202 



Mr. J. Swinburne on a Potentiometer 



range with an accuracy that is quite unattainable by means 

 of any other forms of voltmeter or ampere-meter. It is 

 therefore of the highest scientific importance to be able to 

 extend the potentiometer method into alternating-current 

 work, so that comparisons can be made directly with a 

 standard cell. 



In October 1891* I described two forms of alternating- 

 current so-called ohmmeter. These instruments measured 

 either a quantity R such that E 2 /R was the power when E 

 was the effective or virtual pressure, or a quantity r such that 

 Cr 2 was the power when C was the virtual or effective current. 

 Of course in a circuit with capacity or self-induction, R and r 

 are not equal. One form of alternating ohmmeter was elec- 

 trostatic, the other electromagnetic. The electrostatic ohm- 

 meter can be coupled un in such a way as to compare two 

 pressures, and one of these may be alternating and the other 

 direct. The electrostatic instrument has the advantage over 

 the electromagnetic in requiring only one connexion to the 

 moving system, or needle, and that connexion has not to carry 

 any appreciable current. The needle may therefore be sus- 

 pended by means of a single silk fibre, connexion being made 

 through a hanger dipping into water, and this hanger may 

 end in a vane to make the instrument dead-beat. As the 

 ohmmeter does not need any torsional control, but should be, 

 on the contrary, as free as possible, this form of instrument 

 can be made exceedingly sensitive. For potentiometer work 

 it is best not to use the instrument for giving the ratio of two 

 electromotive forces, but to design it as a differential galvano- 

 meter which shows whether the pressures are equal or not. 



Fig. 1. 



The disposition of the whole apparatus is shown diagram- 

 matically in fig. 1. In this scheme a is a battery maintaining 



* ' Industries,' October 30, 1891. 



