Page:Elementary Text-book of Physics (Anthony, 1897).djvu/326

 270. Fundamental Effects of the Electrical Current.—In 1791 Galvani of Bologna published an account of some experiments made two years before, which opened a new department of electrical science. He showed that, if the lumbar nerves of a freshly skinned frog be touched by a strip of metal and the muscles of the hind leg by a strip of another metal, the leg is violently agitated when the two pieces of metal are brought in contact. Similar phenomena had been previously observed when sparks were passing from the conductor of an electrical machine in the vicinity of the frog preparation.

He ascribed the facts observed to a hypothetical animal electricity or vital principle, and discussed them from the physiological standpoint; and thus, although he and his immediate associates pursued his theory with great acuteness, they did not affect any marked advance along the true direction. Volta at Pavia followed up Galvani's discovery in a most masterly way. He showed that if two different metals, or, in general, two heterogeneous substances, be brought in contact, there immediately arises a difference of electrical potential between them. He divided all bodies into two classes. Those of the first class, comprising all simple bodies and many others, are so related to one another that, if a closed circuit be formed of them or any of them, the sum of all the differences of potential taken around the circuit in one direction is equal to zero. If a body of the second class be substituted for one of the first class, this statement is no longer true. There exists then in the circuit a preponderating difference of potential in one direction.