A-B-C of Electricity

by Wm. H. Meadowcroft

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.