the Self-inductance of Small Loops. 223 



of the circuits. With the solenoid a steady current of 

 2 amperes was used throughout ; the temperature rose from 

 15° to 20° during an experiment, and was allowed for.. 



Copper ring (1) : 



R = 6*68x 10 5 , 6-62 xlO 5 , 6'64xl0 5 , 6'52 x 10 5 : 



mean 6 61xl0 5 absolute E.M.U. 

 L = 85'G, 84'0, 84-3, 78'9 : mean 83*2 cm. 

 L calculated from . the dimensions 8 5' 3 cm. (low 



frequency). 

 Aluminium ring' (2) : 



R = 3-00xl0 5 , 3-03 xlO 5 , 3*12 x 10 5 , 3-18 xlO 5 : 



mean 3*08 x 10 5 . 

 L = 50-2, 54-2, 54-2, 56\5 : mean 53-8 cm. 

 L calculated from the dimensions 54*7 (Weinstein's 



formula) - 

 Solenoid (3). Resistances reduced to 15° C. : 



R = 4-45 xlO 8 , 4-32 xlO 8 , 4*50 xlO 8 , 4*58 xlO 8 : 



mean 4'46xl0 8 . 

 R measured on bridge 4'542 x 10 8 . 

 L = 2-65xl0 5 , 2-88xl0 5 , 2'80xl0 5 , 2'81xl0 5 : 



mean 2*78 x 10 5 cm. 

 L from deflexion experiments, taking bridge value 



of R, 2-88xl0 5 cm. 

 L calculated from the dimensions, with spacing cor- 

 rection * 2-89 xlO 5 cm. 

 L measured with a wavemeter and buzzer, 



2-94 xlO 5 cm. 



It appears that the method is uncertain to about 5 per 

 cent., and in addition there is a correction for wave-form, 

 which may amount to 3 per cent. We are also limited to 

 fairly thick rings in the damping experiment. 



3. High-frequency experiments. 



Much better results are obtained with frequencies of the 

 order used in wireless telegraphy. The results are inde- 

 pendent, to a considerable degree, of the frequency and the 

 resistance of the circuit. Formula (2) is usually very close : 

 for a second approximation we subtract R 2 /Ljp 2 from L. 



* For a convenient collection of formulae and tables, see Nottage, 

 'Calculation and Measurement of Inductance and Capacity' (Wireless 



Press). 



