6 4 2 



CONSTANTS OF MAGNETISATION. 



very high unstable magnetism a kind of magnetic supersaturation, 

 which does not stand mechanical actions, such as vibrations or 

 shocks. It is sometimes sufficient to let a steel bar fall from a 

 height of a metre, to destroy a considerable quantity even half 

 of its magnetism, and its magnetisation is only permanent after a 

 certain number of shocks. 



On the other hand, experiment shows that if we reheat a bar at a 

 given temperature for a given time, at any intervals, remagnetising it 

 each time, it ultimately becomes insensitive, as regards its permanent 

 moment, to any temperature below that of heating. According to 



100 



50 



10 



Resistance 



Fig. 250. 



Barus and Strouhal, magnets which best satisfy the conditions of 

 permanence are obtained by heating them for 20 to 30 hours, at 

 several separate times, to a temperature of 100 in steam, for 

 instance magnetising to saturation, subjecting them anew to the 

 action of steam, and then finally magnetising. Magnets possess 

 then the limiting resistance corresponding to 100, and are no 

 longer permanently modified by lower temperatures. The specific 

 moment thus obtained may reach 45 or 50, corresponding to an 

 intensity of magnetisation of about 300. In these conditions, the 

 temporary variations due to changes of temperature are also very 

 feeble. 



