New Magnet-testing Instrument. 



545 

 During 



is in position on the flux-measuring instrument, 

 winding the number of turns is carefully counted. 



When the coils and magnet are placed in position and the 

 necessary connexions made as shown in fig. 4, the handle is 



Fis\ 4. 



r-v^VvwVV 



+ 



Connexions for Coercive Force. 



turned and the scale-reading gives the total useful flux of the 

 magnet. The current is now switched on in such a direction 

 as to tend to demagnetize the magnet, and the current 

 strength gradually increased while the generator-handle is 

 kept revolving. 



When the flux indicated on the dial has become very small 

 the current is switched off to take the reading of flux, and 

 thereafter the current is increased in small steps and switched 

 off each time to read the flux until the flux is zero. The 

 current so attained is the true demagnetizing current, and 

 from the number of turns and the length of the magnet the 

 demagnetizing field or coercive force can be calculated. In 

 practice the ammeter is graduated so that its readings 

 give the value of H by direct reading for a given size of 

 magnet. 



