in seconds × .1733).] EXAMPLE.--If an arc lamp require a current of 8 amperes, how much electricity does it consume per hour? Since one coulomb = one ampere second, the quantity of electricity consumed per hour is 8 amperes × ( 60 × 60 ) = 28,800 coulombs. Voltameter.--A voltameter is an electrolytic cell employed to measure an electric current by the amount of chemical decomposition the current causes in passing through the cell. There are two classes of voltameter: 1. Weight voltameters; 2. Gas voltameters. Ques. What is the difference between these two classes of voltameter? Ans. In one, the current strength is determined by the weight of metal deposited or weight of water decomposed, and in the other by the volume of gas liberated. Fig. 544 shows a weight voltameter and fig. 545 a gas voltameter. Ques. How should the plates of a weight voltameter be treated before use? Ans. They must be thoroughly cleaned and polished with sandpaper, the sand being afterwards removed by placing them in running water. _The fingers must not be placed on any part of the plate which is to receive the deposit._ Ques. What form of voltameter has been selected to measure the International ampere? Ans. The silver voltameter arranged as here specified: The cathode on which the silver is to be deposited shall take the form of a platinum bowl, not less than 10 cms. in diameter, and from 4 to 5 cms. in depth. The anode shall be a disc or plate of pure silver some 30 sq. cms. in area, and 2 or 3 cms. in thickness. This shall be supported horizontally in the liquid near the top of the solution by a silver rod riveted through its center. To prevent the disintegrated silver which is formed on the anode falling upon the cathode, the anode shall be wrapped around with pure filter paper, secured at the back by suitable folding. The liquid shall consist of a neutral solution of pure silver nitrate containing about 15 parts by weight of the nitrate to 85 parts of water. Ques. What is the value of the International ampere as measured with the silver voltameter? Ans. The International ampere is represented sufficiently well for practical use by the unvarying current which, when passed through a silver voltameter (as described above) deposits _silver at the rate of .001118 gramme per second_. [Illustration: Fig. 546.--Single contact and short circuiting key. This key is intended especially for use with D'Arsonval galvanometers in zero deflection methods. The key is connected in circuit with the galvanometer so that whenever the key is not depressed, the galvanometer is short circuited, and its oscillations quickly damped out by the currents induced in its coil. The back end of the spring is held in a slot in a rubber block attached to the base.]