Part 9

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.