62 



ELECTRICAL MEASUREMENTS 



production changes with the distribution of the current, the 

 change is less than the change in the current distribution itself. 

 The reading of any ammeter which depends on the whole heat 

 production will either remain constant or increase as the fre- 

 quency is increased. 



A two-wire instrument where the total heat production is 

 utilized is shown diagrammatically in Fig. 28. 



The high-frequency current is taken in through the leads A a 

 and Bb which are perpendicular to the active wires ACB and 

 ADB, which are of copper and 0.1 mm. in diameter. 



FIG. 28. Two-wire ammeter for high-frequency currents. 



The points C and D are located so that the arrangement is as 

 nearly as possible a Wheatstone bridge. The high-frequency 

 current is thus kept out of the auxiliary bridge which is used for 

 measuring the resistance. As the location of C and D may not 

 be exact, it is preferable to use a low-frequency alternating cur- 

 rent instead of direct current in calibrating the arrangement. 

 In order to eliminate the effects of thermal e.m.fs. the gal- 

 vanometer is used on closed circuit. This arrangement when 

 immersed in oil has a capacity of 10 amp. 



