A Potentiometer for Alternating Currents. 201 



His method for measuring polarization was somewhat similar 

 to that used in this work. As bearing upon "the change of 

 polarization with time " I would refer especially to the in- 

 vestigation of Dr. E. Eoot upon this subject, discussed by 

 Prof, von Helmholtz, Wisch. Abli. vol. i. p. 835. These ex- 

 periments by Dr. Root seem to prove .clearly that the 

 liberated ions penetrate deeply into the electrode, even when 

 liberated upon but one side of it, as in this case. I take 

 great pleasure in expressing here my thanks and deep 

 obligation to Professor A. Kundt and Dr. L. Arons for their 

 kind sympathy and direction in this work. 



Physical Laboratory, Univ. of Berlin, 

 August 1892. . 



XV. A Potentiometer for Alternating Currents, 

 By James Swinburne*. 



Aegument. 

 A direct pressure is balanced against an alternating pressure by means 

 of a differential electrometer with single-fibre suspension, a null method 

 being employed. An alternating and a direct current are similarly 

 compared by means of a differential dynamometer with the controlling 

 spring removed. 



ONE of the chief difficulties in the way of the accurate 

 measurement of alternating pressures and currents 

 arises in the calibration of the instruments. Some of the 

 alternating-current instruments, such as those on the electro- 

 dynamometer principle, give the same reading whether a 

 direct or alternating current is used ; and electrometers and 

 some forms of electromagnetic instruments with soft iron 

 cores are also adapted equally well for direct and alternating- 

 current work. This is not the case, however, with a large 

 class of instruments, and the only method at present available 

 for calibrating such instruments is comparing them with 

 another instrument which has been calibrated by means of 

 a direct current. Such a practice allows errors to creep 

 in. No careful electrician would trust to direct-current 

 instruments calibrated second-hand, and accuracy is just as 

 important in alternating-current work. In addition to this, 

 the collection of cells and resistances which are now included 

 under the name "potentiometer" is exceedingly useful in 

 any laboratory where it can be permanently set up, as it 

 measures currents and pressures throughout an enormous 



* Communicated by the Physical Society : read December 8, 1893. 



