This is a simple dynamo. A wire "cutting" the invisible lines of force, that a magnet is spraying out into the air, becomes "electrified." Why this is true, no one has ever been able to explain.
The amount of electricity—its capacity for work—which you have generated with the magnet and wire, does not depend alone on the pulling power of that simple magnet. Let us say the magnet is very weak—has not enough power to lift one ounce of iron. Nevertheless, if you possessed the strength of Hercules, and could pass that wire through the field of force of the magnet many thousands of times a second, you would generate enough electricity in the wire to cause the wire to melt in your hands from heat.
Cross-section of an armature revolving in its field
Forms of annealed steel discs used in armature construction
This experiment gives the theory of the dynamo. Instead of passing only one wire through the field of force of a magnet, we have hundreds bound lengthwise on a revolving drum called an armature. Instead of one magnetic pole in a dynamo we have two, or four, or twenty according to the work the machine is designed for—always in pairs, a North pole next to a South pole, so that the lines of force may flow out of one and into another, instead of escaping in the surrounding air. If you could see these lines of force, they would appear in countless numbers issuing from each pole face of the field magnets, pressing against the revolving drum like hair brush bristles—trying to hold it back. This drum, in practice, is built up of discs of annealed steel, and the wires extending lengthwise on its face are held in place by slots to prevent them from flying off when the drum is whirled at high speed. The drum does not touch the face of the magnets, but revolves in an air space. If we give the electric impulses generated in these wires a chance to flow in a circuit—flow out of one end of the wires, and in at the other, the drum will require more and more power to turn it, in proportion to the amount of electricity we permit to flow. Thus, if one electric light is turned on, the drum will press back with a certain strength on the water wheel; if one hundred lights are turned on it will press back one hundred times as much. Providing there is enough power in the water wheel to continue turning the drum at its predetermined speed, the dynamo will keep on giving more and more electricity if asked to, until it finally destroys itself by fire. You cannot take more power, in terms of electricity, out of a dynamo that you put into it, in terms of mechanical motion. In fact, to insure flexibility and constant speed at all loads, it is customary to provide twice as much water wheel, or engine, power as the electrical rating of the dynamo.
An armature partly wound, showing slots and commutator
We have seen that a water wheel is 85 per cent efficient under ideal conditions. A dynamo's efficiency in translating mechanical motion into electricity, varies with the type of machine and its size. The largest machines attain as high as 90 per cent efficiency; the smallest ones run as low as 40 per cent.