378. Electricity Every Where Active.—I have said that there is electricity in all substances, each having its own capacity for it, but that in the usual condition of substances the electricity is in a state of equilibrium, and therefore of quiet. We see this quiet disturbed whenever there is a thunder-storm, when we rub glass or silk, or a cat's back, or when we work an electrical machine. But the active state of electricity is not limited to such palpable demonstrations as these. Electricity is undoubtedly in action every where and always, although we can seldom appreciate and measure its action. Wherever there is motion there is a disturbance of the equilibrium of electricity, and a consequent return to this equilibrium. And this change from the one state to the other must be the constant cause of important changes and operations in the world around us, and in our own bodies. Let us look at some of the indications of this universality electrical action. The friction of any electric upon another awakens it. The friction of the belts upon the drums in cotton factories does it quite freely. Every stroke of India rubber upon paper as you erase a pencil mark excites electricity. The blowing of air upon glass does the same. So, also, does the blowing off of steam from an engine. Electricity has been excited even upon ice by rubbing it when cooled down to 13° below zero. Experiments upon the air have shown that there is usually some free electricity in it, the atmosphere being generally in a positive state, especially when the air is dry and clear. It is constantly generated from one source and another. It is generated every where by evaporation. Every gust of wind, causing friction of the particles of the air upon various substances, generates it. Motion of every kind probably generates it. Chemical action, as you will see in another part of this chapter, generates it every where. It is generated also in the operations of life, and in some animals there are special organs—electrical batteries—for the generation of this agent.
Fig. 259.
379. Induction.—A remarkable influence is exerted by an electrified body upon another body in its usual state when brought near it, and this influence is called induction. I will illustrate this by Fig. 259. Let A be a metallic ball standing on a glass pillar, and charged with positive electricity. Let B be a metallic cylinder supported upon two glass pillars. Now if A be placed near B, but not near enough for the electric spark to pass from it to B, it will destroy the equilibrium of the two electricties in B, the negative electricity being accumulated at the end near A, and the positive at the remote end. This is because the positive electricity in A repels its like in B and attracts the unlike fluid. You observe that there is a pair of pith balls suspended at each end of B, and also at the middle. The two balls at the positive end repel each other because they are charged with the same electricity, and so with the balls at the negative end. But the balls hanging from the middle are not affected, because they are on middle ground between the two electricities. Here is no communication of electricity from A to B, but only an influence upon the quiescent balanced electricities of B. Accordingly, if the surplus electricity of A be discharged by putting the hand or any good conductor upon it the influence will cease, the equilibrium in B will be restored, and the pith balls will all hang straight down. The same effect will be produced if A be withdrawn to a distance from B, and the influence will be renewed if A be brought near again.
Fig. 260.
If instead of one conductor we use two, B and C, Fig. 260, and have them in contact, we shall have the negative electricity on B and the positive on C. Now if we withdraw C from B we may have the two electricities separate, B being charged with the negative and C with the positive.
Fig. 261.
380. Electrical Machine.—You are now prepared to see how the common electrical machine operates. There are two kinds—the plate and the cylindrical. The plate machine, Fig. 261, has at p a large plate of glass, and at r a rubber which consists of two brass plates lined with leather which is stuffed, the pressure of which upon the glass is regulated by a screw. Above this rubber is a brass ball, d, and a brass chain connects the rubber and the ball with the floor, or, in other words, with the earth. At c is what is called the prime conductor—a hollow brass cylinder with rounded ends, having attached to it a rod with points, as seen at a. There is a similar rod attached to it on the other side of the glass plate. The different parts of the instrument are supported on glass pillars, g g g, standing on a wooden platform. The lower part of the plate is covered with a case of silk, which, being a non-conductor, prevents the electricity on the glass from being lost in the air, and also serves to keep the plate free from dust. The rubber is covered with an amalgam of tin, zinc, and mercury, this being found very effectual in exciting electricity. The operation of the machine is this: As the plate revolves positive electricity is collected upon the glass, and negative electricity upon the rubber. The former, as it comes to the points at a, goes to them and passes on by the rods to the prime conductor, while the latter passes from the rubber by the chain to the earth. The points at a are of great service in collecting the electricity, because the fluid is always much more ready to go to points than to conductors of a blunt shape.