42 Dr. J. P. Dalton on a New Continuous-Bala- 



nce 



or simply 



R 2 



(25) 



where r is the resistance which, in series with the single 

 arm, shunts the condenser. 



§ 5. It was thought worth while to test the foregoing as a 

 practical method of comparison. Unfortunately no standards 

 were available, so a very severe test could not be imposed, 

 but it is perhaps of even greater value to experiment with 

 ordinary laboratory equipment, and ascertain how far the 

 method would serve as one for everyday use. 



A small solenoid which happened to be available was used 

 as a test coil. Its mean diameter was 3*9 cm., and it had 

 300 turns of fine copper wire wound in a single layer on a 

 length of 15'9 cm. The coil was not sufficiently well con- 

 structed to justify a very refined calculation of the inductance. 

 Regarding the field as uniform over any cross-section, and 

 equal to its value at the centre of that section, the inductance 

 becomes 



L = 47rVa 2 ( V^H^"-a), . . . . (2S) 



where I is the length and a the mean radius of the solenoid, 

 and n is the number of turns per unit length. In the present 

 instance this gives L = 0'75 millihenry. 



A more reliable value was obtained by direct comparison 

 against a capacity of one microfarad by Anderson's method : 

 this gave an inductance of 0*7 64 millihenry. 



A balance was then arranged in the following fashion : — 



Fig. 3. 



R 2 and R 4 are the ratio arms of an ordinary dial P.O. box. 

 In series with the third set of resistances of the box are 



