ELECTRICAL MEASUREMENT 7S 



in conjunction with the variable capacitance, a frequency range of 

 about 100 cycles to 150 kc. 



Fig. 2 — Single-frequency vacuum tube oscillator used as secondary standard of 



frequency 



The meter is calibrated by balancing the circuit by means of the 

 variable resistance and capacitance with a known frequency input, 

 and recording the coil and condenser settings. It is used for checking 

 frequencies by reversing the process, that is, connecting the source of 

 unknown frequency to the bridge, balancing as before, and determining 

 the frequency by reference to the calibration. There are no input or 

 output transformers connected to this circuit and on this account 

 certain precautions must be taken in connecting the output and input 

 circuits to it; but it is a relatively low impedance circuit, and troubles 

 due to this cause have not been found serious. 



Resistance. A convenient secondary standard of resistance is a dial 

 box having the resistance units designed to meet the same require- 

 ments as the prime standards. Commercial dial boxes are available, 

 having satisfactory stability with variations in frequency and atmos- 

 pheric conditions, and having sufificiently small phase angles for all 

 frequencies but the highest radio frequencies. 



A dial box, requiring, as it does, a certain amount of wiring between 

 dials, and having all of the dials connected permanently whether they 



