130 Dr. S. W. Richardson on the Magnetic 



As the secondary is outside the ring whose temperature is 

 required, care is necessary to insure that the temperature of 

 the ring does not materially lag behind that of the thermo- 

 meter. 



To avoid the error due to this cause as far as possible, 

 readings were only taken when the temperature was prac- 

 tically stationary. 



The close agreement of the results obtained under different 

 conditions shows that the temperature of the ring could not 

 have been greatly different from that of the secondary. As 

 it was impossible to read the magnetic bridge and the 

 resistance bridge simultaneously, the time of each observation 

 was recorded. A time and temperature curve was plotted, 

 and from this the temperature at the time of taking a 

 magnetic observation was inferred. 



§ IV. Methods used for varying the Temperature and 



FOR INSULATING THE COILS. 



(1) Experiments at High Temperatures. 



Asbestos paper was used as an insulator in these experi- 

 ments. The specimen of paper used was obtained from 

 Bell & Sons and was of a very superior quality. After 

 heating to a white heat and subsequently cooling, the dis- 

 coloration was very slight — the paper then having a faint 

 yellow tinge. The paper also held well together after heating, 

 and showed no signs of crumbling. The amount of car- 

 bonaceous matter present must have been very much less 

 than that contained in the paper usually found in the 

 market. 



The paper was cut into strips of about 1 cm. width. 



A double layer of this paper was wound next to the ring. 

 Outside this the platinum secondary was wound, a double 

 layer of paper being twisted round the wire during wdnding ; 

 this thickness of paper being again doubled for the first and 

 last wind. The primary was of copper wire insulated in the 

 same way as the secondary, a double layer of paper being 

 wound between the primary and secondary. 



The ring when wound was placed in a copper box, to which 

 a copper tube of about 8 inches in length was attached. 



The box w r as placed in the muffle of a Fletcher's " Improved 

 Muffle " Furnace, being separated from direct contact with it 

 by means of a thick pad of asbestos board. The mouth of the 

 muffle was closed with another piece of asbestos board, through 

 a hole in which the tube attached to the box passed. 



This copper tube, through which the leads passed, served 



