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



the same diagram represents the effect of heating the iron 

 wire by means of a current flowing through it for one second, 



and of the proper strength to produce ^ calories 



per cub. centim. of the iron. The points which determine 

 this curve are the mean values of several consistent ob- 

 servations. 



The following day the readings were in the wrong direc- 

 tion (curves b and c), nothing having been altered in the 

 mean time. Thinking that the alterations in the field-strength 

 might still be capable of producing some relative movement 

 of the pile and the strands of the test-piece, the effect of 

 which had been altered by the hardening of the paraffin 

 during the night, we caused the paraffin to penetrate the 

 interstices of the iron wire by means of the air-pump, and 

 the apparatus was then placed in position and subjected to a 

 field of about 3000 units for 5 hours while it cooled. Readings 

 taken next day were still negative, and to about the same 

 extent as before (curves d and e). The substitution of a 

 solid cylinder of iron for the iron wire did not affect matters. 

 Relative motion of test-piece and pile seemed therefore to be 

 excluded. 



The clearance between pole-faces and test-piece was next 

 increased from 0*5 to 3*5 millim. without altering the result ; 

 from which we inferred that the values obtained could not 

 be due to a slight yielding of the paraffin slab when the 

 field was altered, the distance from slab to poles being too 

 great to be affected thereby in this case. Neither could 

 they be due to a direct effect of the field upon the resistance 

 or E.M.F. of the pile, for we found that they were indepen- 

 dent of the direction of any permanent current which might 

 be flowing through the pile during the observations. 



It only remained therefore to suppose that our results were 

 attributable to a change in the conductivity for heat of the 

 system pile-testpiece ; the flow of heat through it from the 

 exciting current being alternately checked and accelerated as 

 the field was altered. That a flow was actually occurring 

 from the iron into the pile and on through it to its other end. 

 was shown by the direction of the E.M.F. required in the 

 galvanometer circuit to balance the permanent effect of the 

 pile. It was due to the fact that the pile was too long to 

 allow of its being wholly between the poles at the same time 

 that the test-piece was in a uniform part of the field. 



The correctness of this view was proved by showing that the 

 sign of the effect changed with the direction in which the 



