Abstract :
Evidence is given that the temperature-coefficient inductance of coils is dependent on frequency, owing to the change with temperature of the current distribution over the conductor cross-section. The effect is of importance only over a limited range of section for any particular frequency band. The theory of the effect is examined for circular-section conductors. It is shown that in all self-supporting helical coils suitable for short-wave work, where the conductor radius is greater than 0.1 cm., the effect of change of current distribution with temperature-change is such that no appreciable increase of temperature-coefficient of inductance occurs. For coils wound with very fine wire the temperature-coefficient of inductance is not affected appreciably by change of current distribution. The effect of temperature on the current distribution and consequently on the coefficient of inductance is appreciable only when the conductor radius lies between the limits 0.001 and 0.2 cm. In such cases the coefficient has a component due to this effect which may be quite high over an appreciable frequency band. This condition occurs in practice with deposited or sprayed-on conductors. The temperaturecoefficient of inductance of coils having a conductor section within the defined range reaches a maximum value at a particular frequency and is less at frequencies above and below this value. Experimental verification is given of the conclusion arrived at in the theoretical discussion, together with an explanation of the behaviour of certain coils having ceramic formers. The conclusions arrived at from a study of such coils are applied to the design of coils suitable for use at high radio frequencies, and it is shown that the best type of construction from the point of view of electrical stability is one in which the radial thickness of the conductor is small compared with the radius of the turn while the axial spacing is large.
