﻿Fields on the Electric Conductivity of Bismuth, 491 



was easily calculated and the constant of the galvanometer 

 obtained from it. 



The ballistic coil for the large spiral consisted of one turn 

 of wire mounted on one of the mica plates between which the 

 Bi spiral was cemented, and embraced the whole area of the 

 spiral, which area was determined by accurate measurement. 

 The ballistic coil for the small spiral consisted of four turns of 

 very fine copper wire w T ound side by side on a small wooden 

 cylinder about 6 millim. in diameter and rather less than 

 1*5 millim. long, so that with the thin ends to keep the wire 

 in position the whole coil could pass between the poles. From 

 the exact dimensions of the wooden cylinder and of the wire 

 the constant of the coil was calculated. 



These two ballistic coils were compared in a uniform field 

 between large flat pole-pieces, and the ratio of their constants, 

 experimentally determined, agreed within one half per cent, 

 with the calculated value. 



The resistance of the spirals was measured always by the 

 AYheatstone-bridge with direct currents and galvanometer. 



At first no arrangement was made and no precautions 

 taken to keep the temperature constant, as other experimenters 

 had found no influence due to it ; but after some results had 

 been obtained, it was seen that these varied among them- 

 selves beyond the limits of observational error wherever the 

 temperature had varied, and so a method of keeping it con- 

 stant became necessary. 



For each spiral a watertight copper case was made, that 

 for the large spiral having the dimensions 95 X 26 x 5 millim. 

 and for the small one 95 X 9 x 1 millim., and these were fitted 

 over their respective spirals but so as not to touch the copper 

 rods which served as terminals to them. For each spiral a 

 water-bath was made, that for the large one consisting of a 

 cylindrical copper box with a horizontal section resembling a 

 lemniscate in form, the thin part in the centre going between 

 the poles. Fig. 3 shows a plan of the box between the poles. 

 Since it was necessary to remove the spiral quickly from 

 between the poles to measure the field, the ballistic coil being 

 fastened to it, and still to keep the temperature constant, the 

 spiral was fixed in the narrow part of the box and the box 

 mounted on vertical guides, so that by means of a cord passing 

 over a pulley the whole bath with the contained spiral and 

 ballistic coil could be suddenly raised clear of the magnet. In 

 each side of the bath a stirrer was placed. With this apparatus 

 experiments were made with ice, and with water the tem- 

 perature of the room in the bath, but none could be made 

 with hot water, as the spiral could not be heated above 30° (J., 



2 L 2 



