LAYOUT AND THE CONTROL OF INTERFERENCE 



for a given field, is proportional to the area of the single turn or 'loop'. Thus 

 Figure 42.7a is a bad arrangement but b is good. The interfering e.m.f. is 

 also proportional to the sine of the angle between the plane of the loop and 

 the direction of the magnetic lines of force. If the loop is already small it is 



Magnetic 

 flux 



Figure 42.6 



difficult to minimize the interference by altering the plane of the loop. 

 Fortunately it is often possible to alter the direction of the field. 



Radiated electromagnetic fields — Whenever an alternating electric current 

 flows round a circuit, systems of electric and magnetic fields may be thought 

 of as growing out of, and collapsing back into, the conductors in sympathy 

 with the successive growth and decay of current. These fields represent 

 energy alternatively derived from, and returned to, the electric circuit. How- 

 ever, not all the energy associated with each growth is returned in the subse- 

 quent collapse. A fraction of it is 'radiated' off into space as an electro- 

 magnetic or 'radio' wave. The effect is extremely small at 50 c/s but becomes 

 important as the frequency is raised into the hundreds-of-kilocycles region. 



Amp 



(b) 



Figure 42.7 



and above. An electromagnetic wave, as the name suggests, contains both 

 electric and magnetic field components and cannot exist unless both are 

 present. It follows that any system of protection against either pure electric 

 field- or pure magnetic field-borne interference will be effective against 

 radiation, and no specific remedy is necessary. 



Causes of interference 



The most ubiquitous kind of interference is due to the supply mains, 

 which shows up as the ail-too familiar 50 cycles wavy line on cathode ray 

 tube traces, and as 'hum' on loudspeakers. It may be borne by electric or 

 magnetic fields, often by both. Another variety is 'impulsive' interference, 

 which appears as a sharp spike and is heard as a 'click'. It is usually electric- 

 field-borne and is caused by the sudden appearance of a large back-e.m.f. 

 when a mains circuit is broken which contains an inductive load. Switches, 

 thermostats, commutator motors, etc., are potential sources of trouble here. 



The unexpected reception of the B.B.C. is, of course, interference from 

 radiation. For a biological amplifier to operate as a radio set it is necessary 

 that there be a rectification process somewhere in the input circuit, but this 



658 



