Iron under Alternating Magnetic Force. 



225 



became steady, indicating a temperature of about 53° C, two sets of 

 curves were taken. The points obtained with the heated magnet are 

 indicated by in figs. 3, 4, 5, 6. In fig. 9 the curves obtained at 



Fig. 9. 



53° C. with a maximum magnetising force H of 8*85, and periodic time 

 2*6 minutes, are given in order to enable a comparison to be made 

 with fig. 8. It will be seen that coil No. 1 has an E.M.F. somewhat 

 retarded at the higher temperature. The E.M.F. of this coil also 

 suffers retardation of phase in the experiment with the lower force 

 2*85, when the magnet is at the higher temperature. 



Heating the magnet has had the effect of increasing the maximum 

 average value of B at the centre for the same frequency and slightly 

 smaller magnetising force of the same wave-form. The relation 

 between the surface density (No. 4 coil) and the average obtained from 

 coil No. 5 remains practically the same. In this connection it should 

 be remembered that for the same average over the whole core, a con- 

 siderable increase in the induction density at the centre is com- 

 pensated by a small decrease at the surface. It appears, then, that 

 raising the temperature of the magnet tends to equalise the maximum 



