312 MR. W. DUDDELL ON THE RESISTANCE AND 



B, a battery of from 50 to 90 accumulators which supplied the direct current to 



the arc, 



L, a self-induction, 

 p } , p.,, adjustable resistances, 



A, a Weston ammeter which indicated the direct current through the arc, 

 R, the standard non-inductive resistance, consisting of 12 coils of about 0'5 ohm 



each, described later, 

 t l and 2 , the terminals of the arc lamp, 

 3 and t 4 , terminals which could be moved along R, 

 I, 2, and 3, mercury cups in a wax block, 

 fi,fz, and y^, fine fuses in the connections between the above points. 



The direct P.O. between the terminals of the arc lamp (called " P.D. arc lamp") 

 was measured with a Weston voltmeter (not shown) which was connected between 

 the points 1 and 2. The same voltmeter, which could be connected between points 

 2 and 3, gave direct P.I), between the terminals t. 2 and 1 ?> (called " P.D., R"), from 

 which the resistance of 11 was obtained in terms of the readings of the voltmeter and 

 ammeter, both of which were carefully standardised. 



An image of the arc was projected by means of a lens on to a screen, divided and 

 marked so as to read the actual distance in millimetres between the ends of the 

 carbons ; this distance is called the " arc length." 



The circuit for adding the alternating testing current to the direct current 

 consisted of: 



D, a special high-frequency alternator which supplied the testing current, capable 



of producing small alternating currents having frequencies up to 120,000 -- 



per second, 



p 3 , a variable resistance, 

 S 2 and S 3 , plug switches, 

 f, a fine fuse, 

 T, an Ayrton and Perry reflecting twisted-strip ammeter, having a sensibility of 



400 scale divisions for O'l ampere at a scale distance of 1700 divisions 



(1 division =: -fg inch), 

 F, a condenser to prevent any direct current from flowing through the alternator, 



the capacity of which was 1 mf. for the frequencies from 120,000 to 50,000 -*- 



per second ; 2 - 5 mf. for frequencies from 50,000 to 2000 -- per second; and 



increased up to 15 mf. at 250 -- per second. 



The alternating current supplied to the arc circuit was kept at a constant R.M.S. 

 value as read on T by means of the adjustable resistance p z ; this current flowed 

 through the arc and R in series, and was practically prevented from flowing through 

 the battery by the self-induction L. At the higher frequencies of from 10,000 to 

 120,000 per second this self-induction behaved almost like an insulator; at the 



