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IV. On the Use of an Iodine Voltameter for the Measurement 

 of Small Currents. By E. F. Herroun, F.I.C., Professor 

 of Natural Philosophy, Queen s College, London*. 



THE instruments in general use for the direct measure- 

 ment of currents by electrolysis, as in the determination 

 of reduction factors of galvanometers, comprise the hydrogen, 

 silver, and copper voltameters : each of these possesses cer- 

 tain advantages, but none is free from defect. When occasion 

 arises to make a large number of determinations of relatively 

 small currents, the difficulties attending their use become more 

 manifest. 



Some of the defects of the ordinary types may be briefly 

 stated. The hydrogen voltameter, consisting of platinum 

 plates immersed in dilute sulphuric acid, has trie following 

 disadvantages : — 



1. It acquires a large counter electromotive force of pola- 

 rization, so that the current, on first joining up, is very 

 inconstant. 



2. Its internal resistance is subject to considerable fluc- 

 tuations, according to the rate and mode of disengagement of 

 the electrolytic gases. 



3. Oxygen present in the dilute acid, or migrating from 

 the anode, diminishes the amount of hydrogen collected, and 

 so causes the current to be underestimated. 



4. When small currents are being measured, the bubbles 

 which cling to the electrodes or sides of the graduated tube 

 cause an appreciable error. 



5. Calculation of the current-strength is rendered difficult 

 owing to the necessity for correcting the volume of gas for 

 temperature, barometric pressure, and for the tension of 

 vapour of the dilute acid employed, which last is frequently 

 not exactly known. 



In the silver voltameter the foregoing objections do not 

 hold, with the exception of No. 3 ; for dissolved oxygen has 

 been found to affect the yield of silver, a greater amount 

 being obtained when the electrolysis is conducted in vacuo 

 or hydrogen than in air. But in the silver voltameter the 

 experimental difficulties in washing, drying, and weighing, it 

 may be only a few centigrammes of silver, involve a large 

 expenditure of time ; or if the weight of deposited silver be 

 increased by prolonging the current, the difficulty of main- 

 taining it constant is greater. 



The difficulties which exist with silver are still more 



* Communicated by the Physical Society : read May 10, 1895. 



