ELECTRIC LIGHT

We have now arrived at a very interesting part of the study of electricity, as well as a more difficult part than we have yet told you of, but one which you can easily understand if you read carefully.

You must all have seen electric lights, either in the streets or in some large buildings, for so many electric lights are now used that there are very few people who have not seen them. But perhaps some of you have only seen the large, dazzling lights that are used in the streets, and do not know that there is another kind of electric light which is in a globe about the size and shape of a large pear, and gives about the same light as a good gas-jet.

These two kinds of electric lights have different names.

The large, dazzling lights which you see in the streets are called "arc-lights," and the small, pear-shaped lamps, which give a soft, steady light, are called "incandescent lights." We will tell you later why these names are given to them.

The incandescent lights are generally used in houses, stores, theaters, factories, steamboats, and other places where a number of small lights are more pleasant to the eyes. The arc-lights (Fig. 15) are used to light streets and large spaces where a great quantity of light is wanted.

It would not be pleasant to have one of these dazzling arc-lamps in your parlor—although it would give a great deal of light—because your eyes would soon become tired. But two or three of the small incandescent lights (Fig. 16) would be very agreeable, because they would give you a nice, soft light to read or work by, and would not tire your eyes. So, you see, these two different kinds of lamps are very useful in their proper places.

Now, if you will read patiently and carefully, we will try and explain how both these lights are made.

You have seen that the telegraph, telephone, electric bells, etc., are worked by batteries. Electric lights, however, require such a large amount of current that it is too expensive to produce them in large quantities by batteries. A small number of lamps could be lighted by batteries, but if we were to attempt to use them to light 500 or 1,000 lamps together, the expense would be so enormous as to make it entirely out of the question.

There are many millions of incandescent lamps in use in the United States, but you will easily see that there could not be that number used if we had to depend on batteries to light them. You will understand this more thoroughly when you have finished reading this little book.

Well, you will ask, if we cannot use batteries, what is used to produce these electric lights?

Machines called "dynamo-electric machines," or "generators," which are driven by steam-engines or water-power, are used to produce the electricity which makes these lamps give us light.

You will remember that in the chapter on Magnetism we explained to you how electricity makes magnetism, and now we will explain how, in the dynamo, magnetism makes electricity.

It has been found that the influence of a magnet is very strong at its poles, and that this influence is always in the same lines. This influence has been described as "lines of force," which you will see represented in the sketch above by the dotted lines (Fig. 17). Of course, these lines of force are only imaginary and cannot be seen in any magnet, but they are always present. The meaning of this term "lines of force," then, is used to designate the strength of the magnet.

Many years ago the great scientist Faraday made the discovery that, by passing a closed loop of wire through the magnetic lines of force existing between the poles of a magnet, the magnetism produced the peculiar effect of creating a current of electricity in the wire. If the closed loop of wire were passed down, say from U to D, the current flowed in the wire in one direction, and if it were passed upward, from D to U, the current flowed in the other direction. Thus, you see, magnetism produces electricity in the closed loop of wire as it cuts through the magnetic lines of force. Just why or how, nobody knows; we only know that electricity is produced in that way, and to-day we make practical use of this method of producing it by embodying this principle in dynamo-machines, as we will shortly explain.

In carrying this discovery into practice in making dynamo-machines we use copper wire. If iron were used, there would be a current of electricity generated, but it would be much less in quantity, because iron wire has much greater resistance to the passage of electricity than the same size of copper wire.

Perhaps you can understand it more thoroughly if we state that when a closed loop of wire is passed up and down between the poles of a strong magnet there is a very perceptible opposition felt to the passage of the wire to and fro.

This is due to the influence of the magnetism upon the current produced in the wire as it cuts through the lines of force, and, inasmuch as these lines of force are always present at the poles of a magnet, you will see that, no matter how many times you pass the loop of wire up and down, there will be created in it a current of electricity by its passage through the lines of force.

Suppose that, instead of using one single loop of copper wire, you wound upon a spool a long piece of wire like that in Fig. 18, and that you turned this spool around rapidly between the poles of the magnet, you would thus be cutting the lines of force by the same wire a great many times, and every time one length of the wire cut through the lines of force some electricity would be generated in it, and this would continue as long as the spool was revolved. But, as each length would only be a part of the one piece of wire, you will easily see that there would be a great deal of electricity generated in the whole piece of wire.

All we have to do, then, is to collect this electricity from the two ends of the wire, and use it. If we should attach two wires to the two ends of this wire on the spool, they would be broken off when it turned around, so we must use some other method. We fix on the end of the spool (which is called an "armature") two pieces of copper, so that they will not touch each other (as in Fig. 19), and fasten the ends of the wire to these pieces of copper. This is called a "commutator," and, as you see, is really the ends of the wire on the spool. Now we get two thin, flat pieces of copper and fix them so that they will rest upon the copper bars of the commutator, but will not go round with it. These two flat pieces of copper are called the "brushes," and they will collect from the commutator the electricity which is gathered in the wire around the spool. As the brushes stand still, two wires can be fastened to them, and thus the ampÈres of current of electricity, acted upon by the volts pressure, can be carried away to be used in the lamps, for you must remember that as long as the spool turns around it gathers more electricity while there is any magnetism for the wire on the spool to pass through. The constant revolving of the spool creates so much electricity that it is driven out from the wire on the spool, through the commutator to the brushes, and there it finds a path to travel away from the pressure of the new electricity which is all the time being made.

In this way we get a continuous current of electricity in the two wires leading from the commutator, and can use it to light electric lamps or for other useful purposes.

In explaining this to you, so far, we have used as an illustration of the magnet one of the steel permanent magnets in order to make the explanation more simple, but now that you understand how the electricity is made, we must explain to you something about the magnets that are used in dynamo-machines. We can perhaps make this more clear by giving another example.

Suppose you had a dynamo which was lighting up 100 of the incandescent lamps, each of 200 ohms resistance and each requiring 100 volts pressure. Now each lamp would take just a certain quantity of electricity, say half an ampÈre; so, the 100 lamps would require one hundred times that quantity. But, if you turned off 50 of these lamps at once, the tendency would be for the pressure to rise above the 100 volts required for the other 50, and they would be apt to burn out quicker. It is plainly to be seen, then, that we must have some means of regulating the magnetism so as to regulate the lines of force for the wire on the armature to cut through. We can do this with an electromagnet, but not with a permanent magnet, because we cannot easily regulate the amount of magnetism which a permanent magnet will give.

There is another reason why we cannot use permanent magnets in a dynamo, and that is because they cannot be made to give as much magnetism as an electromagnet will give.

Thus you will see that there are very good reasons for using electromagnets in making dynamo-machines. Let us see now how these electromagnets and dynamos are made, and then examine the methods which are followed to operate and use them.

You must remember, to begin with, that in referring to wire used on magnets and armatures and for carrying the electricity away to the lamps, we always mean wire that is covered or insulated. In electric lighting, insulated wire is always used, except at the points where it is connected with, the dynamo, the lamps, a switch, or any point where we make what is called a "connection."

As the shape of the magnets is different in the dynamos of various inventors, we will take for illustration the one that is nearest the shape of the horseshoe and the shape that is generally used in illustrating the principle of the dynamo. This is the form used by Mr. Edison, whom we have previously mentioned. This form is shown in Fig. 20.

Now, although this magnet appears to be in one piece, it really consists of five parts screwed together so as to make, practically, one piece. The names of the parts are as follows: F, F are the "cores"; C the "yoke," which binds them together; and P, P the "pole pieces," where the magnetism is the strongest. These pole pieces are rounded out to receive the armature, which, as you will remember, is the part that turns around.

The cores, F, F, are first wound with a certain amount of wire, which depends upon the use the dynamo is to be made for. Thus, you will see, there will be on each core two loose ends of the wire that is wound around it—namely, the beginning of the wire and the end where we leave off winding, which on the two cores together will make four ends of wire. We will tell you presently what is done with them.

After the cores are wound, they are screwed firmly to the yoke and to the pole pieces, so as to make, for all practical purposes, one whole piece pretty nearly the shape of a horseshoe magnet.

Now, to make the dynamo complete, we must put in the armature between the poles, which are rounded off, as you will see, to accommodate it. The armature is held up by two "bearings," which you will see in the sketch of the complete dynamo above. (Fig. 21.)

The armature in a practical dynamo-machine consists of a large spool made of thin sheets of iron firmly fastened together and having a steel shaft run through the center, upon which it revolves.

This spool, or armature, is wound with a number of strands of copper wire. The commutator, instead of consisting of two bars, is made in many dynamos with as many bars as there are strands of wire, and the ends of these wires are fastened to the bars of the commutator so as to make, practically, one long piece of wire, just as we showed you in explaining how the electricity was produced.

The brushes, resting upon the commutator, carry away the electricity from it into the wires with which they are connected.

Now we have our dynamo all put together and ready to start as soon as we properly connect these four loose ends of wire on the cores.

If you will turn back to Fig. 20 you will see that two of the wires are marked I, and the other two O. The letter I means the inside wire, or where the winding began, and the letter O means the outside wire, or where we left off winding.

Now, if we fasten together (or "connect") the two ends of wires, I and O, near the top of the magnet, we make the two wires round the cores into one wire, which starts, say, at I near the poles, goes all around one core, crosses over and around the other core down to the other end of the wire to O, near the poles.

So far we have called the iron a magnet, although it is not a magnet until electricity is put into it; so, when the dynamo is started for the first time, these two ends of wire, I and O, are connected to a battery or other source of current for the purpose of sending electricity through the wire on the cores. When the electricity goes into this wire the iron immediately becomes a magnet, and the lines of force are present at the poles.

Now, the armature is turned around rapidly by a steam-engine, and, as the wire on the armature cuts the lines of force with great rapidity and so frequently, there is quickly generated a large quantity of electricity, which passes out as fast as it is made through the commutator and the brushes to the lamp. And so long as the armature is revolved and the battery attached, the electricity will be made, or, as it is usually termed, "generated."

As we stated above, a battery is used the first time the dynamo is run, and now we will explain why it is not needed afterward.

Although iron will not become a permanent magnet, like steel, it does not lose all its magnetism after it has been once thoroughly charged. When the dynamo is stopped, after the first trial, and the battery is taken away, you will discover only traces of magnetism about the poles. They will not readily attract even a needle or iron filings; but there is, nevertheless, a very small amount of magnetism left in the iron. Small as this magnetism is, however, it is enough to make very faint and weak lines of force at the poles of the magnet.

After the battery is taken away, the ends of the wire on the cores, which were connected to the battery, are connected, instead, to the wires which carry away the electricity from the brushes to the lamps. Thus, you will see, if any electricity goes from the dynamo to the lamps, part of it must also find its way through the wires which are around the cores.

We will now start up the dynamo without having any battery attached and see what happens. The armature turns around and the wires upon it cut through those very faint lines of force which are always at the poles. This, as you know, makes some electricity; very little, to be sure, but it comes out through the brushes to the wires leading to the lamps, and there it finds the wires leading back to the cores. Well, part of this weak current of electricity goes into these wires and travels back round the cores and so makes the magnetism stronger. The consequence of this is that the lines of force become stronger and, as the armature keeps turning around, the electricity naturally becomes stronger, and so there is more of it going through the wires back to the cores and increasing the strength of the magnet all the time, until the dynamo becomes strong enough to generate all the current it was intended to give for the lamps.

Of course, you understand that the stronger the magnet becomes, the greater will be the lines of force and the greater the amount of electricity made by the turning of the armature. Now, there is naturally a limit to what can be done with any particular dynamo; so, while the electricity continues to strengthen the magnetism and the magnetism increases the electricity, this cannot go beyond what is called the "saturation" point of the magnet.

Saturation means that the iron is full of magnetism, and will hold that much but no more. You will learn more as to the saturation of magnets when you study electricity more deeply, and we therefore do not intend to enter into that subject in this book. We will only state, however, that the magnets of dynamos are not always charged up to their saturation point.

THE LAMPS

So far you have learned how the current of electricity is produced, and now we will follow along the wires to find out how it makes the lamps give out both strong lights and the smaller, pleasant ones.

Suppose we take first the large, dazzling lights we see in the streets, which, as you know, are called

ARC-LIGHTS

Those who have seen the arc-lamps will readily recognize them from the picture in Fig. 22.

You will see that there are two sticks, or "pencils," of carbon. Now you will remember that in the chapter on Magnetism we told you that in order to have electricity do work for us we must put some resistance or opposition in its way. When we get light from an electric lamp it is because we make the electricity do some work in the lamp, and this work is in pushing its way through a resistance or opposition which is in the lamp.

When we generate electricity in the dynamo and put two wires for it to travel in, the current goes away from the dynamo through one of the wires and will go back to the dynamo through the other one if it can possibly get a chance to get to this other one. Now, the electricity which is constantly being made fills the wires and acts as a pressure to force the current through the wires back to the dynamo, and, if we put no resistance or opposition in the way, it would have a very easy path to travel in and would do no work at all. The wires leading to an electric lamp should have very little resistance, not sufficient to require any work from the current in passing through.

So, if we bring the two carbons in an arc-lamp together they really form part of the wire, and do not interrupt the current in its travels, but, if we separate the carbons, we make a gap which the current must jump across if it wants to go on. As the volts, or pressure, is so great, the current must jump, and this against the resistance or opposition in an arc-lamp is that which gives the current so much work to do. Indeed, so hard is it for the current to jump across this gap that it breaks off from one carbon a shower of tiny particles as fine as the finest dust, and makes them white hot in passing to the other. This shower of fine carbon dust, together with the ends of the carbons, being white hot, of course makes a light, and this is the dazzling light which you see in the arc-lamp.

Of course, when the electricity has jumped over from one carbon to the other, it goes through it to the wire, and so passes on to the next lamp, where it has to jump again, and so on until it has gone through the last lamp, then it has an easy path to get back to the dynamo.

Now, we want you to understand more thoroughly how that much resistance or opposition will cause heat, so we will try to give you a simple example.

Most of you know that if you were holding a rope tightly in your hands and some one pulled it through them quickly and suddenly, it would get very hot and your hands would feel as though they were being burned. This is heat caused by your hands resisting or opposing the passage of the rope through them, and if you could hold on tightly enough and the rope was drawn through quickly enough, it would take fire. This fire would, therefore, cause heat and light.

It is just this principle of resistance to the passage of the current which causes the light in an arc-lamp, as we have shown you.

INCANDESCENT LAMPS

You have just learned that the light in an arc-lamp is caused by the current forcing off from the carbon sticks tiny particles and heating them up until they give a brilliant light. So, you see, in an arc-light there is a wearing away of carbon by electricity, and therefore these sticks, or pencils, of carbon in time are all burned away. In practice the carbon pencils last about eight or ten hours, and then new ones must be put in.

Now, in the incandescent lamp there is also carbon used, but the light is not produced by the combustion or wasting away of the carbon, as we will show you.

The picture below will show you the appearance of an incandescent lamp. (Fig. 23.)

You will see that this lamp consists of a pear-shaped globe, and inside is a long U-shaped strip of carbon no thicker than an ordinary thread. This is a strip of bamboo cane[1] which has been carbonized to a thread of charcoal. It is joined to two wires which come through the glass. These two wires come down through the bottom of the globe, and one is fastened to a brass screw-ring, while the other wire is fastened to a brass button at the bottom of the lamp. These two (the ring and button) must, as you know, be separated from each other by something which will not carry electricity, or they would make a short circuit when the electricity was applied. We separate the ring and the button in various ways.

Now, if we took the ends of two wires which were charged with the proper amount of electricity and put one wire on the screw-ring and the other on the button, the lamp would light up, because there would be a complete path for the current to travel in.

It will, however, be plain to you that it would be awkward to light the lamps in this way, so we use a "socket" into which the lamp is screwed. (Fig. 24.)

The wires from the dynamo carrying the electricity are connected in the socket, one wire with the screw thread into which the screw-ring fits, and the other with a button which the button on the lamp touches when the lamp is screwed into the socket. Thus we have a connected path for the current to travel in, or, as it is termed, a complete circuit.

You will notice that in the incandescent lamp the electricity does not need to jump, as it does in the arc-light, because we give it one continuous line to travel in.

In order, however, to get the current to do work for us, we put some resistance in its path, which it must overcome in order to travel back to the dynamo. The resistance in an incandescent lamp is the U-shaped carbon strip (or, as it is called, "filament"). This charcoal filament has so much greater resistance than the wires that it opposes, or resists, the passage of the electricity through it; but the electricity must go through, and, as it is strong enough to force its way, it overcomes this resistance and passes on through the carbon to the wire at the other end. You see it is a struggle between the carbon and the electricity, the current being determined to go on and the carbon trying to keep it back; and, in the end, the electricity, being the stronger, gets the best of it; but the struggle has been so hard that the carbon has been raised to a white heat, or incandescence, and so gives out a beautiful light, which continues as long as the current of electricity flows.

You will remember that in the arc-light the carbons are slowly consumed and new ones must be put in. If the carbon in the incandescent light were consumed, it would not last many minutes, because it is only about the size of a horsehair. Now, you will naturally inquire why this fine strip is not burned up when it is raised to so high a heat. Well, we will tell you.

You know that if you light a match and let it burn the wood will all be consumed. But did you ever light a match, put it into a small bottle, and put the cork in? If you never did, do so now as an experiment, and you will see that the match will keep lighted for an instant and then go out without consuming the wood.

The reasons for this are very simple. In order to burn anything up entirely it is absolutely necessary to have the gas called oxygen present, and, as the air you live in contains a very large amount of oxygen, there is more than sufficient in your room to cause the wood of the match to be entirely consumed after it is lighted. But there is such a small quantity of oxygen in the bottle that it is not enough to keep the fire going in the match, and, consequently, it will not burn up the wood.

The reason the filament in an incandescent lamp is not burned up is because there is no oxygen inside the globe. After the carbon is put in its place all the oxygen is drawn out through a tube, and the glass is sealed up so that no more oxygen can get in. This is called obtaining a "vacuum," and vacuum means a space without air.

There being no oxygen in the globe, it is impossible for the carbon to burn up; so the incandescent lamp will continue to give its light for a very long time, some of them lasting for thousands of hours. Some day, however, from a great variety of obscure causes, the filament becomes weak in some particular spot and breaks, and the light ceases. When this happens, we unscrew the lamp and put another one in, and the light goes on as usual.

Now you have learned how the incandescent lamp is made to give light. We will add that it is a beautiful, soft, white light, almost without heat, it will not explode, throws off no poisonous fumes like gas or oil lamps, and has many other points of comfort and convenience which make it very desirable.

ELECTRIC-LIGHT WIRES

Before closing the subject of electric light you would perhaps like to know something about the way in which we place the wires leading to the lamps.

If you remember what we told you about measurements in the beginning of this book, it will be easy to understand what follows:

You know that if you have a very great pressure you can force a quantity through a small conductor. This is the principle upon which the arc-lamps are run. Every arc-lamp takes about 40 to 50 volts and from 5 to 10 ampÈres to produce the light, and they are connected with the wires as shown in Fig. 25.

This is called running lamps in "series," and, as you will see from the sketch, the wire starts out from the dynamo and connects with one carbon of the first arc-lamp, and to the other carbon is connected another wire which goes on to the next lamp, and so on until the last lamp is reached, and then the wire goes back to the dynamo. This forms, practically, one continuous loop from one brush to the other of the dynamo.

The current starts out, makes its way through the first lamp, goes on to the next, makes its way through that, and so on till it has jumped the last one; then it goes back to the dynamo.

Now, as each of these jumps requires a pressure of 40 or 50 volts, you will easily see that the total pressure, in volts, of the electricity must be as many times 40 or 50 volts as there are lamps to be lighted; so, if there were 60 lamps in circuit, there would be 2,400 to 3,000 volts pressure, which, while it gives very fine lights, might cause instant death to any one touching the wires.

Suppose anything happened to the first lamp, which stopped the current from jumping through it. There would be no path for the current to travel farther, and, consequently, all the lights would go out. To get over this difficulty there is sometimes used what is called a "shunt," which only acts when the lamp will not light. This shunt carries the current round the lamp to the other wire, so that it may travel on and light up the other lamps.

WIRES FOR INCANDESCENT LAMPS

The wiring for incandescent lamps is carried out in an entirely different way, which you can see by comparing Fig. 25 A with Fig. 25 which shows the wiring for arc-lamps.

This is called connecting in "multiple arc."

You will notice that the two wires running out from the dynamo (which are called the main wires) do not form one continuous loop as in the arc-light system, but that a smaller wire is attached to one of the main wires and then connected with the screw-ring in the lamp-socket; then another wire is connected with the button in the socket and afterward to the other main wire. Every lamp forms an independent path through which the current can travel back to the dynamo.

Now, if we turn one of these incandescent lamps out, we simply shut off one of these paths and the electricity travels through the other lamps, and, if we wish, we can turn out all the lamps but one and there will still be a way for the electricity to go back to the dynamo.

In the arc-lamps we must have a very high number of volts pressure, because the electricity has only one path, and it all has to pass through the first and other lamps till it comes to the last one. In the incandescent light the electricity has as many paths as there are lamps, so we only need to keep one certain pressure in volts in the main wires all the time. This pressure is even all the way through the main wires, and, therefore, it is ready to light a lamp the instant it is turned on, because, as you have seen, electricity will always get back to the dynamo if there is a possible chance, and the lamp opens a path.

The volts pressure used to operate any number of incandescent lamps is altogether very much less than for a number of arc-lights. For example, in the Edison system the pressure (sometimes called "electromotive force") is only about 110 volts, which is very mild and not at all dangerous. This electromotive force would be the same if there were one lamp or ten thousand lighted.

While this Edison current would not hurt any one, you should remember that it is much the better plan not to touch any electric-light wires until you have learned a great deal more on this subject.

We may add that each of the standard incandescent lamps requires only about one-quarter of an ampÈre of current to make them give a light of 16 candle-power, which is about the light given by a very good gas-jet, and while the electromotive force, or pressure, would only be about 110 volts, whether there were one lamp or ten thousand lighted, there must be sufficient ampÈres in the wires to give each lamp its proper quantity.

SWITCHES

We have made mention several times of turning on or off one or more lights, and now, perhaps, you would like to know how this is done.

Suppose the electricity was traveling through wires to one or several lamps, it would light up those lamps as long as the wires provided a path to travel in, but if you were to cut out one of them, which is called "breaking the circuit," there would be no road for the electricity to follow, and, consequently, its course would be stopped short and the lamps would go out. You will remember that electricity must have a complete circuit or it can do no work, and in electric lighting it is always a metallic circuit that is used.

Now, the switch is simply a device which is used to break the circuit so that the current cannot pass on. The simplest form of switch is seen in the sketch. (Fig. 26.)

You will see that there is a wire cut in two, and to one piece is attached a metallic piece, A, which turns one way or the other, and when it is turned so as to touch the other part of the wire the circuit is closed and the electricity goes from the lower part of the wire through the metallic piece A to the other part of the wire, thus making a complete circuit or path for the electricity to travel in.

If we turn the piece A away from the upper wire this breaks the circuit and cuts off the path, and, of course, the lamps would go out.

This is the principle of the switch, and, although they are made in thousands of ways, switches all have the same object—namely, the closing and breaking of the circuit, whether it is for one or a hundred lamps.

WIRE ON DYNAMOS

In explaining to you the construction and working of dynamo-machines, we did not state anything about the amounts of wire used in winding the machine.

It is not our intention to say exactly how much is used on any one dynamo, because that is among the things you will have to learn when you come to study the subject of electricity more deeply.

We simply want to have you understand that upon the number of turns of wire on any one machine depends the effect that that amount of wire, carrying electricity, will have upon a certain weight of iron when the armature is revolved a certain number of turns per minute.

A certain number of strands of wire on an armature will only do a certain amount of work at the most, so you will see that a small dynamo will not produce as much electricity as a larger one containing more iron and wire. For high pressure there must be more strands of wire cutting the lines of force more frequently than would be required for low pressure; and, to produce a great many ampÈres, the armature must be larger and the wire upon it thicker than it would need to be if only a small number of ampÈres were wanted.

This of itself is a very deep and complicated subject, and many books have been written upon it alone. We shall, therefore, not attempt to go more deeply into it in this little book, but simply content ourselves with giving you the general idea, which will be sufficient until you make a thorough study of the subject.

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