CHA1TKK XXI 



THE MEASUREMENT OF PERMEABILITY AND 

 THE ALLIED QUALITIES IN FERRO- 

 MAGNETIC BODIES 



Magnetometer method Ballistic method Magnetisation in \<-r\ weak 

 fields Magnetisation in very ptrong fields Ewing's inthin': 

 Nickel and cobalt The hysteresis loop and the energy dissipated in ft 

 cycle Mechanical model to illustrate hysteresis. 



A KNOWLKDCK of the permt ability <t' iron, and of the way in which 

 B varies a- II i- chaniM <!, is <>f the highest importance at A guide 

 to dynamo construction. In this chapter we shall u r i\r an account 

 of the principles upon which are based *onu- of tin- methods used 

 to investigate the relation between H and II for iron and steel, and 

 we shall indicate the general nature of tlic rcsulK* It \\ill be 

 necessary to assume some knowledge of electro-magnet i-m. 



Magnetometer method. In tin- m<t!iod. applicable to 

 along thin rod or \\iie. the rod i^ placed v. ithin a sol 



FIG. 204. 



what longer than itself and is magnetised by a knoun current 

 through the solenoid. One end of the \\ire i> near a magnetic needle 

 the magnetometer and from the deflection of this needle the 

 magnetisation of the rod is calculated. A verv convenient 

 arrangement, due to Ewing, is represented in Fig. 204. 



* The reader will liml th. details <>f tin- methods and th*-ir ndapta: 



t-cluiir:il nrtMls ill Kwinp's M,i>in,ii,- Indurtnn, in Iron and t>( ! 



