274 Dr. C. R. A. Wright on the Determination of 



spoken of as the " density of the current ") is constant. The 

 following experiments show that this is so. 



The voltameters used were mostly those described above 

 (§ 78), or others of similar construction, only differing in 

 that the measuring-tubes were replaced by ordinary tine 

 tubes bent over and dipping under mercury or water, so as 

 to allow of the evolved gas escaping without introduction of 

 air — a similar arrangement being applied at the limb containing 

 the + electrode (as described in Fart III. § 71). They were 

 filled with boiled dilute sulphuric acid of such strength that 100 

 cubic centims. contained 27 # grammes of H 2 S0 4 (in other 

 experiments different strengths of acid were employed), and 

 then submitted to the action of a current of 25 to 50 micro- 

 webers for several days, or even weeks, until the rate of fall 

 of the polarization noticed after breaking battery-contact be- 

 came sensibly constant at its minimum value for the par- 

 ticular current passing (§ 70), at which time, as already stated 

 (§ 79), the difference between the hydrogen actually collected 

 and that calculated from the quantity of electricity passing 

 also became reduced to the constant minimum. During the 

 time that elapsed since first starting the experiment, obser- 

 vations of E, the difference of potential between the electrodes, 

 were frequently made : the current C passing being also 

 determined by observing the difference of potential set up 

 between the ends of a known resistance also in circuit (Part 

 III. § 69), and the resistance R of the voltameter being known 

 from the dimensions of the column of liquid between the 

 electrodes and its specific resistance (as determined by Kohl- 

 rausch and others), the value of e=E — OR, the counter E.M.F. 

 set up, was known. It was invariably found that the value 

 of e continually rose after first starting the experiment until 

 it finally reached a constant maximum, the time when this 

 maximum' was reached being precisely coincident with the 

 epoch when the minimum rate of fall of polarization and the 

 minimum deficiency in hydrogen collected were attained. 



Thus, for example, the following numbers were obtained with 

 voltameter No. 1, when filled with acid containing 40'0 

 grammes of H 2 S0 4 per 100 cubic centims., the values being 

 calculated by interpolation from experiments in which the 

 current was uniformly near to the value 1000 microwebers 

 = 0*0001 C.G.S. current-unit. 



Counter E.M.F. set up for a current of 1000 microwebers. 



Freshly boiled acid just cool ; values"! Values gradually in- 

 obtained during first few minutes > creasingup to 1'965 

 of passage of current. J volt. 



