Page:Conservationofen00stew.djvu/83

Rh in, without, however, diminishing the amount of our original stock. Now, this distant action or help, rendered by the original electricity in separating that of and, is called electric induction.

88. The experiment may, however, he performed in a somewhat different manner—we may allow and  to remain together, and gradually push them nearer to. As and  approach, the separation of their electricities will become greater and greater, until, when  and  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.

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