ELECTRICAL MEASUREMENT 



77 



In measuring impedances, we have found that those methods which 

 determine the unknown in terms of circuit constants are superior to 

 those requiring the measurement of current and voltage. Accordingly, 

 bridge methods are used almost exclusively and, furthermore, the 

 bridge type which is used wherever possible is the equal ratio arm 

 bridge in which a direct comparison is made of the unknown impedance 

 with a known impedance adjusted to that same value. This type of 

 measurement has the disadvantage of requiring standards of the same 

 value as the quantity measured over the whole range of impedances 

 used, but it has the compensating advantages that, having standards 

 whose value is known, this circuit is extremely simple, very easy to 

 check at any time, and may be made extremely accurate. 



Auxiliary Apparatus. Without going into details regarding the 

 auxiliary apparatus used in connection with bridge measurements, 

 we may state briefly that vacuum tube oscillators are used almost 

 exclusively for furnishing all frequencies, and that the telephone 

 receiver is used almost exclusively as a detector, due to its simplicity 

 and the rapidity with which it may be used. For frequencies below 

 200 cycles, it is used with a chopper to give a tone of about 1000 cycles, 

 and above 3000 cycles, it is used with a heterodyne detector to give 

 a beat note of about 1000 cycles. In the audio frequency range, it is 

 used alone or with an amplifier, if necessary. 



Fig. 4 — Shielded impedance bridge circuit 



While it is impossible to draw a distinct line between the methods 

 of measurement of different types of impedances, certain bridge 

 circuits have been designed primarily for certain types of measure- 



