31(5 



MR. W. DUDDELL ON THE EESISTANCE AND 



also made to see if the condenser G really prevented M from being deflected by steady 

 direct P.D.'s ; for this purpose steady P.D.'s of from 100 to 180 volts were applied 

 to the terminals of the arc, the carbons being separated, and M was connected in the 

 ordinary way as if to measure V A . A deflection of M might have occurred due to 

 leakage or electrostatic forces, but no such deflection was observed. 



The Arc Lamp. 



A hand-fed arc lamp, enclosed in a,n iron case, was used in all the experiments. 

 The sliding contacts were shunted by flexible leads, so as to avoid any uncertainty in 

 the resistance of these contacts. The resistance of the contacts and of the carbons 

 was determined by adjusting the carbon holders to 10 centims. apart as in use, short- 

 circuiting them with a brass rod and the different carbons in turn, and then measuring 

 the drop in volts between the terminals of the lamp when 10 amperes direct current 

 flowed round the frame and holders, etc. The results are given below : 



TABLE II. 



Carbon holders short-circuited with 



Brass rod 



11 millims. solid "Conradty Noris" carbon. 



11 ,, cored ,, ,, , 



11 ,, solid " Lc Carbon" electrographitic carbon 



Resistance. 



ohms. 



0-0017 

 0-13 

 0-22 

 0-048 



The self-induction of the loop formed by the frame of the lamp with the carbon 

 holders 10 centims. apart and short-circuited by the brass rod was approximately 

 determined by passing an alternating current rpund the lamp and measuring the 

 P.D. between its terminals, at a frequency of 30,000 per second. The value 

 obtained was 2'4 x 10~ 7 henry, which was used as a correction. All the above tests 

 were taken with the lamp in place in its case, and with the carbon holders, etc., in 

 exactly the positions they occupied when commencing a test on the impedance of 

 the arc. 



The. Standard Non-Inductive Resistance, R. 



The essence of the test of the arc consisted in comparing its behaviour to alternating 

 currents of various frequencies witli that of the standard resistance R in series with 

 it. It was, therefore, necessary that R should be as free as possible from self- 

 induction and capacity. The type of resistance adopted was that described by 

 Professor AYIITON and Mr. MATHER before the Physical Society* in 1891. 



As both the self-induction and the capacity depend on the size of the resistance, it 



Philosophical Magazine,' 1892, vol. 33, p. 187. 



