=43. The Earth's Inductive Influence.= The earth's magnetism acts inductively upon pieces of steel or iron upon its surface. If a piece of steel or iron, like a stove poker, for example, be held in a north and south line with its north end dipping considerably, it will be in the best position for the magnetism of the earth to act upon it; that is, it will lie in the direction taken by the earth's lines of force. If the poker be struck two or three times with a hammer to shake up its molecules, we shall find, upon testing it, that it has become magnetized. By this method we can pound magnetism right out of the air with a hammer. If the magnetized poker be held level, in an east and west direction, it will no longer be acted upon to advantage by the inductive influence of the earth, and we can easily hammer the magnetism out of it again. (For experiments on magnets and magnetism see "Study," Part I.) CHAPTER III. HOW ELECTRICITY IS GENERATED BY THE VOLTAIC CELL. =44. Early Experiments.= In 1786 Galvani, an Italian physician, made experiments to study the effect of static electricity upon the nervous excitability of animals, and especially upon the frog. He found that electric machines were not necessary to produce muscular contractions or kicks of the frog's legs, and that they could be produced when two different metals, Fig. 35, like iron and copper, for example, were placed in proper contact with a nerve and a muscle and then made to touch each other. Galvani first thought that the frog generated the electricity instead of the metals. [Illustration: Fig. 35.] Volta proved that the electricity was caused by the contact of the metals. He used the condensing electroscope as one means of proving that two dissimilar metals become charged differently when in contact. Volta also carried out his belief by constructing what is called a _Voltaic Pile_. He thought that by making several pairs of metals so arranged that all the little currents would help each other, a strong current could be generated. Fig. 36 shows a _pile_, it being made by placing a pair of zinc and copper discs in contact with one another, then laying on the copper disc a piece of flannel soaked in brine, then on top of this another pair, etc., etc. By connecting the first zinc and the last copper, quite a little current was produced. This was a start from which has been built our present knowledge of electricity. Strictly speaking, electricity is not generated by combinations of metals or by cells; they really keep up a difference of potential, as will be seen. [Illustration: Fig. 36.] [Illustration: Fig. 37.] [Illustration: Fig. 38.]