LAYOUT AND THE CONTROL OF INTERFERENCE 



conjunction with a microelectrode would produce an excessive high-frequency 

 loss. 



A powerful weapon against interference of this type is the electrostatic 

 screen, a conductor interposed between source and amplifier, and earthed 



i 



Figure 42.3 



{Figure 42.3). Lines of electric force emanating from P in the direction of 

 the amplifier now alight on the screen and, even though it be of mesh con- 

 struction, are prevented from reaching the amplifier. The equivalent circuit 

 is Figure 42.4. Notice that the earthing of the screen must be sound, other- 





"Q 



_^ Amp ' 



•"in 



Cjn 



_l 



Figure 42.4 



wise the equivalent circuit degenerates to Figure 42.5, we have a high-pass 

 filter, and interference proceeds as before. 



Magnetic fields — According to the laws of electromagnetic induction a 

 steady magnetic field cannot cause interference but a fluctuating one can. 



Amp' 



Figure 42.5 



and in practice such fields are set up by alternating currents flowing through 

 devices possessing solenoidal windings such as transformers, heaters, and 

 electric motors, and to a lesser extent by apparatus possessing what is 

 virtually a single turn winding, notably electric fight bulbs. The magnetic 

 field so produced threads the single turn made by the input circuit of the 

 amplifier, Figure 42.6, inducing e.m.f. into it. The magnitude of this e.m.f., 



657 



