﻿470 



Mr. A. Campbell on the Use of 



slipped off the bar from the mid position and replaced without 

 removing the bar from the solenoid. By sending a measured 

 current through the solenoid the bar was subjected to a known 

 demagnetizing field, This field was gradually increased 

 until the search-coil when slipped off gave no throw on the 

 galvanometer, thus showing that B had been reduced to zero. 

 The value (H ) of the magnetic force when this took place is 

 a good indication of the permanence of the remanent 

 magnetism. 



If H is large, we should expect the bar to hold its magne- 

 tism very obstinately. The following table gives the results 

 for four of the rods (Nos. 1 to 4z). For the sake of comparison 

 the corresponding numbers are given for exactly similar rods 

 (M and A) of hardened magnet steel of well known makers, 

 M being supplied by Marchal of Paris, and A coining from 



the Allevard Forge. 



Table I. 



Marks. 



Material. 



Maximum 

 Remanence. 



Coercivity. 



No. 1 



No. 2... 



Chilled Cast Iron. 



»> )) 

 Mfcgnet Steel. 



1775 

 1670 

 1690 

 1850 

 2550 

 2950 



52-8 

 48-9 

 50-4 

 52-1 

 55-5 

 73-0 



No. 3 



No. 4 



M 



A 





The above results show that the hardened cast iron rods 

 are not very much inferior to the ordinary magnet steel (M) 

 either in original strength or in power of resisting demagne- 

 tization. 



Tests on Rings. — Primary and secondary coils were wound 

 on each of the chilled rings, and the (H, B) curves shown in 

 fig. 1 were determined by the ordinary ballistic method. In 

 addition to these the coercivities (H ) were found for a 

 number of different values of B, and the (H , B) curves are 

 also shown. 



Fig. 2 gives the (H, B) curves for the thin ring before 

 and after chilling. 



We see from fjg. 1 that, after chilling, the thick ring is 

 magnetically very similar to the thin one; and thus it is 

 evident that, by the treatment already described, a quite 

 heavy casting can be satisfactorily hardened throughout. 



