THE MEASUREMENT OF CURRENT 



73 



an absolute instrument the arrangement and proportions must 

 IK* such that the constant of the instrument may be accurately 

 calculated from the measured dimensions. The Helmholtz ar- 

 rangement is sometimes adopted for both the fixed and movable 

 members, but in the instrument shown in Fig. 37, which was espe- 

 cially designed for absolute measurements, the coils are wound 

 on cylinders. 



!'!<;. 37. Absolute elcctrodyuaiinmirtrr. 



The fixed coil of N f turns consists of a single layer of insulate* 1 

 wire wound on a plaster of paris or marble cylinder, which is very 

 accurately turned. 



The movable coil of N M turns is wound in a single layer on a 

 carefully ground porcelain cylinder. The coils are placed con- 

 centrically. The diameter of the movable coil is about one-fifth 

 that of the fixed coil. 



The movable member is suspended by a torsion wire and the 



