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



electrostatic voltmeter E. The reading of the electrostatic 

 voltmeter is thus unaffected by the continuous current, while 

 it is, at a given frequency, a measure of the alternating current 

 flowing. 



By means of the commutator C, the air-transformer T can 

 he thrown into either circuit, the resistance S being by the 

 same operation thrown into the other circuit. The resistance 

 S is equal to that of the thick wire coil of T, so that when S 

 replaces T the continuous current is unaffected by the change. 



Method of Experimenting . 



The arc was run at the required current and P.D. by alter- 

 ing the number of cells in B, K being always kept the same. 

 The current was kept constant by adjusting the arc by hand. 

 Under normal conditions the current could be kept constant 

 to within 1 per cent. R was now adjusted tilt the deflexion 

 of E was the same when T was in either circuit. When 

 balanced, a change of 0*01 ohm in R caused an appreciable 

 difference in the deflexions of E. It was found to be useless 

 to adjust more accurately than this, since the small variations 

 in quality always found in carbons produced differences of 

 this order in the resistance of the arc. 



Let the value of R when a balance is obtained be R x . This 

 is equal to the resistance, to alternating currents, of the bat- 

 tery (&]), the resistance (k), the arc lamp and connexions (7), 

 and the arc (x). 



.-. B.^k + hi + 1 + x (i.) 



The carbons are now firmly screwed together and the number 

 of cells in B reduced till the continuous current is the same 

 as before. R is again adjusted till the deflexions of E are 

 equal ; and if R 2 is the new value thus obtained, 



R 2 = k + b 2 + l; (ii.) 



b 2 being the resistance of the portion of the battery now used. 

 The cells are next cut out, the mains leading to them short- 

 circuited, and a third value R 3 obtained, 



R 3 = £ + ? , . (iii.) 



From (ii.) and (iii.) we obtain the resistance of b 2 , and by 

 proportion of any number of cells. Putting these values in 

 (i.) we obtain the value of x in ohms. 



The advantages of this method are that no calibration of 

 the electrometer is required, the speed of the alternator need 

 not be constant for long periods together, various alternators 

 giving different frequencies and wave-forms can be used 

 without materially altering the circuit, and especially that 



