574 BELL SYSTEM TECHNICAL JOURNAL 



The shielding may, in special cases, limit the maximum frequency 

 at which the circuit will operate; but in such cases it can usually be 

 taken for granted that even if the circuit would operate at higher 

 frequencies without shielding, the accuracy of the results would be 

 highly questionable. Examples of limitations of shielding may be 

 given using the apparatus already described. Take the case of the 

 resistor shown in Fig. 5. The admittance across the resistances 

 results in objectionably high phase angles and an effective change in 

 the resistances at very high frequencies. While this effect would be 

 present even though no shielding were used, the shielding increases it 

 and therefore limits the maximum frequency at which the apparatus 

 can be used from the standpoint of this type of error. The same 

 limitation occurs in the case of the inductance standards only it is 

 more serious due to the large physical size of these standards. The 

 exact type of limitation here is that the individual units increase in 

 inductance due to the admittance across them to such an extent that 

 the difference between them cannot be bridged by the next lower 

 decade, thus rendering it impossible to obtain certain values of in- 

 ductance by any dial combination. 



In the case of a symmetrical bridge the principal limitation is the 

 shunting effect of the admittance introduced across the impedance 

 arms. This becomes so large that, at frequencies in the order of 

 100 kilocycles, difficulties are encountered in measuring the current 

 through the unknown impedances by the method of measuring the 

 total current input to the bridge. If the meter question is eliminated, 

 the actual loss in sensitivity, which is the only other objectionable 

 feature of this admittance, may be made of no serious consequence 

 up to frequencies as high as 2,000 kilocycles. In all other respects, 

 the shielding functions as satisfactorily at this frequency as at the 

 ower frequencies. 



Auxiliary Equipment 



While auxiliary apparatus such as oscillators and detectors is not 

 strictly speaking measuring apparatus, its operation is essential to 

 the satisfactory operation of the measuring circuit and so a few words 

 may be added regarding the shielding of this apparatus. 



Provided they are separated sufficiently from the measuring circuit 

 and from each other, there is no need to shield the oscillator or detector 

 from the standpoint of operation of the measuring circuit. However, 

 for maximum flexibility it is desirable that they be so constructed that 

 no special precautions are necessary in placing them relative to the 

 measuring circuit. If, as is usually done, the individual apparatus 

 included in these circuits is adequately shielded it is only necessary to 



