of Silver and of Copper. 411 



supported all along their length by a strip of wood. The length 

 of rod in each U most of course be sufficient for the metal not 

 to become much heated by the potential of cell, one or two volts; 

 the length actually used in the apparatus is four metres. The 

 troughs t are partially filled with mercury and have thick copper 

 bottoms, against which the ends of the plantinoid rods press 

 by their own weight. The thin wires have thick terminal 

 pieces fixed to the wooden strips for this purpose. The con- 

 ductivities of the different bridges, beginning at the end t, t, 

 fig. 7, are graduated so as to be nearly in the ratio 1, 1, 2, 4, 

 8, 16, &c; so that any conductivity between the highest and 

 the lowest can be put in by steps equal to the lowest conduc- 

 tivity in the set. To bridge over these steps and thus make 

 the variation from the highest to the lowest perfectly con- 

 tinuous, the rheostat R, which has a minimum conductivity 

 somewhat less than the lowest of the bridges, is introduced 

 and forms in fact another bridge which can be readily varied. 

 The wire of this rheostat is formed of a strand of fine copper 

 wires, and is capable of carrying a current of ten amperes. 



The galvanometer G and the current-balance B may be of 

 any form which it is desired to standardize ; they are simply 

 put into the figure by way of illustration. The electrolytic 

 cells E are rectangular earthenware vessels fitted in the 

 manner illustrated in figs. 4-6, and described on page 392, 

 above. The larger of these cells is fitted to receive plates 

 the aggregate surface of which is sufficient for two hundred 

 amperes, but when the current to be measured is smaller the 

 plates are not all introduced. In this cell the plates are so 

 large that there is considerable danger of their touching each 

 other when they are left freely suspended in the liquid, and 

 hence two iTs of thin glass rod are hung over each of the 

 anode plates, so as to keep the cathodes from touching them. 

 The small cell is convenient for currents of from 5 to 25 

 amperes. 



All these operations connected with the treatment of the 

 plates for these cehVhave been already described, and hence 

 it only remains to indicate how the apparatus is generally 

 used for standardizing instruments. Suppose, for example, a 

 current-balance is to be standardized, and that it is known, by 

 preliminary trials or by comparison with another instrument, 

 to require about fifty amperes to bring its index to the zero 

 mark when a certain weight is put on the beam. Arrange 

 the electrolytic cell so that the cathode plates expose a surface 

 of about twenty-five hundred square centimetres, and join up 

 as shown in the diagram, fig. 7, putting a bridge across from 

 4 to 5. Introduce conductivitv into D until the index of the 



