While the two outer ends are in contact, there is that current formed in the metallic plates, which is termed galvanic. If the contact is broken, the action ceases; if it is again renewed, the action is recommenced. Another very simple experiment, and within the power of every one to demonstrate for themselves, is that of applying a piece of zinc to the underside of the tongue, and to the upperside, a silver coin, and then by bringing their projecting ends in contact, a sensible and curious effect is experienced upon the tongue. It is a feeble galvanic shock, and is proof of the presence of that fluid termed galvanic. We will now proceed to describe the battery used for telegraphic purposes; the same in principle, but in arrangement more complicated, and far more powerful than those in common use. Two distinct acids are employed; two metals and two vessels. Each part will be described separately, and then the whole, as put together ready for use. First. A glass tumbler of the ordinary size is used, or about three inches high and two inches and three quarters in diameter. Second. The zinc cylinder, made of the purest zinc, and cast in an iron mould, represented by figure 1. [Illustration: FIG. 1.] It is three inches high, and two inches in diameter. The shell I is three-eighths of an inch in thickness. D is an opening in the cylinder, parallel with its axis, and is of no other use than to aid in the operation of casting them, and facilitating the access of the fluid to the interior. A A represents the body of the cylinder. B is a projecting arm, first rising vertically from the shell, and then projecting horizontally one and three quarters of an inch. To this arm, at C, is soldered a platinum plate of the thickness of tin foil, and hanging vertically from the arm B, as seen at O, and of the form shown in the figure. This constitutes the zinc cylinder and platinum plate, the two metals used in the battery. [Illustration: FIG. 2.] Third. The porous cup. To avoid an erroneous impression in the use of the term porous, it will suffice to state, that it is a cup of the form represented by figure 2, made of the same materials as stone-ware, and baked without being _glazed_.[1] A represents the rim surrounding the top. From the under side of the rim to the bottom, it is three inches long, and one and one-quarter in diameter. The rim projects one-quarter of an inch, and the shell of the cup is one-eighth of an inch thick. These several parts are placed together thus. The porous cup, fig. 2, is set in the hollow of the zinc cylinder, fig. 1, represented by H, with the rim of the cup resting upon the top of the zinc at I. The zinc cylinder is then placed in the glass tumbler. The whole is represented in figure 3. [1] These are made at the American Pottery, in Jersey City, opposite New York. [Illustration: FIG. 3.]