112 Lord Rayleigh and Mrs. Sidgxvick. [June 19, 



A battery composed of plates 10 inches long and ^ an inch wide 

 gives a current of 0*122 ampere at first starting, but as polarisation 

 takes place, after five minutes, only 0*079 ampere. The cost of a 

 battery of this size is 0'40 fr. (4rf. about), it remains in action for six 

 days without the renewal of the sodium. Batteries of larger 

 dimensions, as for example 10 inches long and l inches wide, last 

 four weeks, because the action is chiefly on the edges of the sodium 

 plate, and the broader the plate the longer the sodium lasts without 

 renewal. 



X. " On the Electro-chemical Equivalent of Silver, and on the 

 Absolute Electromotive Force of Clark Cells." By LORD 

 RAYLEIGH, D.C.L , F.R.S., and Mrs. H. SIDGWICK. Received 

 June 18, 1884. 



(Abstract.) 



The paper contains a record of a long series of experiments, 

 extending over nearly two years. 



The measurement of the electric currents is direct, not depending 

 upon a knowledge of the force of terrestrial magnetism. Three 

 horizontal coils are traversed in succession by the electric current. 

 Of these two of large diameter are fixed, and at a distance apart 

 equal to the radius of either. Symmetrically between them a smaller 

 coil is suspended in the balance. When the current passes, the 

 suspended coil is pressed down, or lifted up, according to the 

 connexions, and the observations relate to the double force called into 

 operation when the direction of the current in the fixed coils is 

 reversed. In a paper read before the British Association at South- 

 ampton it was shown that this construction presents special advantages, 

 and in particular that the calculation of the result does not require 

 an accurate knowledge of the radii of the coils, but only of the ratio 

 of the radii of the small and large coils. In this way one of the 

 principal difficulties, the measurement of the small coil, is evaded. 



The ratio of the radii is found by the electrical method of Bosscha. 

 A large and small coil being adjusted so as to be concentric and 

 coaxal, a very small magnet with attached mirror is suspended at the 

 common centre. The two circuits are connected electrically in 

 parallel, and resistance is added to one of them until no effect upon 

 the suspended magnet follows a reversal of the battery current. The 

 ratio of the resistances, to be found immediately by comparison with 

 standards, is the ratio of the galvanometer constants of the two coils, 

 and from this the ratio of the radii may be obtained by the introduc- 

 tion of small corrections relating to the finite dimensions of the 



