SHIELDING IN HIGH-FREQUENCY MEASUREMENTS 563 



to other parts of the whole system and in particular to ground. These, 

 of course, act to modify the effective impedance between the terminals 

 and as they vary with the location of the resistor, the result is that its 

 effective impedance is variable and known only for the location in 

 which it has been calibrated. One of the first objects to be accom- 

 plished by shielding is to remedy this type of indefiniteness of value. 

 This is done by mounting the elements within a shield of conducting 

 material and in fixed space relation thereto, as shown in Fig. \h. 

 Thus, the circuit element has direct admittances only to the shield 

 and as these are of fixed value the terminal to terminal impedance 

 becomes independent of the location of the shielded element. 



If, then, we connect the shield to any fixed point in the circuit 

 element such as one terminal, all of the current transferred by the 

 shield admittances passes to or from the circuit at this particular 

 point. This concentration of admittance enables the ready evaluation 

 of the effect produced by it when the element is used in conjunction 

 with others in a complete measuring system. We may summarize 

 all of this to form a fundamental rule of shielding, viz. : " the association 

 of an element of a system with a shield so that all admittances from 

 the element to other parts of the system or to ground are confined to 

 one terminal." 



If it is possible to connect such an element in a circuit so that the 

 terminal to which the shield is connected is grounded, all variable 

 admittances will be eliminated completely. 



Series Impedances 



In the case of two impedances in series as shown in Fig. Ic, shielding 

 may be accomplished by connecting one shield to terminal A and 

 the other shield to B. In addition to the effects described for a single 

 impedance, there will then be admittance between the two shields 

 which will depend on the position of the apparatus. This admittance 

 is slightly more objectionable than admittance from shield to ground 

 since, while we may ground either A or B, there will always be an 

 admittance from one shield to ground which will be variable. 



The shields may also be connected as shown in Fig. \d, in which 

 case the admittance between shields appears across the first impedance. 

 Now if we extend the shield connected to A to include the other shield 

 as shown in Fig. le, we have introduced a fixed admittance across 

 A C and have variable admittances to ground from A . The admittance 

 across ^4 C is not objectionable in the case of a capacitor since it may 

 be considered simply as an addition to it, but it has the effect of 

 increasing the phase angle of a resistor and in the case of an inductor 

 37 



