40 BELL SYSTEM TECHNICAL JOURNAL 



high as 100,000 cycles. On the other hand, in a certain frequency 

 range the reflection loss at iron-air boundaries may be very low. Since 

 the attenuation loss of a complete shield made of coaxial layers of 

 copper and iron is independent of the sequence of the layers, consider- 

 able gain in shielding may be secured by placing an iron layer between 

 two copper layers rather than a copper layer between two iron layers 

 so as to take advantage of the added reflection loss, assuming of course 

 that the amounts of copper and iron are the same in both cases. 



Since the high-frequency impedance ratio is proportional to the 

 diameter of the shield, the size has a substantial influence upon the 

 efi"ectiveness of the shield. Each time the diameter of a non-magnetic 

 shield is doubled, the shielding is increased by 6 decibels. In the 

 case of magnetic shields, this is true only at frequencies considerably 

 higher than the critical frequency at which the reflection loss is mini- 

 mum. Considerably below this frequency, the effectiveness of a mag- 

 netic shield is decreased by 6 decibels with each doubling of the diam- 

 eter of the shield. For the transition region we can say that with 

 increasing size the effectiveness of the magnetic shield decreases below 

 the critical frequency and increases above it. 



"Electrostatic Shielding" 



If the cylindrical wave is originated by two parallel oppositely 

 charged wires, alternating with a given frequency/, the radial imped- 

 ance in free space is l/icoep provided p is small compared with the 

 wave-length. ^2 As in the preceding case, in metallic media the radial 

 impedance is Vtaj/x/g provided the frequency is not too low ; for very 

 low frequencies the radial impedance becomes 1/gp. 



It is clear at once that for these waves the reflection loss is tre- 

 mendous. Thus in air e — (l/367r) 10"^ farads per meter ; if / = 10*^ and 

 p = 0.01 m., then the radial impedance is 1,800,000 ohms. The corre- 

 sponding impedance in copper is only 0.000369 ohms. At lower fre- 

 quencies the disparity between the radial impedances becomes even 

 greater. The impedance ratio tends to infinity as the frequency 

 approaches zero. 



In the elementary theory a metal shield is regarded as a perfect 

 shield against this "electrostatic field." An "electrostatic " field alter- 

 nating 1,000,000 cycles per second is probably a misnomer. And the 

 shielding is excellent but not perfect. Nevertheless the distinction 

 between two possible types of waves is a valid one, at least in the 

 frequency range usually employed in the communication art. In one 

 wave the electric field is normal to the direction of propagation and 



1^ Equation (13). 



