DIRECT CAPACITY MEASUREMENT 29 



methods, and here negative resistances make a one-setting method 

 at least theoretically possible, as explained above. Of possible varia- 

 tions it will be sufficient to refer to the ammeter method Fig. 8. Termi- 



Fig. 8 — Ammeter Circuit for Determining Direct Capacity 



nals 1 and 2 of the required direct capacity Cn are connected to the 

 voltmeter and ammeter terminals, respectively, and all other terminals 

 go to the junction point at 3. Then 



Cl2 — 



2n-/£' 



provided the ammeter actually has negligible impedance. The 

 method is well adapted for rapid commercial testing. The ammeter 

 impedance may be reduced to zero by a variable negative impedance 

 device ( — Z), adjusted to reduce the shunted telephone to silence. 



Shielding 



In the discussion of the bridge, it has been assumed that the several 

 pieces of apparatus forming the six branches of the bridge have no 

 mutual electrical or magnetic reaction upon each other, except as 

 indicated. In general, however, a balance will be upset by changes 

 in position of the pieces of apparatus, or even by movements of the 

 observer himself, whereas these motions cannot affect any of the 

 mutual reactions which have been explicitly considered. The skillful 

 experimenter, understanding how these variations are produced by 

 the extended electric and magnetic fields, will anticipate this trouble 

 and take the necessary precautions, possibly without slowing down his 

 rate of progress. 



Where hundreds of thousands of measurements are to be made, 

 however, substantial savings are effected by arranging the bridge so 

 that reliable measurements can be made by unskilled observers, and 

 here it is necessary to shield the bridge so that any possible movements 

 of the observer and of the apparatus will not affect the results. Mag- 

 netic fields of transformers are minimized by using toroidal coils 

 with iron cases. Electrostatic fields are shielded by copper cases; 



