88 ELECTRICAL MEASUREMENTS 



rents, up to 5,000 amp., by the wattmeter method. It can, ol 

 course, be used as an ordinary wattmeter for power measure- 

 ments. Its distinctive feature is the means taken to avoid erron 

 due to the skin effect in the very massive conductors which musl 

 be used for the current coil. 



As seen from Fig. 46 the current "coil" is made in the forrr 

 of two coaxial tubes. When they are traversed by the current 

 strong field will exist in the space between them, while the fielc 

 external to the tubes will be nil Stray-field effects due to th( 

 heavy current in the instrument are thus avoided. 



The movable system consists of two rigidly connected coilj 

 one above, the other below the central tube. The working 

 position of the movable system is approximately 90 from thai 

 shown in Fig. 46. As the movement takes place in a strong field 

 it is essential that the movable system be entirely free from mag 

 netic impurities. If this is not the case, the zeroes with the cur 

 rent on and with the current off will not coincide. 



The suspension strip is of phosphor-bronze and air damping ii 

 provided. Diaphragms between the tubes are necessary te 

 prevent disturbance of the movable system by air currents. 



Theory of the Tubular Electrodynamometer. Suppose thai 

 two coaxial circular tubes are arranged as shown in Fig. 47 

 the directions of the current being as indicated. Consider an} 

 point, P, in the space between the tubes and distant r from the 

 axis. A direct current will distribute itself uniformly over the 

 cross-section of the tubes. The work done in taking a unit pole 

 around the indicated path is 



2irrH = 4irl 



where / is the current encircled by the path, that is, the currenl 

 which flows in the central tube, and H is the field strength at 

 any and all points in the path. Therefore, 



H-T 



It is obvious that this relation will hold as long as a symmetrical 

 arrangement of the current around the central axis is preserved, 

 It is thus evident that the generating lines of the surfaces of the 

 coaxial tubes need not be straight but may have any form what- 



