Magnetic Permeability, $c., of Iron at Low Temperatures. 91 



of the current passing through the primary coil, and it was found 

 that the mean magnetising force to which the iron was exposed was 

 closely expressed by the value 20'219, multiplied by the ampere 

 current flowing through the primary coil. This coil had its second- 

 ary circuit connected* up to the galvanometer, as above described, 

 and a series of observations were taken with this coil by reversing a 

 constant magnetising current passing through the primary coil, and 

 observing the throw of the ballistic galvanometer connected with the 

 secondary circuit. The ring coil, together with the platinum thermo- 

 meter, was embedded, as above described, in a mass of paraffin wax, 

 and the whole mass, after having been cooled down to the tempera- 

 ture of liquid air, was slowly allowed to heat up again. Observations 

 were taken with two different magnetising forces over the range of 

 temperature from 185 C. up to the ordinary temperature, and 

 from the calculated induction in the ring determined for each mag- 

 netising force, the permeability was found corresponding to each 

 particular force and. temperature. The results of these observations 

 are given in Table IV, and are delineated in fig. 2, in the form of 

 two curves marked unannealed iron. 



Table IV. Variation of Magnetic Permeability of Unannealed ' 

 Swedish Iron with Temperature. 



Temperature measured in platinum degrees by standard thermo- 

 meter P t . 



Permeability. 



Temperature. 







- 20 



- 40 



- GO 



- 80 

 -100 

 -120 

 -140 

 -160 

 -180 

 -200 



The results of the observations, as indicated in fig. 2 in the 

 curves marked Unannealed Iron, show that for this unannealed iron 

 the permeability increases as the temperature falls, and is exactly 

 the reverse in the case of the same quality of iron carefully annealed. 

 The difference, also, between the two materials is very marked 



