whole wagon load of kegs of nails from the wagon to the hold of a ship.

"Electro-magnets are our only means of utilizing electricity for power. It is the pull of electro-magnets that moves the electric car. Electro-magnets are now used for pulling all the trains out of the Grand Central Depot in New York City.

"Let us now compare the strength of our electro-magnet with that of the bar magnet used in our former experiment."

I opened and closed the switch, which sent the electric current through my magnet coils at frequent intervals, and the two boys, each with a compass needle, searched the field for magnetic effects. They found that the magnetic field extended six or eight feet, but this piece of research was broken up by a new idea which appeared to strike them both at the same instant, for they shouted both together, "Let's use this electro-magnet in place of the bar magnet for our dynamo experiment!"

Photograph by Helen W. Cooke

Wiring

"That is surely the next step in our programme," said I, "but you will need a steam-engine to move an armature in this magnetic field, will you not, judging from the struggle we had with that iron bar a few minutes ago?" The boys looked quite hopeless until I said, "The best thing about the electro-magnet remains yet to be told. You have perfect control of its strength by changing the amount of electricity which you send around the coil.

"By means of an instrument which works like the motorman's controller on the electric car, I may control the amount of electricity which flows, just as well as you may control the flow of water by a faucet or stop-cock. By this means I will control the strength of the magnet so that you may move the armature in your dynamo experiment.

"In 1821, Faraday, at the Royal Institution, London, learned that he could produce magnetism by means of the electric current, and, in 1831, he learned that the reverse was also true, namely, that he could produce electricity from magnetism. This idea coming as the result of ten years of incessant search made him shout and dance like a child. You are feeling a little of the pleasure of his discovery."

Fig. 4

I then fastened one of the coils upon the table underneath a small bench (Fig. 4) and sent an electric current around it. The other coil, B, connected with the ammeter was pushed back and forth along the surface of the bench over this coil. The boys found that the more electric current I sent around the coil A, that is, the stronger I made the magnetic field, the harder it was to move the coil B. They found that the nearer B was to A the harder it was to move it. They found that the faster they moved B the more electricity was produced. They tried laying B upon its side upon the bench and thus moving it. They tried taking B off the bench and moving it on all sides of A. They found it much harder to move in some ways than in others, but in all cases they found that the harder they had to work the more electricity was developed, as was shown by the ammeter.

"The dynamo is any machine which will convert mechanical work into electricity. The magneto is one form of a dynamo which you have used much at the summer cottage, but have never seen the inside of. Here are several (see Figs. 5, 6, and 8) which I will let you examine inside and out, and with these I must leave you to yourselves for a time."

When I returned I asked the boys why these dynamos were called magnetos. "Because they have steel magnets for their fields," they replied. "There are several magnets bent in the shape of a horseshoe."

"Yes," I said, "in this case the field is made stronger by taking several magnets. Have you noticed any armature?" "Yes, it is made of iron with insulated copper wire wound around it."

"Please recall that the amount of energy you expend in going upstairs depends on two things: (1) your weight and (2) the speed with which you move. Also recall that the amount of electricity you could generate with a dynamo depended upon the amount of energy you expended. Therefore, the strength of the electric current which this machine may produce depends upon two things: (1) the strength of the magnetic field against which you must pull and (2) the speed of the motion of the armature. Evidently this field is made as strong as it is possible to make it with steel magnets. Now is there any device for giving high speed to the armature?"

"Yes, indeed," said the boys, "one has a pulley so that it may be connected by a belt with a gas engine, and the others have each a large cog-wheel working into a smaller one. We found in one of them that a single revolution of the crank gave six revolutions to the armature."

I found that the boys had made large-sized drawings of the parts, and were preparing to report on the magneto as a form of dynamo at the next meeting of the Science Club, which we had started among the boys in school.

Fig. 5

"I will loan you some apparatus so that you may give a very interesting demonstration on that subject," said I, "only let me show you how to use it first. Connect the binding posts D and E of this magneto (Fig. 5) with my ammeter. Turn the crank very slowly and notice that the needle of the ammeter swings to and fro with each revolution of the armature. That shows that you have not only a dynamo, but an alternating current dynamo.

Fig. 6

"Now connect the binding posts d and e of this magneto (Fig. 6) with a short piece of copper wire. Turn the crank and you notice that this dynamo rings two electric bells. Turn slowly and you notice that the alternations of the current are numbered by the strokes on the bells. The hammer swings to and fro just as the needle of the ammeter did. Each bell therefore receives one stroke of the hammer for each revolution of the armature. Now try to turn the crank steadily at the rate of one revolution per second. The armature is making six revolutions, or cycles, per second and you now have not only an alternating current dynamo but a six-cycle alternating current dynamo. The lighting circuit used in our apartment is a sixty-cycle alternating current. To be sure the armature of the dynamo which generates that current revolves only once a second, but it carries coils enough upon its rim to make that number of alternations.

Fig. 7

"Now connect this telephone receiver with the binding posts D and E of this magneto (Fig. 7). Unscrew the cap of the receiver. Move to one side the iron diaphragm and turn slowly the crank of the magneto. Notice that the diaphragm vibrates in time with the alternations of the dynamo. Replace the diaphragm, screw on the cap, hold the receiver to your ear and turn the crank as fast as you can. You will probably be able to make about sixteen cycles per second. The receiver in that case is giving forth a sound of the same pitch as a sixteen-foot closed organ-pipe.

Fig. 8

"Connect the telephone receiver to the binding posts D and E of this magneto (Fig. 8), and by means of a belt connect the pulley to this series of cog-wheels. Now you may turn the crank and readily make the armature revolve at the rate of sixty cycles per second, and you notice that you get the same tone that we heard in the dynamo room of the power station and the same tone the telephone receiver gave when I connected it to a coil in our apartment. The tone which is produced by sixty vibrations per second is very nearly that of the C two octaves below middle C on the piano. Try it along with the piano and you will find it a little flat. This string on the piano is making sixty-four vibrations per second.

Fig. 9

"Now connect this miniature telephone switchboard lamp with the magneto (Fig. 9) and turn the crank fast. The lamp lights up to full brilliancy and you notice that the light is steady, although it is made by an alternating current passing through the filament in one direction, stopping entirely, and then passing in the opposite direction. The filament has no time to cool off, provided you turn fast enough, but try turning a little slower and you will notice the flickering of the lamp."

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Fig. 10

At the last meeting of the Science Club so many questions were asked, which the demonstrators could not answer, that a programme committee, to whom such questions might be referred thereafter, was appointed. It was made the duty of this committee to assign to various members the task of searching for satisfactory answers, and when the material was ready to be reported to the club, the programme committee determined the time and order of presentation. I found that I had been made an honorary member of this committee and that it was expected that I should steer the committee. I told them that I accepted this appointment with the understanding that the fellow who steers is always the smallest man in the crew, and if they would do all the work I would enjoy the honorary title of cockswain. Secretly, however, I appreciated that this was in effect adding several courses to my already rather heavy programme. I must, under the régime, direct a large number of inexperienced students in library research, in laboratory research, and in the art of giving demonstrations with apparatus and experiments to audiences.

The most urgent questions, as also those which were next in the natural order, concerned the ammeter. I told the committee to make that the subject of the next meeting and to send to my laboratory on a certain day the person or persons whom they might appoint to report upon it.

Fig. 11

I find that the boys never come singly, but generally in pairs. When the boys came they found lying upon the table an ammeter (Fig. 11).

I told one of them to take out the three screws in the front and remove the face of the instrument. I had told the boys that the instrument cost sixty dollars and that letting them open it was like letting them open my watch. As soon as the face came off one of the boys exclaimed that from my reference to the watch he had expected to see very complicated machinery with many wheels, but from the exceeding simplicity of the mechanism he could not see why it should cost sixty dollars. I told him that although it was a fine piece of workmanship it was fortunately very easy to understand, and I asked them if it reminded them of anything else that they had ever seen. After a few moments of reflection they agreed that it was very much like one of the magnetos. "Well," said I, "where is the field?"

Fig. 12

"Is this horseshoe arrangement a magnet?" they inquired.

"There is a compass needle right at your hand waiting to answer that question," I replied. They immediately found that it was a magnet. "Well," I said, "to be really sure that it is a magnet you must find a portion of it that will repel a portion of your compass needle as well as other portions in both horseshoe and needles which attract each other." Whereupon, they found that the portion marked N (Fig. 13) repelled the blue end of the compass needle and attracted strongly the bright end of the needle, while the portion marked S did the reverse. "We will call N and S the poles of the magnet. This is simply a steel bar magnet bent into the shape of a horseshoe."

Fig. 13

"You told us," remarked one of the boys, "that steel magnets gradually lose their strength. How then can this be correct as a measuring instrument?"

"It is the purpose of the iron case to enable this magnet to retain its magnetism, and if you will examine its field, as we did that of another magnet upon a former occasion, you will find that although this is a strong steel magnet its field does not extend outside of the iron case. It is as though we could box up magnetism and keep it from escaping.

"Now if this is like the magneto, where is the armature? The spool-like thing between the poles of the magnet looks just like the armature in one of the magnetos.

"Yes, it has an iron core with a coil of insulated wire around it, and you remember that when an electric current is sent around a piece of iron, that iron is made into a magnet, and if it is a magnet it must have poles. It is very delicately poised upon a pivot and will act exactly like your compass needle, which is also a little magnet with poles. I will send an electric current through the wire which surrounds this armature, and you notice that the needle which it carries moves to the right. Notice that the lower end of this armature acts like the blue end of your compass needle in that it is repelled from the pole N of the field and is attracted toward S of the field. In like manner, the upper end or pole of the armature is repelled from S and attracted to N of the field. The blue end of the compass needle is called its north pole because it points north under the magnetic influence of the earth, and so we may call the lower end of the armature its north pole.

"The electric current which I am sending through the armature comes first through one ordinary 16-candle-power electric lamp which you see lighted on this 'resistance board,' as it is called, and you notice that the needle points to .5. This means that half an ampere of electricity is passing through this lamp. I will now send the current through a 32-candle-power lamp, and you notice that the needle points to one, indicating that one ampere is required to light that lamp. But what prevents the needle from going farther, and what brings it back to zero each time?" The boys discovered a very small spring, like the hair spring of a watch, coiled around the pivot of the armature. "So, then, one ampere of electricity gives magnetism to this armature so that it may pull against its coiled spring hard enough to carry the needle to the point one. Twice as much electricity will give it magnetism enough to carry it to two, and so on across the scale.

"The full name of this instrument is Ampere meter, which by usage has been shortened to ammeter. It was named in honour of André Marie Ampère, who was born at Lyons, in France, in 1775, the year our Revolutionary War broke out. He died in 1836. When Oersted made his famous discovery of the action of an electric current upon a magnetic needle, in 1819, Ampère was in middle life (forty-four), and took up the same line of research with great vigour. The next year, 1820, he discovered what you will doubtless enjoy rediscovering now.

"You will notice that the binding posts on the bottom of this ammeter are marked, one positive, +, and the other, negative -. The electric current now enters the instrument by the post marked + and after passing around the armature leaves by the post marked -. I will reverse the connections and thus send the current around the armature in the other direction, and you notice that its poles are now reversed. The lower end which was formerly the north pole of the armature has now become the south pole, as proven by the fact that it is repelled from the south pole of the field and attracted to its north pole. This carried the needle to the left, and inasmuch as the zero is in the middle of the scale we may with this instrument both measure the amount of current and tell its direction. You will recall that when we connected the magneto with this instrument, it indicated that the magneto sent the current first in one direction and then in the other, which we call an 'alternating current.' But you notice that the current which I am using in this laboratory flows continuously in one direction. This is called the 'direct current.' We shall find out how a dynamo may produce a direct current at another time. Let us not forget, however, that we have repeated Ampère's discovery, and found out that the direction in which we send the current around an electro-magnet determines which end shall be its north and which its south pole. If you will note carefully which way the wire is wound around the armature you will see that when I send the current in at the positive post it is passing around the north pole of the armature opposite to the direction in which the hands of a clock move. If I reverse the current it passes around the lower end of the armature in the same direction as the hands of a clock move and then this end becomes a south pole. This is 'Ampère's rule,' and it is what candidates for admission to college are very careful to learn.

"Before we replace the face of this ammeter I must call your attention to a wire running by a short cut from one binding post to the other, s (Fig. 14). Suppose a represents the wire around the armature. Electricity, like water, goes more readily through a big conductor than a small one and more readily through a short than a long conductor. If s and a were water pipes, each having a stop-cock, we might easily adjust the cocks so that one tenth of the water would go through a and nine tenths through s. Or, indeed, without stop-cocks, the size and length of s and a might be so apportioned that one tenth of the water would flow through a and nine tenths through s. This is precisely the adjustment which has been made with reference to the flow of electricity through this instrument. s is called a 'shunt.' When the shunt is out all the current goes through a and when the shunt is in only one tenth of the current goes through a. I have two other shunts, each of which may be put in the place of s. With the second only one hundredth of the current goes through a and with the third only one thousandth of the current goes through a. Thus I have an instrument which will measure anything from one thousandth of an ampere up to ten amperes.

Fig. 14

"In this laboratory we pay about one cent for an ampere of electricity for one hour. Twice as much coal must be consumed to furnish two amperes as one, and twice as much coal must be consumed to furnish an ampere for two hours as for one hour. Hence we need an instrument which will keep account of time as well as amount of current. Such an instrument we must look into next.

"Just before we pass to that, however, let me ask if you have ever heard of a 'shunt-wound' dynamo. Can you guess from the way we have just used the word 'shunt' what the expression could mean with reference to a dynamo?" Without hesitation the boys told me that it meant that the field and armature were wound parallel to one another, as shown by diagram in Fig. 15. In which case the electric current which the machine generates divides, part of it going around the field and part around the armature. Another type, called series-wound dynamos, is indicated by diagram in Fig. 16, in which case the electric current goes through field and armature in succession. Under either of these circumstances, how can the armature move with reference to the field? The answer will appear in the next chapter.

Fig. 15

Fig. 16

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