146 Mr. H. Tomlinson on the Effect of Change of 



coil ; the temperature-coefficients of the platinum coil had 

 previously been very carefully determined*. The iron wire 

 used was 1 millim. in diameter ; it had been thoroughly 

 annealed, and the loads employed were not sufficient, even at 

 the highest temperature, to produce any very sensible per- 

 manent extension. As a preliminary the wire was heated 

 to 300° C, tested with all the loads and magnetizing 

 forces to be presently mentioned, and again tested when 

 cooled to the temperature of the room. This treatment was 

 repeated several times until constant results were obtained at 

 both temperatures ; the observations recorded in figs. I.-VI. 

 were then made. In these figures the abscissae represent the 

 number of kilogrammes in the load and the ordinates the per- 

 centage alteration of permeability at the temperature indicated 

 on each curve, + signifying increase and — decrease of per- 

 meability. The Villari critical point is the value of the 

 magnetizing force for which loading does not affect the per- 

 meability and may be ascertained by noting the point at which 

 each curve cuts the line of loads ; it varies with the value 

 of the load and with the temperature. In some instances the 

 curves cut the line of loads twice, and there are consequently 

 two loads which for the magnetizing forces pertaining to the 

 curves have no effect on the temporary magnetization. In 

 the following table is given the load (or loads if there are 

 two) which for each particular magnetizing force and tem- 

 perature is without effect on the permeability. 



Table I. 



Magnetizing 



force in 

 C.G.S. units. 



2-84 

 3-70 

 4-83 



Load for which 

 the permeability 



is the same as 



for the unloaded 



wire. 



Temp. 12° 0. 



4-6 

 2-5 

 1-8 



Ditto. 



Ditto. 



Ditto. 



Ditto. 



Temp. 



Temp. 



Temp. 



Temp. 



76° C. 



167° 0. 



244° C. 



285° C. 



4-8 



5*1 and 



5-8 and 







12-0 



10-1 





3-2 



36 



4-2 and 



47 and 







11-5 



9-9 



25 



2-7 



not tested. 



3-1 and 

 123 



* For further information concerning the platinum coil and the 

 primary and secondary coils, see Phil. Mag. [5] xxv. p. 372. 



