The specification of the properties of the thermistor as a circuit element in very-low-frequency systems

An analysis based on sinusoidal applied voltages shows that a conductor whose resistance is a function of temperature may be represented by an equivalent circuit having a semicircular impedance locus. An expression for the distortion of the waveform is also obtained, and this is found to be small provided that the alternating current is less than a quarter of the steady polarizing current which flows in the conductor. The impedance loci of a bead-type thermistor are plotted by means of a null technique. A typical impedance varied from a negative resistance of 3 000 ohms at very low frequencies to a pure inductance of 2 000H at 0.3c/s and then to a positive resistance of 5 000 ohms at high frequencies. The use of the equivalent circuit is illustrated by designing phase-shift networks suitable for use in the stabilizing of very-low-frequency control systems. These circuits may be used in systems where either a.c. or d.c. data transmission is employed.