THE MEASUREMENT OF CURRENT 



35 



long time and then magnetized to saturation. The ageing is 

 effected by alternately heating in steam and cooling in tap water. 

 If the magnets are magnetized to saturation, the strength is 

 reduced about 20 per cent, by this process. 



When properly prepared, chilled cast-iron magnets have a small 

 temperature coefficient. For magnets of the forms used in instru- 

 ments, the average decrease of field strength per degree rise of tem- 

 perature, between 10 and 100, is about 0.04 per cent. Within 

 the ordinary range of room temperatures it is much smaller, 

 about 0.013 per cent, per degree. 



The strength of aged cast-iron magnets is less than the strength 

 of those of equal weight, made of special magnet steel. 



FIG. 16. Pole pieces for producing a radial field and the resultant field. 



Effect of Magnetic Impurities in the Coil. Practically it is 

 found that the increase in sensitiveness may not be proportional 

 to the increase in H, for, as first pointed out by Ayrton and 

 Mather, the coil itself may be slightly magnetic due to the pres- 

 ence of minute amounts of iron as an impurity in the metal 

 or which have been worked into the insulation during the process 

 of winding. 



