﻿and some Applications to Physical Measurements. 485 



wound on a short length of glass tubing and had an effective 

 diameter of 2'10 cm. The self-inductance, employing the 

 exact formula of Nagaoka *, viz., 



L s = 47r 2 <2 2 >i 1 2 b. K, 



where L s is the self-inductance of a current sheet of the 

 same dimensions as the coil, n 1 the number of turns per cm., 

 a the effective radius, b the total length, and K a factor 

 depending on the ratio of the diameter of the coil to the 

 length, to which was applied the correction for spacing, was 

 calculated to be 39,160 cm. The small frequency correction 

 was neglected. 



Coil L 2 . 



The coil was 36*70 cm. long, and consisted of 541 turns 

 of copper wire, no. 24 s.w.g., silk covered. It was wound 

 on a long glass tube, of external diameter 1*00 cm., and 

 separated therefrom by means of a layer of paraffined paper. 

 The effective diameter (2a) of the coil was 1*105 cm., and 

 self-inductance, calculated as above, was 92,430 cm. 



The total inductance L ( = L 1 + L 2 ) is thus 131,600 cm. 



The coil L 2 was outside the box containing the rest of 

 the circuits, and was shielded from external electrostatic 

 influences by means of an enveloping earthed metal cylinder. 



Coil L 3 . 



The length was 9 cm., and the coil consisted of 90 turns 

 of copper wire. no. 22 s.w.g., double cotton covered. It was 

 wound on a short, length of glass tubing and had an effective 

 diameter 4*13 cm. The self-inductance was similarly cal- 

 culated to be 124,600 cm. 



The capacities employed had a range of 100 to 1200 

 microfarads and were provided with a slow movement. 



Changes in the frequency of the oscillations of Set 1 

 brought about by the insertion of a specimen within the 

 coil L 2 may be due to three causes : — 



(a) In the first place, if the coil is not shielded from the 

 electrostatic effect of the specimen, the self capacity of 

 the coil will be changed. In order to observe changes 

 of inductance alone, it is necessary to guard against this 

 possibility. This was done by depositing a thin layer of 

 platinum f on the outside of the glass tube on which the 

 coil L was wound, and earthing. The thickness of the deposit, 

 obtained by weighing, was 7 x 10 ~ 6 cm. It is necessary to 



* Nagaoka, Jour. Coll. Sci. Tokyo, xxvii. art. 6, p. 18 (1909). 

 t The function of the paraffined paper was to prevent any possible 

 short-circuiting 1 of the coil through the layer of platinum. 



