395. =Volta's Pile.=--The observations of Galvani awakened much interest in all scientific minds, and of course there was much of inquiry, observation, and experiment. Professor Volta, of Pavia, went a step farther than Galvani toward the true explanation, in referring the effects to the contact of dissimilar metals, and he was led by this view of the subject to construct his _pile_ or battery--called after him the voltaic pile--the object of which was to produce a much greater amount of electricity than could be obtained by the contact of only two pieces of metal. The pile is made of circular pieces of copper, zinc, and cloth, the cloth being moistened with salt-water. They are arranged as represented in Fig. 278. First a disk of copper is laid down, then upon this one of zinc, then one of cloth, and so on in the same order, the top of the pile ending in a plate of zinc. If you touch one end of the pile with a moistened finger and the other end with a finger of the other hand, you will feel a shock like that from a Leyden jar. The communication between the two ends of the pile may be made by wires, as seen in the figure. Volta afterward changed this to the form of a cup battery, the plates of metal being immersed in a series of cups in a mixture of sulphuric acid and water. There have been various improvements from time to time, but the arrangement is in all the different batteries essentially the same. Although Volta accomplished so much he did not arrive at the truth in full. His "contact theory," as it is called, so long received as the true theory, gradually gave way to the true explanation, viz., that the electricity produced is owing to chemical action. 396. =Difference Between Frictional and Voltaic Electricity.=--The electricity produced by the friction of the electrical machine is more intense than that of the voltaic battery. Voltaic electricity, on the other hand, is much more abundant, and is more continuous and lasting. As it is therefore more steady and more easily controlled than frictional electricity, it is used in the working of the Telegraph. CHAPTER XVI. MAGNETISM.