THE MEASUREMENT OF CURRENT 89 



soever. Symmetry is the all-important thing. It is also appar- 

 ent that any symmetrical redistribution of the current in the 

 conductors will not alter H. 



If an alternating be substituted for a direct current, the dis- 

 tribution of the current over the cross-section of the tubes will be 

 altered, due to the skin effect, but from the symmetry of the 

 conductors the new distribution of the current will be symmetrical 

 about the axis and therefore the field due to the changed distribu- 

 tion of the current is the same as before. 



In the actual instrument, owing to the manner of taking the 

 current into the large outside tube, the natural distribution will 

 not be quite uniform. For this reason, a groove about 5 cm. wide 

 is turned eccentrically in the tube. By filing, the groove may 

 be so adjusted that the stream lines are uniformly distributed. 



FIG. 47. Pertaining to the Agnew tubular electrodynamometer. 



It is essential that the symmetrical distribution of currents 

 about the axis be maintained; therefore, any springing of the 

 slender inner tube must be avoided. A test for distribution 

 errors may be made by the method described on page 316. 



The principal dimensions of the instrument, as used at the 

 Bureau of Standards, are 



Outer tube, length, 101 cm.; radii, 6.41 and 7.07 cm. 



Inner tube, length, 125 cm.; radii, 0.50 and 1.27 cm. 



Current capacity, air-cooled, 1,200 amp., (water-cooled) 5,000 arnp. 



Field at center of movable coils at full load, 300 gausses. 



Movable coils, 116 turns of 0.2-mm. silver wire, diameters 2.5 and 5.0 

 cm., weight of each coil 7.3 gm.; total resistance of movable coil circuit 

 14.3 ohms; current capacity 0.06 amp.; inductance 1.4 millihenries. 



Sensitiveness, 100-cm. deflection at 86-cm. scale distance requires 100 amp. 

 in tubes and 0.06 amp. in movable coils. 



The Current Balance. In this class of instruments the current 

 is measured by weighing, with a gravity balance, the pull exerted 



