The dynamo is a mechanical engine, like the steam engine, the water turbine or the gas engine; and it converts the mechanical motion of the driven wheel into electrical motion,with the aid of a magnet. Many scientists say that the full circle of energy that keeps the world spinning, grows crops, and paints the sky with the Aurora Borealis, begins and ends with magnetism that the sun’s rays are magnetic rays. Magnetism is the force that keeps the compass needle pointing north and south. Take a steel rod and hold it along the north and south line, slightly inclined towards the earth, and strike it a sharp blow with a hammer, and it becomes a magnet feeble, it is true, but still a magnet.
Take a wire connected with a common dry battery and hold a compass needle under it and the needle will immediately turn around and point directly across the wire, showing that the wire possesses magnetism encircling it in invisible lines, stronger than the magnetism of the earth.
Insulate this wire by covering it with cotton thread, and wind it closely on a spool. Connect the two loose ends to a dry battery, and you will find that you have multiplied the magnetic strength of a single loop of wire by the number of turns on the spool concentrated all the magnetism of the length of that wire into a small space. Put an iron core in the middle of this spool and the magnet seems still more powerful.
Lines of force which otherwise would escape in great circles into space, are now concentrated in the iron. The iron core is a magnet. Shut off the current from the battery and the iron is still a magnet weak, true, but it will always retain a small portion of its magnetism. Soft iron retains very little of its magnetism. Hard steel retains a great deal, and for this reason steel is used for permanent magnets, of the horseshoe type so familiar.
A dynamo consists, first, of a number of such magnets, wound with insulated wire. Their iron cores point towards the center of a circle like the spokes of a wheel; and their curved inner faces form a circle in which a spool, wound with wire in another way, may be spun by the water wheel.
Now take a piece of copper wire and make a loop of it. Pass one side of this loop in front of an electric magnet.
As the wire you hold in your hands passes the iron face of the magnet, a wave of energy that is called electricity flows around this loop at the rate of 186,000 miles a second the same speed as light comes to us from the sun. As you move the wire away from the magnet, a second wave starts through the wire, flowing in the opposite direction. You can prove this by holding a compass needle under the wire and see it wag first in one direction, then in another.
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.
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.
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.