The stability of inductance coils for radio frequencies

The paper gives an account of an investigation into the stability of inductance of coils used in radio transmitters and valve oscillators. It is shown that variation of temperature is chiefly responsible for instability, and data are obtained showing the value of the temperature coefficient of inductance for a number of modern coils of representative types, from which it is observed that this coefficient differs appreciably from the coefficient of expansion of the metal conductor. A theoretical investigation is made of the inductance variation produced by small changes in self-capacitance and in dimensions, together with an examination of the causes of mechanical distortion with temperature change. A critical study of existing methods of temperature compensation is presented and conditions for stability are established. As a result of the investigation, an experimental coil is described in which high stability is obtained by the application of these principles.