the voltmeter connection is reversed so that the needle will read up scale. This affords a method of determining the + and - terminals if the voltmeter is marked.
Let’s try moving the wire parallel to the lines of force (flux). Nothing happens, there is no evidence of voltage at all--the needle of the voltmeter has not moved. Then we move the wire diagonally through the field--the voltmeter needle has moved--but not as far as when we cut the flux at right angles. There is an e.m.f. but a small one.
What conclusions can we draw from these experiments? Clearly the greatest e.m.f. is produced when the wire cuts the flux at right angles. At any other angle the e.m.f. is less. It gradually becomes less as the cutting becomes parallel to the flux lines.
Fig. 17
Keeping these facts in mind, we can go on with our experiment and learn more about generators. We want to see what effect moving the wire through the flux at different speeds will have on the e.m.f. We move the wire at a certain speed (let us say at 10 ft. a second*) through the magnetic field at right angles to the lines of force. We note the reading. Let us say it reads 2. Then we move it twice as fast--we read 4. Moving it 3, 4, 5 times faster will cause a correspondingly higher voltmeter reading, 3 x 2; 4 x 2; 5 x 2 (6, 8, 10).
We can do the same thing, cutting the flux diagonally. Suppose that at the first speed the meter reads 1. Double the speed--the reading shows 2, etc.
*This does not mean that a wire must travel 10 ft. to have this speed (velocity).. It means that if this wire was allowed to continue its travel for one second without going faster or slower it would cover 10 ft.; 10 ft. per (for each) second.
