﻿il6 Prof. Chattock and Mr. F. B. Fawcett on the 



in the magnetizing force experienced by the iron under test ; 

 the latter being contained in a zinc box C, which was filled 

 and surrounded with cotton-wool and placed between the 

 poles DD. 



The test-piece consisted of a bundle of fine iron wires 

 lying parallel to the lines of force, into the centre of which 

 was introduced one end of a " line " thermopile. The other 

 end of the pile was surrounded by a coil of silk-covered 

 copper wire all in one length, which was similar in size and 

 shape to the iron test-piece. 



The actual size and shape of the mould in which the separate 

 plates of the alloys forming the pile were cast is shown in 

 fig. 2. The plates were about ^ millim. thick, and were com- 

 posed alternately of the alloys * 32 parts bismuth + 1 part 

 antimony, and 12 parts bismuth + 1 part tin. They were 

 placed side by side with mica to separate them, and were 

 soldered at their pointed extremities, which were then covered 

 with goIdbeaterVskin. Fourteen pairs were used to form 

 the pile. 



By this means the junctions formed a blunt knife-edge and 

 were brought well into the centre of the iron wire ; the mass 

 of the pile being at the same time kept small at that point, 

 while it w r as large enough outside to prevent undue resistance 

 to the thermal current. 



The galvanometer, connected through a mercury commu- 

 tator with the pile, was at a distance of nearly 20 yards from 

 the electromagnet, and its circuit was provided with a small 

 variable E.M.F. to counterbalance accidental thermal currents. 

 The scale was read by a telescope, and a motion of a tenth of 

 a division could be easily detected. 



Results of the Experiments. 



The constant saturating field between DD (tig. 1), pro- 

 duced by exciting the main coils A, was first measured by 

 the earth-inductor method and found to be about 2800 C.G.S. ; 

 and a separate current was then sent through the coils B of 

 such a strength that on reversing it the main field was altered 

 by 166 C.G.S. units (H in equation iii.). The currents re- 

 quired to produce these fields wee also measured, so that 

 they might be reproduced for the experiment. 



After passing current through the exciting coils for a 

 couple of hours to obtain a steady temperature, the method of 

 experiment was as follows : — 



Galvanometer-readings were taken every 20 seconds, the 

 current in B being at the same time reversed every 5 minutes, 



* Boys, ' Cantor Lectures,' 1889, p. 18. 



