THE MEASUREMENT OF CURRENT 



69 



amount of the terminal rod in series with it. Up to a frequency 

 of 750,000 cycles no distribution error could be detected. 



A theoretically perfect arrangement of the terminal blocks is 

 that shown in Fig. 33. Everything is symmetrical about a 

 central axis along which the current enters and leaves. 



The conductors are thin strips or wires soldered between the 

 terminal blocks; this makes the mutual inductance of each wire 

 with respect to the others the same. Of course the self-induc- 

 tances of all the wires will be equal. 



This symmetry insures that high-frequency currents will 

 divide equally among the wires. The practical difficulty is to 



FIG. 33. Arrangement of parallel-wire conductor for avoiding distribution 

 errors in hi^h-froqueney ammeters. 



ct all the fine wires of the same resistance. If this is not 

 done, there will be changes of distribution in passing from low to 

 high frequencies. This construction is used by Hartmann and 

 Braun. 



SOFT-IRON INSTRUMENTS 



The first ammeters and voltmeters used for commercial meas- 

 urements on electric light and power circuits were of the soft- 

 iron type, instruments in which an iron core is drawn into a 

 solenoid, in opposition to the action of either gravity or a con- 

 trolling spring. 



As the induced pole reverses in sign with reversal of the cur- 

 rent, a soft-iron instrument deflects in but one direction, irre- 

 spective of the direction of the current through it. 



The law connecting the current and the pull of a solenoid on 

 an iron core depends on the degree of saturation of the iron. 



