Let us consider the state due to these twisting strings of particles.
Place two pennies lying on the table before you, and suppose them to be the sections in which two strings of a conductor are cut across, so that you are looking at two particles, represented by the pennies in the interior of a conductor; the strings, of which the pennies are sections, come up towards your eye. Now twist the two pennies each in the same direction—say that of the hands of a watch. From the outer edges you can take the motion off; the edges are moving in the same direction. But where the two pennies meet you will see that the edge of each is going in a contrary direction to the other. And if one penny tends to move an object in one way the other tends to move it in the contrary direction. Hence these motions tend to neutralize each other in the interior of a conducting wire.
Having now formed a conception of the state of the particles in an electrified poker, suppose another poker likewise held by an insulating handle is brought near the first. Let the pokers be so arranged that the handles both point one way, the black ends another way, and let the second poker be in the same line as the first, with its handle towards the black end of the first.
Now the first poker is charged, it contains electricity, its particles are twisted. What effect will it have on the second poker?
It is found that the second poker undergoes a certain change, but when it is removed to a distance from the first poker all trace of this change disappears.
On the end nearest the first poker—on the handle—is found silk electricity; on the end furthest from the first poker—on the black end—is found glass electricity.
Let A B be one poker, the + representing the charge of glass electricity. Let C D represent the other poker, the - representing the induced silk electricity, the + the glass electricity in it.
Let A be the handle of the first, B its black end. Let C be the handle of the second, and D its black end. To explain this let us bring in our imaginary principle. Let us suppose that when a charged body is brought near an uncharged body, but is separated from it by some medium through which electricity cannot pass—let us suppose that by some agency the twist in the charged body calls up an image twist in the body opposite it. Thus, due to the twist in the first poker there will be an image twist in the handle part of the second poker.