RICHARDS AND LEWIS. — ZINC AND CADMIUM AMALGAMS. 91 



with a Leclanche cell through such external resistance as to maintain a 

 total fall of potential of 0.1 volt. This was adjusted before each series 

 of measurements by comparison with a standardized Helmholtz cell which 

 remained constant in potential throughout the course of the experiments. 



The direct measurement of potential as far as three figures when de- 

 sirable was made possible by a device, the principle of which is shown 

 in the accompanying diagram, ^i'^ represents a potential box containing 

 three sets of resistances: AB, 9 times 100 ohms; CD, 11 times 10 

 ohms; JSF, 10 times 2 ohms. These resistances are fitted with pegs 

 for connection as in the ordinary potential box. ABCD forms a simple 

 circuit between the terminals A and D. EF'xi, a shunt whose terminals 

 can be connected with movable caps to the pegs in the row CD. If 

 they are connected to two pegs, G and H, for example, between which 

 there is 20 ohms resistance, the current between the pegs is evenly 

 divided by the shunt. Thus arranged the total resistance of (7— Z^ is 

 100 ohms, that of A—B is 900 ohms, making 1,000 ohms for the whole 

 box. A definite potential is maintained between A and D. \i for 

 example this is 0.1 volt, then the fall between two adjacent pegs is in 

 ^5 0.01 volt; in CD 0.001 volt (except between G and H, where 

 the total fall is 0.001 volt); in EF 0.0001 volt. The cell, P, whose 

 potential is to be measured, is connected through the galvanometer to 

 the box by means of the caps /and K. Its potential is compensated, and 

 there is no current through the galvanometer when it is equal to the 

 total fall from B to I, from G to H, and from E to K. This value can 

 be read directly from the box ; thus the arrangement in the figure indi- 

 cates a potential of 0.0374 volts. Of course even the next decimal place 

 may be estimated from the deflections of the galvanometer when two 

 adjacent pegs in EF on each side of the true point are connected in 

 succession, or might be measured directly by yet another shunt similar to 

 E F, with a total resistance of four ohms. In this case E F mxxsi be 

 provided with an extra peg. 



The external resistance of 13,000 ohms was calibrated during the 

 course of the experiments. Owing to an error found in this box, it was 

 necessary to correct the direct readings of potential of some of the early 

 cadmium amalgam cells by multiplying with a constant factor. The 

 corrected readings are given below without comment. 



Results. 

 Four classes of cells were studied: — (1) electrodes of cadmium 

 amalgam of different concentration ; (2) electrodes of zinc amalf^am of 



