Part 10

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 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