and the line above the horizontal line represents the e.m.f. in the opposite direction.
Naturally, it takes the conductor a definite length of time to go from 1 to 2. Since the length of 2 to 3 is the same as 1 to 2, it should take the same length of time. Let us assume it to be 1/10 of a second. From point 1 on the horizontal line, let us lay off 16 equal distances and each of these divisions will represent 1/10 of a second. From point 1 on the horizontal line, let us lay off 16 equal distances and each of these divisions will represent 1/10 of a second. At position 9, just as in position 1, the length is zero. At 10 we must naturally measure off below the line. Points 5 and 13 represent the maximum voltages. Now draw a line connecting all these points. The result is what is known as a sine curve and any A.C. voltage or A.C. current, the curve of which is like this sine curve, is called a sine A.C. voltage or sine A.C. current.
Alternating current machines, naturally, do not use permanent magnets and a single loop of wire for an armature as shown in these elementary illustrations. They use two or more electromagnets, called field coils, the armature having an iron core, on which a number of wires are wound in the form of loops insulated from the core and from each other. (See Fig. 20.)
Fig. 20
Fig. 21
One end, or half, of every loop on the armature is connected to a collector ring, the other end, or half, to the other collector ring. When a loop revolves in the magnetic field from the field coils, one-half of it cuts down on lines of force while the other half is cutting up. Every half revolution the loop changes because the half that was cutting up is now cutting down, and the half that was cutting down is now cutting up. This causes the induced current to reverse its direction and flow to the other collector ring. Every loop on the armature experiences this and
