But no matter how fast we move the wire parallel to the magnetic lines of force, we can’t get a reading--no e.m.f. will be set up in the circuit.
If we substitute electromagnets with greater magnetic fields for the permanent magnet, we shall find that we can get much larger e.m.f.’s.
We can set up, in our imagination, a more complete experiment. We have 2 electromagnets; we pace them so that the North pole of one faces the South pole of the other at a distance of about 2 inches. Then we get a piece of wood about an inch and a half square, a quarter of an inch thick. We mount this piece of wood on a shaft and place it so that we can make it rotate in the magnetic field of the electromagnets. We fasten a turn (loop) of wire to the edges of the wood and connect the two lead wires to two separate copper rings mounted on the shaft so they will turn with the piece of wood and loop of wire. They are insulated from each other and from the shaft. (This whole assembly is an armature--and the poles of the electromagnets are called the field poles, the lines of magnetic force between the poles is the flux.)
Contact is made between the rings (slip rings) and the external circuit (with a special voltmeter in it) by brushes, pieces of copper mesh (metal cloth) or small blocks of carbon.
With this, we can start making our observations of an elementary (simple) generator. The armature is rotated by means of a small crank handle (it can be made by bending a wire to the shape of a crank). Watch the meter--it shows a good strong e.m.f. at times and the needle is always moving, first on one side of the “zero” line, then on the other. The e.m.f. is reversing in direction all the time. Turn the armature more quickly--the needle will swing back and forth faster--and farther. The maximum e.m.f.’s are greater and the reversals (alternations) are more frequent.
We can determine by experimental observations why this e.m.f. should be alternately in one direction and then in the other. First, however, we must understand more completely, the results of our previous analysis. Apparently, the direction of the motion and the direction of the lines of force have a great deal to do with the direction of the voltage.
Let us use an ordinary voltmeter having two terminals marked “-” and “+.” In the study of batteries, we recall that
