When a thundercloud charged with electricity passes over the ground, it induces a charge of an opposite kind upon it.  The cloud and earth with air between are analogous to the charged foils of the Leyden jar separated by the glass.  The two electricities of the jar, we know, attract each other, and if the insulating glass is too weak to hold them asunder, the spark will pierce it.  Similarly, if the insulating air cannot resist the attraction between the thundercloud and the earth, it will be ruptured by a flash of lightning.  The metal rod, however, tends to allow the two charges of the cloud and earth to combine quietly or to shunt the discharge past the house.

CHAPTER II.

The electricity of chemistry.

A more tractable kind of electricity than that of friction was discovered at the beginning of the present century.  The story goes that some edible frogs were skinned to make a soup for Madame Galvani, wife of the professor of anatomy in the University of Bologna, who was in delicate health.  As the frogs were lying in the laboratory of the professor they were observed to twitch each time a spark was drawn from an electrical machine that stood by.  A similar twitching was also noticed when the limbs were hung by copper skewers from an iron rail.  Galvani thought the spasms were due to electricity in the animal, and produced them at will by touching the nerve of a limb with a rod of zinc, and the muscle with a rod of copper in contact with the zinc.  It was proved, however, by Alessanjra Volta, professor of physics in the University of Pavia, that the electricity was not in the animal but generated by the contact of the two dissimilar metals and the moisture of the flesh.  Going a step further, in the year 1800 he invented a new source of electricity on this principle, which is known as “Volta’s pile.”  It consists of plates or discs of zinc and copper separated by a wafer of cloth moistened with acidulated water.  When the zinc and copper are joined externally by a wire, a current of electricity is found in the wire One pair of plates with the liquid between makes a “couple” or element; and two or more, built one above another in the same order of zinc, copper, zinc, copper, make the pile.  The extreme zinc and copper plates, when joined by a wire, are found to deliver a current.

This form of the voltaic, or, as it is sometimes called, galvanic battery, has given place to the “cell” shown in figure II, where the two plates Z C are immersed in acidulated water within the vessel, and connected outside by the wire W. The zinc plate has a positive and the copper a negative charge.  The positive current flows from the zinc to the copper inside the cell and from the copper to the zinc outside the cell, as shown by the arrows.  It thus makes a complete round, which is called the voltaic “circuit,” and if the circuit is broken anywhere it will not flow