462 



ELECTRICAL MEASUREMENTS 



secure this result Elihu Thomson, after experimenting with many 

 materials, was led by a knowledge of the mechanical and elec- 

 trical properties of silver, and experience with contacts made of 

 it, to adopt the pure metal for both the commutator and the 

 brushes. This solved the greatest problem in the design of 

 commutating meters. 



The magnets used in connection with the retarding disc or 

 brake must retain their strength. This involves the choice of 

 a proper magnet steel, a correct design (a nearly closed magnetic 

 circuit) and the artificial ageing of the magnets by partial de- 

 magnetization and by the proper heat treatment at moderate 

 temperatures. From equation (2) in the demonstration already 



Normal distribution. 



Distribution after a short circuit. 



FIG. 266. ^Showing effect of a short-circuit on the distribution of magnet- 

 ism in the drag magnets of a direct-current watt-hour meter. 



given (page 460), it will be seen that a 1 per cent, change in the 

 strength of the retarding magnets affects the accuracy of the 

 meter by 2 per cent. 



The magnets should be so placed that their strength is not 

 likely to be altered by the field due to the current coil. 



Besides the natural deterioration of the magnets there is the 

 chance of an accidental change due to short-circuits, and the 

 magnets should be so arranged that this effect will be minimized. 

 The enormous and sudden rush of current through the current 

 coils during a short-circuit sets up a magnetic field which may be 

 strong enough to change entirely the distribution of magnetism 

 in the drag magnets and cause the meter to over-register. Such 

 a change in the distribution of magnetism is illustrated in Fig. 266. 



The field coils must be firmly held and kept apart by spacing 



