414 Messrs. J. Frith and C. Rodgers on the 



The alternator was run at a speed corresponding to this fre- 

 quency, at which the arrangement is most sensitive to changes 

 of R. " 



An experiment consists in adjusting R till the deflexions 

 of E are the same whether R or S + # is in circuit. Then 

 R = S + ,z. 



The experiment was first tried without a resistance at S, 

 but it was found impossible when using solid carbons to 

 obtain a balance even when R was zero. In order to make 

 it possible to obtain a balance with R greater than nothing, 

 a resistance S numerically greater than the negative resistance 

 of the arc had to be put in series with the arc. In our expe- 

 riments we kept S constant and adjusted R. The experiment 

 might have been performed by making R = 0, and adjusting 

 S till the readings of E were equal. Then S 4- x must be 

 equal to zero, and therefore x= — S, or the resistance of the 

 arc is equal to the resistance in the box S with its sign changed. 



This method gives results which agree well with those ob- 

 tained by Method I. ; but on account of the necessity of 

 keeping the speed of the alternator constant the arrangement 

 is more troublesome to manage, and therefore was used only 

 as a check method. 



Description of the Apparatus. 



The arc-lamp had a hand adjustment for each carbon with 

 centering arrangements for the positive. The arc-length was 

 measured by projecting an image of the arc on to a screen by 

 means of a lens in the usual way. 



The resistance K consisted of broad platinoid strips bent 

 back on themselves so as to avoid self-induction*. It did not 

 heat appreciably with the largest currents used. 



The battery consisted of 50 10-ampere E.P.S. cells arranged 

 in 4 groups of 10 cells each, 4 of 2, and 2 groups of 1. 

 These groups could be connected up by means of mercury 

 cups. 



The ammeter and voltmeter were Weston instruments of the 

 horizontal type. 



The air-transformer T was made in two sections which were 

 usually used in series. The constants were as follows :— 



Section No. 1 : 



Fine wire coil. 



Diameter of wire (uncovered) = 4*3 mils. 



„ „ (covered) =^6'8 „ 



Number of turns .... =9730. 



Resistance =4707 ohms. 



* Described in Phil, Mag. February 1892. 



