87. Let us now suppose that we have set in action a machine of this kind, and accumulated a considerable quantity of positive electricity in its prime conductor at A. Let us next take two vessels, B and C, made of brass, supported on glass stems. These two vessels are supposed to be in contact, but at the same time to be capable of being separated from one another at their middle point, where the line is drawn in Fig. 6. Now let us cause B and C to approach A together. At first, B and C are not electrified, that is to say, their two electricities are not separated from each other, but are mixed together; but mark what will happen as they are pushed towards A. The positive electricity of A will decompose the two electricities of B and C, attracting the negative towards itself, and repelling the positive as far away as possible. The disposition of electricities will, therefore, be as in the figure. If we now pull C away from B, we have obtained a quantity of positive electricity on C, by help of the original electricity which was in A; in fact, we have made use of the original stock or electrical capital in A, in order to obtain positive electricity in C, without, however, diminishing the amount of our original stock. Now, this distant action or help, rendered by the original electricity in separating that of B and C, is called electric induction. 88. The experiment may, however, be performed in a somewhat different manner--we may allow B and C to remain together, and gradually push them nearer to A. As B and C approach A, the separation of their electricities will become greater and greater, until, when A and B are only divided by a small thickness of air, the two opposite electricities then accumulated will have sufficient strength to rush together through the air, and unite with each other by means of a spark. 89. The principle of induction may be used with advantage, when it is wished to accumulate a large quantity of electricity. [Illustration: Fig. 7.] In this case, an instrument called a _Leyden jar_ is very frequently employed. It consists of a glass jar, coated inside and outside with tin foil, as in Fig. 7. A brass rod, having a knob at the end of it, is connected metallically with the inside coating, and is kept in its place by being passed through a cork, which covers the mouth of the jar. We have thus two metallic coatings which are not electrically connected with one another. Now, in order to charge a jar of this kind, let the outside coating be connected by a chain with the earth, while at the same time positive electricity from the prime conductor of an electrical machine is communicated to the inside knob.