56 Mr. W. H. Preece. Effects of Temperature on [Feb. 22, 



Received February 20, 1883. 



Note. — The method adopted to measure the electromotive force and 

 the resistance of the different cells was very simple, and, as it is 

 believed to be very accurate and free from any of the disturbing 

 influences due to polarisation, a description of it may be useful. 



The charge or discharge of a small condenser through a galvano- 

 meter of comparatively low resistance is an accurate measure of the 

 electromotive force present, for the current is practically instantaneous, 

 und therefore the whole quantity present acts as a balista upon the 

 needle. 



In the figure C is a condenser of ^-microfarad capacity, G a sensi- 

 tive reflecting galvanometer, b the cell to be measured, r a shunt of 

 small resistance, and and K 2 simple keys. 



1 



1 Ka' 





1 







The condenser is first charged with a standard Daniell cell and the 

 charge deflection (D) noted. The standard cell is then replaced by the 

 cell b to be examined, and the charge deflection (d) produced by it 

 noted. The key K 2 is then depressed (K l9 which had been previously 

 depressed to charge the condenser, being still held down), and the 

 cell is thus shunted through r. The electromotive force affecting the 

 condenser is thus reduced and a discharge (d r ) is noted — the deflection 

 being in the reverse direction. 



Now if b be the resistance of the cell under test, then 



This particular mode of measuring was devised by Mr. H. E,. 

 Kempe and modified by Mr. Munro. Condensers are much used in 

 telegraphy for measuring electromotive forces. [Vide Kempe's 

 " Handbook of Testing," p. 195.] 



