He describes his greatest experiment as follows: "After we had investigated the power of atmospheric electricity in storms, our hearts burned with the desire to investigate the daily quiet electricity of the atmosphere. Therefore, as the prepared frogs, hung on an iron railing which surrounded a hanging garden on our house, with brass hooks inserted in the spinal cord, fell into convulsions not only when it lightened, but when the sky was calm and clear, I thought that the cause of these contractions was the changes in the electricity of the atmosphere. Then for hours, yes, even days, I observed the animals, but almost never a movement of the muscles could be seen. At last, tired with such fruitless waiting, I began to press the brass hooks, which were fastened in the spinal cord, against the iron railing to see if such a trick would cause the muscles to contract, and if instead of changes in the atmospheric electricity any other changes would have any influence. I observed, indeed, vigorous contractions, but none which could be caused by the condition of the atmosphere." It was pressing the brass hook against the iron railing, thus forming an electric battery, that caused electricity to pass through the muscles of the frog. Galvani did not know that he had discovered a new source of electricity. He never arrived at a correct explanation of his results, and never knew the value of his discovery. Volta and the Electric Battery It was left for Alexander Volta to show that, in Galvani's experiment, the muscles of the frog, together with the brass hook and the iron railing, formed an electric battery. Volta showed that an electric charge can be produced merely by bringing two different metals into contact. He found that, if he placed copper and zinc in sulphuric acid, or a solution of common salt, he could, produce a continuous flow of electricity (Fig. 18). [Illustration: FIG. 18--VOLTA EXPLAINING HIS ELECTRIC BATTERY TO NAPOLEON BONAPARTE From a painting. Photo by Dubray.] In the beginning of the year 1800 Volta made the first electric battery (Fig. 19). It was made of copper and zinc disks placed alternately, with a piece of wet cloth above each pair of disks. With his column of disks he could obtain a strong shock; indeed, many shocks, one after the other. This first battery of Volta's was a form of "dry battery." Later Volta devised his "crown of cups," a form of wet battery similar to some batteries in use to-day. Each cup contained a strip of copper and a strip of zinc in dilute sulphuric acid. [Illustration: FIG. 19--THE FIRST ELECTRIC BATTERY No. 1--A battery of one hundred pairs of copper and zinc disks. No. 2--Two such batteries connected. By permission of the Italian Institute of Graphic Arts, Bergamo.]