Part 5

"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.

"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.

THE WATTMETER

We were able to maintain connections between the binding posts of the ammeter and the movable armature of flexible wires because the armature never moves more than one third of a revolution, but we now wish to examine an instrument in which the armature must not only make a complete revolution but must continue to revolve in the same direction indefinitely. How are connections made so that an electric current may pass from the fixed binding posts to the wire of the moving coil? I will lift the cover off this instrument, which is called a wattmeter, and let you find the answer to that question.

I sent through the instrument the current from a 32-candle-power lamp. According to the ammeter, which was also in circuit, the amount was one ampere.

The armature of the wattmeter revolved slowly and it was not long before the boys reported that connections for the current were made by strips of metal sliding on metal plates. The ends of the armature wire were fastened one to one plate and the other to the other plate, and the metal strips brush along over the surfaces of the plates. (That is why they are called "brushes," I said.) And the brushes slide from one plate to the other each time the armature makes half a revolution. (That is, the brushes change the connection and thus change the poles of the armature at the proper instant so that they are always attracted to the poles of the field toward which they are moving.) This is called a commutator.

Notice that while the ammeter was like the magneto in having a steel magnet for its field, the wattmeter is like the dynamo in having electro-magnets for both armature and field. Notice in the second place that this instrument is an electric motor since it is made to revolve by an electric current. If it were made to revolve by some other power it would generate electricity and would then be called a dynamo. Indeed, let me tell you something which must at present be nothing more than a puzzle to you. Every machine, while it is being driven by an electric current as an electric motor, is, at the same time, acting as a dynamo to generate a current in the opposite direction. Notice in the third place that this is a shunt-wound instrument. The current which is sent into the instrument divides, and part of it goes through the field, while part goes through the armature. Motors, as well as dynamos, are either shunt-wound or series-wound. But notice finally that the axle on which the armature is carried has a cyclometer arrangement which keeps account of the number of revolutions. The armature is going slowly enough for us to count the revolutions. With watch in hand we found that it made one hundred and twenty revolutions per minute. I next brought the current to the wattmeter through a 16-candle-power lamp and the ammeter, connected in series, showed that half an ampere was passing. We counted the revolutions of the wattmeter and found them to be sixty per minute.

Here, then, is a simple electric motor which will register the amount of electricity we use. It will register the same amount whether we use one ampere for one hour or half an ampere for two hours or two amperes for half an hour. In any case this product is called one ampere hour. But the words printed upon the dials of this instrument are not ampere hours, but watt hours and the name of the instrument is wattmeter. This next requires explanation. Follow me in a little roundabout journey and the matter will be readily understood when viewed from another approach.

When we were estimating the energy required to climb the stairs of an apartment house, we needed to take into account two factors, (1) our weight and (2) the time which we took in climbing them. The amount of coal burned, steam generated, electricity produced, to run our elevator depends upon two factors, (1) its weight and (2) its speed. That idea is fundamental. Let us get at it in still another way. Suppose we have a mill pond, (Fig. 17, A). We construct a penstock p and install a water-wheel, S, to operate a mill. Our business increases and we install more machinery in our mill and must have more power to run it. We have two ways of getting it, (1) we may lengthen our wheel and enlarge our penstock so that a greater weight of water will fall upon the wheel, or (2) we may lengthen our penstock and move the wheel farther down so that the water will fall upon the wheel with greater velocity. It is just so with the electric current. Like water it is driven on in its course by pressure. The unit for electric pressure is called a volt. If we wish to drive the wattmeter or any other electric motor twice as fast as now, we may choose whether we shall do so by doubling the volts of pressure or by doubling the amperes of quantity.

The electric pressure on our mains is about one-hundred and ten volts. We three together weigh 330 pounds. Our elevator brought us up stairs at the speed of 100 feet per minute. It requires one horse-power to raise 330 pounds 100 feet in a minute. The ammeter in the engine room showed that 7 amperes of electricity were sent through the motor of the elevator to bring us up. That is, seven amperes at 110-volt pressure give one horse-power. In the office building across the street where they use a 220-volt current 3-1/2 amperes are required to take us up stairs at the same speed. It is necessary that the same amount of coal be consumed to furnish the horse-power of energy whether we supply it by means of seven amperes at 110 volts or 3-1/2 amperes at 220 volts. You notice that the product is 770 in each case. The name given to this product is watts. More accurately 746 watts of electrical power are equivalent to one horse-power. The name of this unit commemorates the famous inventor of the steam engine, James Watt (1736-1819). His monument now overlooks the Clyde at his native town, Greenock, Scotland.

To light a certain lamp, to heat a certain laundry iron, to furnish a certain amount of power for an electric motor, we must have a definite number of watts. We may choose whether we will have it at high or low voltage with correspondingly low or high number of amperes.

We will now connect with our laboratory current a 32-candle-power lamp, an ammeter, and a wattmeter, all in series, Fig. 18, and in parallel with these a volt meter. This last instrument indicates the electric pressure. Its mechanism will be examined later. The volt meter indicates 110 volts and the ammeter shows that one ampere is passing. The filament in the lamp resists the passage of the current. It gets quite hot and gives forth as much light as thirty-two candles. Its resistance is just such that 110 volts of pressure send one ampere through it. We will now take the reading of the wattmeter, note the time and read it again later. One hour later its index showed that 110 watt hours of electrical energy had been converted into light and heat. This at the usual rate, costs 1.1 cents, one cent per hundred watt hours or ten cents per thousand watt hours, called a kilowatt hour. The more common 16-candle-power lamp costs about half a cent an hour to operate. It requires one horse-power to keep fourteen of them burning.

I will now take you to see the wattmeter which measures all the electric energy used in this building. You note down its reading and the date and the next time you come we will read it again and thus find out how much electricity has been used for electric lights, for electric ventilating fans, for electric elevators, for electric ovens, and electric irons in the school of household arts, for electric motors to run lathes and other machines in the school of technical arts, for electric experiments in my laboratories and lecture room, for electric vacuum cleaners and, lastly, for pumping the pipe organ in chapel.

I saw by the boys' faces as they departed what would be the next question that they would bring to me. Knowing, however, that the hour was up, they were too polite to press it then.

THE ELECTRIC MOTOR

In a few days I received a telephone message, asking if I could appoint an hour to meet the programme committee in my laboratory. I must confess that my pleasure in these meetings had increased so much that I was quite ready to slight other duties, if need be, to engage in them. Moreover, since my business was education it was not difficult for me to regard these meetings in the light of a duty quite as important as my regular class instruction--perhaps more effective. At any rate the boys and I managed to get together. May God forgive the man who essays to teach boys, but does not love to be with them.

Of course at the last meeting of the Science Club every one wanted to know how we ran a pipe organ by electricity. Moreover the Electrical Show was coming on in the city, and cows were to be milked by electricity, dishes were to be washed by electricity, rugs and furniture were to be cleaned by electricity, and innumerable distracting and distressing things were to take place. I told the boys that really only two kinds of things were to be done by electricity at the show, and if they would give me two one-hour appointments I would furnish them with the key to the whole show. We might as well begin to-day with the pipe organ question.

A pipe organ is operated by air. It has bellows which are simply one form of an air pump. A boy is often employed to turn a crank which works the bellows. Down in the basement underneath our pipe organ I will show you how a half-horse-power electric motor takes the place of a boy. We found a dark and dirty corner where a boy used to stand and turn a crank every time æsthetically inclined people enjoyed an organ recital in the room above. Science, which has not been given credit for being humanitarian, put an electric motor into that dark corner and sent the boy up stairs to hear the music. The motor grumbled at the dirt in the corner and compelled the janitor to keep it clean.

The electric motor, better than any device I know, enforces justice, but never requires mercy, or at least rarely receives it. It comes nearer than any other machine to paying back all that you put into it. It is most economical when working up to its full capacity. I recommend that you look it over carefully and after a few minutes tell me what you have seen in it.

The boys said that it looked just like a dynamo. We must not forget that it is a dynamo, but is here used as a motor by sending an electric current through it. This fact, that a dynamo might be driven by an electric current and serve as a mover of other machinery, was first publicly exhibited in 1873 at the Vienna Exhibition, and by many believed to have been discovered by accident at that exhibit. But why does it look like a dynamo? It has a field whose magnetism is produced by an electric current sent through coils of wire, and it has an armature whose magnetism is likewise produced by the electric current. If it were used as a dynamo, where would it get the electric current to magnetize its field? From its own moving armature. Is it adapted for direct current? Yes. It has a commutator and brushes. Is it shunt- or series-wound? Shunt-wound, as shown by diagram in Fig. 20.

Suppose we treat the machine as a dynamo. Bring the ends of the line wire together, thus, as we say, closing the circuit. By some external force let us cause the armature to rotate and under the influence of the magnetic field it will generate an electric current, part of which will pass through the field and part through the line circuit. We may adjust the relative amount of wire in field and line so that any portion of the current we choose will pass through the field. The amount of current it will generate depends, (1) upon the strength of the field and (2) upon the speed of the armature. Its field, although never entirely without magnetism, is very feeble at first, and hence in the first instance a very small current will be generated in the moving armature. This, however, will strengthen the field slightly, and as the field is strengthened the armature will generate more current, and thus by a mutual reaction the machine gradually "builds up" to full strength.

When now we use the machine as a motor, an electric current must be sent along the line wires in the opposite direction (Fig. 21) from which it would come out of the machine when acting as a dynamo. It will then be noticed that, although the direction of the current through the field is the same, whether the machine is used as a dynamo or a motor, the direction through the armature, when used as a motor, is the reverse of that when used as a dynamo.