82 Mr. H. Tonilinson on the Effect of Heating and 



It will be noticed that the specific resistance both at 17° C« 

 and at 100° 0. is diminished with each repetition of the 

 heating and cooling process, until, finally, it becomes about 

 4J per cent, less than at first. Thus, though the permanent 

 effect on the electrical resistance is very much less than that 

 on the permeability for very minute magnetizing forces, it is 

 quite sensible. 



At the sixth heating the wire was kept at 100° C. for 26 

 hours and. moreover, the battery- current was applied first in 

 one direction and then in the opposite. These reversals of the 

 current served to increase the rate of diminution of the re- 

 sistance. 



The maintaining the temperature at 100° C. for 26 hours 

 did not diminish the resistance as much as the next two 

 heatings and coolings taken together ; evidently, therefore. 

 the cooling exerts an influence as well as the heating. It is 

 not. in fact, of much use prolonging each period of heating 

 beyond 8 hours, whilst, on the contrary, a less time will hardly 

 suffice, heating and cooling in rapid alternation being com- 

 paratively ineffectual. 



Though the specific resistance of the annealed iron is per- 

 manently diminished by the heating and cooling process, this 

 is not so with the temporary change of resistance arising from 

 change of temperature; the sixth column of the Table shows 

 that the temporary increase of resistance produced by raising 

 the temperature from 17 c O. to 100° C. is not affected by the 

 heating and cooling process, whilst the temporary increase of 

 resistance per unit becomes greater in proportion as the 

 specific resistance itself becomes less. 



The values of E 10o — R 17 and K *L were calculated so 



K i 7 ... 

 as to avoid, as much as possible, error arising from the per- 

 manent changes consequent on the heating and cooling. Let 

 C 1? C 2 . C 3 . &c., represent the resistances of the cold wire after 

 the first, second, third. &c., heatings, and H 1? H 2 . H 3 , &c, 

 the corresponding resistances of the hot wire, then the num- 



TT . TT 



bers in the sixth column were obtained from -^ — 2 — Cj, 

 jj C 1 + C 2 H 2 + H 3 n „ C 3 + C 4 - , , 



H 9 a — j — o ^3- -°-3 i — j & Q -> an d those m 



C +C 



the last column by dividing these bv C\. — ^-= — -. C 3 , 



C +C\ - . 



-^ — -, &c., respectively. Of course, the temperature of the 



room was not always 17° C. but the resistance of x at 17° C. 

 conld be calculated from its actual resistance at the temperature 



