and Electrical Resistance of Iron. 227 



have shown that though the susceptibility of that oxide rapidly 

 decreases with rise of temperature, yet it may be comparable 

 with that of iron, which is also very feebly magnetic in the 

 neighbourhood of 1000° C. The author's methods were, 

 however, not adapted to the determination of the magnetic 

 qualities of feebly magnetic bodies, and any determinations 

 by them of the permeability of the iron core when /a < 10 

 have in any case little or no worth ; so that errors of such 

 values need hardly be considered. 



The only serious effect of the oxidation was to alter the 

 sectional area of the iron. By directly measuring this section 

 both before and after the experiments with each ring-magnet, 

 uncertainties in the absolute values, both of the permeability 

 and resistance measurements, were practically eliminated in 

 all the series of observations, except that taken after annealing 

 specimen B at 840° ; and even in this case the alteration of 

 resistance which took place gave the means of determining 

 with but little uncertainty that sectional area which was not 

 directly measured. Whilst cooling from 1150° a pause of 

 half-an-hour was made at 1071°. During this time the 

 resistance of the iron did not increase, and it is therefore con- 

 cluded that from this time onwards no further change of 

 section due to oxidation took place. 



Method of taking Observations at each Temperature. — 'The 

 experiments with specimen B were as follows (with specimen 

 A they were similar but not so full) : — 



When the temperature had become quite stationary, and 

 the " standardizing throw " had been observed, the iron core 

 was de-magnetized by repeatedly reversing the magnetizing 

 current w r hilst diminishing its strength. 



Observations were then taken, by the method of reversals, 

 of the magnetic induction in the core for each of eight values 

 of the field; beginning with H = '078 and *153 to get the 

 permeability under very small forces, and ending with H = 9*20. 

 Next, the iron core was carried through a cyclic process of 

 magnetization between definite limits of magnetic field 

 (H= +6*83), observations being taken by the "step-by-step" 

 method — sixteen steps round the cycle. This process had, 

 however, been previously gone through several times till the 

 iron had reached a cyclic state. Finally, the iron was 

 carried through a further cyclical magnetization, this time 

 between definite limits of induction (e. g. B = + 4550 lines/cm 2 ). 



The last experiment was by far the most difficult ; for the 

 magnetizing currents had to be so adjusted that the sum of 

 the ballistic throws during one half of the cycle might add up 

 to a certain previously calculated amount depending on the 



