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



cross- wires to the right resulted. Hence, if the differences 

 observed were due to the iron, an increase of the magnetic 

 field must have warmed it — just the reverse of what theory 

 required. 



The volume and resistance of the copper in the coil upon 

 the pile were known. By sending the proper current through 

 the coil for one second, it was possible to produce in the copper 

 the number (0*006) of calories per cub. centim. which ought 

 to have been produced in the iron by weakening the field, 



viz. —^-. The result was a deflexion which reached a maxi- 



u 



mum value of 3*8 at the end of about 75 seconds after the 

 copper had been warmed, and died away in less than 6 minutes. 

 As the two ends of the pole were as nearly alike as possible, 

 this value gave a rough idea of the deflexion to be expected. 

 The observed effect of 0*44 was thus about nine times too 

 small and in the wrong direction. 



These results were obtained at the end of the Christmas 

 vacation, and at this point we were obliged to remove our 

 apparatus for the term's work. 



Two possible causes for the effect we had observed occurred 

 to us. One, that the alteration in field-strength had altered 

 the resistance of the pile, and therefore of the current passing- 

 through it. The other, that the iron had not been quite 

 saturated throughout its length, and that hysteresis effects 

 had been superposed upon the one we were looking for. We 

 therefore undertook a fresh series of measurements this 

 summer with better appliances. The galvanometer was ren- 

 dered absolutely dead-beat with a large mica vane, so that 

 each reading was complete in itself. The distance between the 

 pole-pieces was decreased from 3*0 to 2*65 centim., by which 

 means the strength of the main field was increased to 3200 

 C.G.S., and the length of the iron test-piece was increased 

 until there w r as only 0*5 millim. clearance between its ends 

 and the pole-faces, instead of 7 millim. as before. 



Direct measurement now showed that the iron was 

 thoroughly saturated, and that the value of I for it was 1640. 



A further improvement consisted in the fact that the iron 

 wire was itself silk-covered and all in one length, so that the 

 artificial heating for calibration could be performed upon the 

 specimen itself, and a far more accurate indication obtained 

 of w r hat to expect from the variations of the magnetic field. 



Slight movements of the test-piece on altering the field 

 were now found to give rise to small thermal effects, due 

 doubtless to an alteration in the flow of heat from the exciting- 

 coils through the pole-pieces into the pile brought about 



