On the construction of a standard high-frequency inductive resistance and its measurement by a thermal method

Abstract

The ordinary methods of measuring the radio-frequency resistance of inductance coils depend on the measurement of the resistance of a complete circuit, containing, usually, the coil, a thermo-ammeter, a condenser and leads. The lack of knowledge of the effective resistance of the circuit outside the coil, of the losses induced in the surrounding objects, and of the distribution of parasitic currents, leave some doubt on the reliability of the value of the resistance obtained by these methods. To obtain reliable measurements it is necessary to fall back on some thermal method by means of which the power lost in the coil itself can be measured quite independently of the circuit and neighbouring bodies. The method described in this paper was carried out essentially as a check to the usual methods of measurement. A number of investigators have used thermal methods to measure the effective resistance of coils. Amongst them may be mentioned T. P. Black, L. W. Austin, H. Abrahams, Gr. W. O. Howe, and L. Lehrs. T. P. Black (1) compared the effective resistance of long solenoids with the effective resistance of straight wires, employing a method resembling that of J. A. Fleming (2) for measuring the high-frequency resistance of straight wires by means of a differential air thermometer. L. W. Austin (3) compared the heat given out by a coil when current at radio frequencies and direct current were passed through two similar coils. The coils were immersed in oil, and the equality of temperature noted by thermo-junctions connected in opposition in the two vessels. H. Abrahams (4) compared the rise of temperature of coils after passing radio-frequency current and then direct current by quickly finding the change in the direct-current resistance measured by a Wheatstone bridge. G. W. O. Howe (5) found the rise of temperature of long solenoids by means of a thermojunction placed near the centre, and compared the effect of direct and highfrequency currents. He thus assumed that the loss is uniformly distributed along the coil, an assumption which will hold only if the coil is long. L. Lehrs (6) enclosed the coil in a vessel connected to a very sensitive manometer, and adjusted the alternating and direct currents so that no change could be observed on switching from one to the other.