34 ELECTRICAL MEASUREMENTS 



determined by calibration; but the formula serves to direct atten- 

 tion to certain quantities which are involved in its action. 



The Magnets. It is seen that if there were no modifying con- 

 ditions the current sensitivity would increase proportionally to H: 

 This indicates that the magnet should be very strong. How- 

 ever, strength is not the only thing to be considered, for it is well 

 known that so-called permanent magnets gradually lose their 

 strength, that is, they "age." The ageing depends upon the 

 quality of the steel, the design of the magnetic circuit and upon 

 the temperature variations and mechanical jarring to which the 

 magnet is subjected. Any deterioration will influence the sensi- 

 tiveness of the instrument, so in this and in many other cases, 

 for instance, in the magnets used in direct-current ammeters and 

 voltmeters, and in watt-hour meters, it is necessary to resort to 

 artificial ageing. As pointed out by Strouhal and Barus this 

 may be done by the proper heat treatment at moderate tempera- 

 tures. Their procedure was, after the magnet had been hardened, 

 to heat it in a steam bath at 100C. for 20 or 30 hours, then magnet- 

 ize it strongly and afterward heat it again in the steam bath for 4 

 or 5 hours. In addition, some makers resort to a partial demag- 

 netization. The net result is that while the strength of the mag- 

 net is reduced, the remaining magnetization is very permanent. 



The temperature coefficient of magnets such as are used in 

 galvanometers and in direct-current ammeters and voltmeters is 

 about -0.025 per cent, per degree C. rise of temperature; it 

 varies with the magnet. 



Chilled cast-iron magnets may be employed. 8 In general, 

 they are useful where it is necessary to employ forms so compli- 

 cated that forging would be difficult. 



After the gray iron castings have been machined, they are 

 heated to a bright red (just under the melting point) in a gas 

 furnace provided with a power blast, and then plunged into a 

 cold acid bath which is violently stirred. Care must be taken in 

 the manipulation of the heated castings, for they are lacking in 

 tenacity. It is important that the entire mass of the casting be 

 hardened; consequently the heating must be prolonged until it 

 is certain that the temperature is practically uniform through- 

 out the mass. 



After hardening, the magnets are heated in a steam bath for a 



