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.
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.
(B) Remove the connector from Cu, swing it over into the position of the dotted line ([Fig. 115]), slip the connector upon M P and watch the needle. This cuts the cells out of the circuit; but, if you desire, also remove wire 3 from M P. Does the storage cell, S C, produce any current? Does it pass through A G in the same direction as that which came directly from the two cells?
(C) Try the dry cell in place of the two simple cells. Try 2 other cells in series if you have them.
382. Secondary or Storage Cells must be charged by a current before they can give out a current. Electricity is not really stored. Chemical changes are produced in the storage cell by the charging current, as in the voltameter or electroplating bath; and it is, then, potential chemical energy that is stored. When the new compounds are allowed to go back to their original condition by joining the electrodes of the charged cell a current is produced. In other words, an electric current[157] produces chemical changes in the cell by electrolysis, and these new compounds have an E. M. F. of polarization because they are constantly willing and anxious to get back to their old state. The plates are lead and are usually coated with compounds of lead. Hydrogen and oxygen are given out at the electrodes. The current from a dynamo is used to charge secondary batteries. (See "Things a Boy Should Know About Electricity.")