_=379. Electrotyping=_ consists in making a copy in metal, of a woodcut, page of type, etc. A mould or impression of the type is first made in wax, or other suitable material (the pages of this book, for example, as set up by the printer). These moulds are, of course, the reverse of the type. They are coated with graphite to make them conduct electricity, and hung as the cathode, in a bath of copper sulphate. After a thin coat of copper has been deposited by an electric current, the wax is removed and the thin copper backed with soft metal. The metal surface next to the wax will be just like the type, only made of copper. These plates or _electrotypes_ can be printed from, the original type being used to set up another page. (See "Things a Boy Should Know About Electricity.") _=380. Voltameters=_ are cells used to measure the strength of an electric current. In the _Water Voltameter_ the hydrogen and oxygen produced are measured. The H acts like a metal and goes to the cathode, two parts of H being formed to one of O. _Copper Voltameter._ This cell measures the amount of copper deposited in a given time by a current. The copper cathode is weighed before and after the current flows. The weight of Cu deposited is then divided by the number of seconds during which the current passed, and this result, in turn, by .000328, which will give the average strength of the current in amperes. (See § 351.) Other forms of voltameters are also used. In all voltameters the quantity of metal deposited is proportional to the time that the current flows, and to its strength. [Illustration: Fig. 115.] =EXPERIMENT 151. To study the construction and action of a simple "storage" cell.= _Apparatus._ Two lead plates, L P, (Nos. 77, 78) fastened to a wooden cross-piece (§ 275). The spring-connectors should not be forced upon the thick lead. Fasten one end of the wire under the screw-head. A tumbler two-thirds full of dilute sulphuric acid (§ 258); the astatic galvanoscope, A G; wires to form connections; the two simple cells joined in series. _Arrange_ as in Fig. 115. One L P is joined to binding-post, L, of A G by the wire marked 1; wire 2 connects the other L P to the copper Cu. Wire 3 joins the zinc to any thin metal plate, M P, which is used for convenience, so that the spring connectors can be quickly slipped on or off. Wire 4 joins M P with binding-post R of A G. =381. Directions.= (A) Get clearly in mind the direction in which the right-hand end of the astatic needle is deflected when the current passes, remembering that it passes into A G at L and leaves at R. Allow the current to flow for 10 or 15 minutes through the circuit, at the same time watching the needle to see whether the strength of the current remains constant.