94 Profs. J. A. Fleming and J. Dewar, On the 



iron was, therefore, under these circumstances, case-hardened, and 

 will be referred to as the hardened iron ring. Having been formed 

 into a transformer in the above-described manner, a long series of 

 observations were taken with this coil to determine its permeability 

 at different temperatures and with different magnetising forces. The 

 results of these observations are shown in the Table V below, and are 

 delineated graphically in the curves in fig. 2, marked Hardened Iron. 

 The results show in a remarkable manner that the iron so 

 treated undergoes a very considerable increase in magnetic perme- 

 ability, when it is cooled to the temperature of liquid air; for certain 

 magnetising forces the permeability at the lowest temperature 

 reached may be increased as much as five times. In this respect, 

 therefore, this iron presents in an exaggerated degree the same 

 qualities found in the unannealed iron. 



Table Y. Variation of Magnetic Permeability with Temperature of 



Hardened Iron. 



Temperature measured in platinum degrees by standard thermo- 

 meter PI. 



Experiments with Steel. 



We have also examined the behaviour of a ring coil made of steel 

 pianoforte wire. We have found in this case the curious result that 

 pianoforte steel behaves in the same manner as the annealed soft 

 iron; its permeability is decreased as the temperature is lowered. 

 The results of the measurements with this steel-core ring are shown 

 in Table VI, and graphically in the curves in fig. 2, marked steel. 



