LAYOUT AND THE CONTROL OF INTERFERENCE 



is probably the rule rather than the exception. A small amount of non- 

 linearity in the amplifier will suffice, but even without this, in a circuit con- 

 taining brass stereotaxic apparatus, animal preparation, silver wires, agar 

 bridges, etc. it is hardly surprising if a rectifying junction exists. It is not 

 sufficient to be remote from a B.B.C. transmitter to avoid interference of 

 this kind. Most laboratories are near towns large enough to support radio- 

 controlled taxis, police cars, road patrols, and in addition there are uses for 

 powerful radio-frequency sources other than for communications, e.g., di- 

 electric heaters and induction furnaces. 



Prevention of interference 



Preventive techniques for interference fall under three headings; those 

 which seek to cure it at its source, those which protect the preparation, and 

 those which protect the amplifier. 



Prevention at source — electric field — For minimizing 50 cycle pick-up, the 

 golden rule is to screen everything possible which is fed from the mains. 

 The laboratory wiring should be in lead-covered cable or in steel conduit, 

 and power points should be of the 3 pin metal-clad variety. All flexible 

 leads should be of screened cable, possessing an outer protective PVC sheath* 

 with the braiding connected to the earthing pin on the power plug. All the 

 appliances fed by these cables should as far as possible have outer cases of 

 metal — rather than plastic or some other insulating material — and the cases 

 earthed via the braiding on the cable. Bench lights should have metal, not 

 bakelite, lamp holders, again properly earthed, supporting a spun-metal 

 lampshade. The reader will be able to continue in this vein for himself. 



The rule that, if an appliance is to be switched in one pole only then the 

 switch must be connected in the phase, not the neutral pole, is sound engineer- 

 ing practice and is made in the interests of safety. It is also extremely impor- 

 tant in interference reduction, since the phase side of the mains is the one 

 which causes all the trouble; the neutral lead is very nearly at earth potential. 

 By switching off appliances at the power socket in the phase side the whole 

 appliance is then earthed. If the switch is in the neutral lead, the appliance 

 continues to cause interference — though the screening will do much to 

 reduce it — besides being unsafe. 



The electric fields responsible for impulsive interference are contained 

 automatically by the techniques which are applicable to 50 cycle interference 

 prevention. As an additional safeguard it is as well to reduce the back- 

 e.m.f's which cause them, and this can be done by connecting across offend- 

 ing contacts the simple series CR suppressor circuit of Figure 42.8. The best 

 component values should be found by trial and error; those shown in the 

 figure are suitable for a first attempt. The circuit may be effective across the 

 brushes of a commutator motor, but if possible it is better to change over to 

 the induction variety, if necessary with variable speed gearbox. 



Prevention at source — magnetic fields — The most reliable ally here is dis- 

 tance, and every effort should be made to keep gear carrying alternating 

 current as far away from the amplifier input circuit as possible — say, at 

 least 6 feet. If a bright light is needed immediately over the preparation it 



* Exposed braiding collects dust. 



659 



