568 BELL SYSTEM TECHNICAL JOURNAL 



The decades are ordinarily connected in series in ascending order of 

 magnitude. The shield should always be connected to the low end, 

 that is, to the terminal to which the decade of smallest value is con- 

 nected. The reason for this may be explained as follows: the ad- 

 mittance from any decade to the shield is a function of the dimensions 

 of the dial switch rather than of the resistance value. Consequently, 

 all dials have approximately equal admittance to ground. It is 

 desirable that the total admittance to ground be a minimum across the 

 higher resistance settings. This requires that the low resistance dials 

 be connected between the high dials and the shield, that is, the shield 

 should be connected to the low end of the box. 



In the actual constructon of such a resistor it is essential that the 

 shielding be complete, particularly at the dials. Since the efifect of 

 the hands in operating the dials is more variable than any other 

 coupling, it is of very little value to place an unshielded dial box in a 

 metal shield which allows admittance from the hand of the operator 

 to the circuit. 



An example of a six-dial resistor in a complete single shield is shown 

 in Fig. 5. The box itself and the panel are of metal, and the con- 

 struction of the dials is such that there is a continuous metal shield 

 between each dial head and the switch proper which it controls. 



As stated earlier, the effect of the shield on the performance of the 

 resistor is to increase the phase angle of the higher resistance values. 

 In the case shown the admittance introduced by the shielding is of 

 about the same value as the total admittance distributed in the coils 

 themselves and from the coils to the switch parts. 



Adjustable Inductor 



The same considerations apply to an inductor as to a resistor except 

 that on account of the larger physical size of the former, larger ad- 

 mittances are associated with it and for that reason it is usually 

 necessary to use nested shields. Fig. 6 shows a standard inductor 

 consisting of three decades and an inductometer using four shields. 

 The three top panels have been removed showing the method of 

 nesting the shields, and the construction used to bring the dial controls 

 through the shields. The shielding of this unit is not complete in 

 that the fourth shield (the outer one) does not extend over the top. 

 This is allowable as the admittance from the third shield to ground is 

 across the inductometer and any variation in it has little effect, 

 particularly as the final balance is obtained with the inductometer, 

 thus eliminating variations due to the hands of the operator, which 

 occur in operating the other dials. 



