86 ELECTRICAL MEASUREMENTS 



of FM, the required product of the fixed and moving turns for t he 

 new winding, is readily determined. 



Electrodynamometers for Heavy Currents. For several 

 reasons it is difficult to apply the ordinary electrodynamometer 

 to the measurement of very large currents. If the coils are used 

 in series, trouble is experienced in getting the current into and out 

 of the movable member. Where any considerable current is to 

 be carried, the necessary flexible connections are made by means 

 of mercury cups, and no dynamometer in which they are em- 

 ployed can be considered a portable instrument as that term is 

 now generally understood. There are also the structural diffi- 

 culties due to the necessity of supporting a heavy movable coil 

 on pivots which are practically free from friction and are suffi- 

 ciently strong so that the instrument will stand handling. These 

 considerations preclude the use of the electrodynamometer with 

 its coils in series as an alternating-current ammeter, and are 



the reasons for the survival of soft iron 

 ammeters as alternating-current in- 

 struments. 



Again, it is essential that the field at 



no. 45,-Wattmeter [he movable coil be the same with 

 method for measuring large both direct and alternating currents, 

 alternating currents. f or direct currents will be used in cali- 



brating and alternating currents in the 



subsequent use of the instrument. This equality of fields may be 

 attained either by arranging the metal of the coil so that the 

 current distribution will be the same in both cases, or by adopt- 

 ing an arrangement which from its symmetry is such that the 

 change in distribution in passing from direct to alternating cur- 

 rent does not affect the field in which the movable coil swings. 



It is also necessary that there be no error due to eddy currents 

 induced in the mass of the coils, or in the frames by which they 

 are supported. For this reason, in instruments of moderate 

 capacity recourse is had to stranding the conductors. This must 

 be very carefully done so that all the strands will have the same 

 effective inductance and resistance and the same increase of 

 resistance due to heating. 



Wattmeter Method for Measuring Large Alternating Currents. 

 -To obviate the necessity of taking a large current into the 



