THE MEASUREMENT OF CURRENT 



77 



coiled spiral spring. A pointer, which normally stands at zero, 

 is attached to the movable coil; on the passage of the current, 

 this pointer deflects against a stop and is brought back to its 

 original position by turning the torsion head. The amount of 

 twist which it is necessary to give the spring in order to return 

 the coil to its zero position is read from the graduated circle. 

 If the spring be perfect, the moment exercised by it will be pro- 

 portional to this angle of twist. As springs cannot in general 

 be relied upon throughout the whole range of twist, the instru- 

 ment should be calibrated at a 

 number of points and a calibration 

 curve drawn. 



The current is led into the mov- 

 able coil by two stout wires which 

 dip into mercury cups. 



Setting up the Siemens Elec- 

 trodynamometer. The instrument 

 must be levelled and the same rela- 

 tive position of the coils maintained 

 during calibration and subsequent 

 use. Though the instrument is to 

 be used with alternating currents, 

 it is convenient to employ direct 

 currents in the calibration. If this 

 is done it is desirable to place the 

 instrument so that the local field 



will have no influence. This may be accomplished by turning 

 the dynamometer in azimuth until a position is found where the 

 strongest current which is to be used produces no deflection 

 when sent through the movable coil alone. 



The Law of the Electrodynamometer. The law of the electro- 

 dynamometer is dependent on the method of reading. Two 

 cases will be considered: 



1. When the movable coil is always brought back to its initial 

 position by the use of a torsion head, as in the Siemens instrument. 



2. When the movable coil is allowed to deflect, as in an ordinary 

 reflecting galvanometer. 



1. When an elect rodynamometer, set up without regard to the 

 local field, is used with direct currents, a part of the turning 



FIG. 38. Siemens electro- 

 dynamometer. 



