Volta was led to discard the frog by observing that no electric results followed when the two plates were of the same metal. Suspecting from this that the frog was merely a conductor (instead of the generator) of the electric fluid, he tried the experiment with a wet card placed between two pairs of plates, and thus discovered that the secret lay in the metals being heterogeneous. But it cost thousands of experiments to reach this result, and ten years of ceaseless thought and exertion to arrive at the invention of the "pile," which merely consists of many pairs of heterogeneous plates, each separated by a moist substance. The weight of so much metal squeezed the wet cloth dry, and this led to various contrivances for keeping it wet, resulting at last in the invention of the familiar "trough-battery," now employed in all telegraph offices and manufactories of electro-anything. Instead of Galvani's frog or Volta's wet rag, the conductor is a solution of sulphuric acid, which Volta himself suggested and employed. The negative electricity is conveyed to the earth by a wire, and the positive is conducted from pair to pair, increasing as it goes, until, if the battery is large enough, it may have the force to send a message round the world. And the current is continuous. The galvanic battery is an electrical machine that goes without turning a handle. By the galvanic battery, electricity is made subservient to man. Among other things, it sends his messages, faces his type with copper, silvers his coffee-pot, and coats the inside of his baby's silver mug with shining gold.