164 BELL SYSTEM TECHNICAL JOURNAL 



Shields and Wiring. The shields have sufficient rigidity and are 

 supported so as to maintain a definite and constant space relation to 

 the part shielded and to the other shields. They are also of sufficiently 

 high conductivity to maintain a common definite electrical potential 

 at all points with respect to the part shielded. 



The supports of shields or of bridge elements within the shields 

 are as nearly as possible of constant specific inductive capacity, have 

 low dissipative and leakage losses and are restricted to the minimum 

 in number and size consistent with meeting the required rigidity of 

 support. 



Interconnecting conductors are shielded within brass tubes of 

 approximately 3^-in. diameter, the conductor which is of No. 10 

 gauge copper being supported at the axis of the tube by means of glass 

 beads fitting snugly within the tubes and having holes through which 

 the conductor passes. These beads are located longitudinally on the 

 conductor by means of a small lump of solder placed on each side. 



Standards. The impedance standards consist of adjustable self- 

 inductance elements used in series with an adjustable non-inductive 

 resistance. Each self-inductance element consists of a series of induc- 

 tance coils and a low range inductometer of the Brooks type,^ arranged 

 in three decade formation and connected to dial switches by means 

 of which any series combination of the coils can be selected. The 

 inductometer is always in circuit and permits of balancing inductance 

 values that fall between consecutive steps on the dials. Fig. 15 shows, 

 schematically, the connections used for these standards and also the 

 way in which they are shielded. It will be noted that the parts com- 

 prising each decade have a shield enclosing them and also all preceding 

 decades of higher value. This makes a rather complicated mechanical 

 arrangement but results in very important advantages from the stand- 

 point of electrical performance. Due to the individual decade shields, 

 each decade has effective values that are entirely independent of the 

 settings of either of the other decades. Hence, once each individual 

 setting of each dial has been calibrated, the value for the standard 

 as a whole for any possible combination is obtained by simple addition 

 of the separate dial values. This saves an immense amount of work 

 in calibrating and also simplifies the reading of the standard. Without 

 these shields the inter-coil and coil-to-ground admittances, at the 

 higher frequencies, are sufficiently large to make the effective imped- 

 ance of each decade setting depend to an appreciable extent upon the 

 settings of the other decades. Under such conditions a calibration of 



" H. B. Brooks and F. C. Weaver: "A \'ariable Self and Mutual Inductor," 

 Scientific Paper of the Bureau of Standards, No. 290. 



