LAYOUT AND THE CONTROL OF INTERFERENCE 



can be supplied by a motor-car headlamp bulb fed from an accumulator 

 (but not the same accumulator as that used to heat direct-coupled pre-ampli- 

 fier valves) and mammalian or avian preparations can be kept warm by low 

 voltage heaters similarly fed. If the near presence of some mains-driven 

 items is absolutely necessary it may be feasible to reduce the interference 

 caused by it by reorienting it so that its field cuts the pick-up loop at a 

 shallower angle. If there is any freedom of choice in its construction, 'astatic' 

 winding may be possible ; the winding being split into two halves, arranged 

 so that the fields set up by the two parts oppose one another and cancel 

 {Figure 42.9). 



Finally, of course, all mains-driven apparatus must be supplied via the 

 usual twin cable and not by two distinct insulated wires. With the twin 



100 SI 0-OluF 



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4XH 



Figure 42.8 



Figure 42.9. One method of 



astatically winding a heater 



element 



Figure 42.10 



cable the two conductors lie close together and the magnetic fields due to 

 each largely cancel one another. If the two conductors are allowed to sepa- 

 rate an interfering magnetic field is created proportional to the area of the 

 loop so formed. 



Prevention at source — radiation — Unless one is a person of considerable 

 influence there is very little which can be done about this. So far as mobile 

 radio users are concerned, the messages passed are usually brief, and the 

 research worker need only exercise a little patience. 



Amplifier protection — electric fields — The use of aluminium or steel cases 

 for electronic gear is now general and is usually sufficient to screen adequately 

 the internal input wiring of the amplifier. The only point to watch is that the 

 pieces of sheet metal composing the cases are not isolated electrically from 

 one another by paint; Figure 42.10 shows two such pieces joined together by 

 a self-tapping screw. The hole in the lower member is drilled such that the 

 screw thread grips it tightly, but the hole in the upper member is sufficiently 

 large to allow the thread to pass through easily. Under these circumstances 

 there is little risk that there will not be good electrical connection from screw 

 to lower member, but that from screw to upper may be very poor. It is 

 usually recommended that the paint be scraped away from the region under 

 the screw head to cure this, but the procedure is liable to produce messy 

 looking work. A better practice is to fit an ordinary cheese-headed bolt into 

 the clearance hole before painting, securing it with a nut. After painting, 

 the screw is removed, leaving a neat circular area of bare metal. 



Outside the body of the amplifier the input leads should be of screened 

 cable, polythene insulated if a low capacitance is necessary, and if cathode 

 follower probes are used, the leads from microelectrode to cathode followers 



660 



