A LARGE number of different types of cryostat which can be used for physico-chemical studies of matter has been described in the literature. '-9 Without going into constructional details, we can distinguish the following main components of any present-day cryostat; thermostatted chamber, heater, coolant, temperature regulating system (manual or automatic), and temperature measuring arrangement. Most such equipments have disadvantages making it difficult to use them for studying the electrical properties of crystals at different temperatures. The limiting accuracy of the temperature control (_+ 0-5-1 ยฐ C) and the complexity of construction are such drawbacks. The existing methods of automatic temperature control come down, in the main, to two types. In one the constant temperature is maintained by a photoelectric regulator which controls the current through a heater, while in the other the heating is determined by a thyratron regulator. These methods achieve temperature control with an accuracy of + 0.01ยฐC.
Sarakhov 2 has obtained an accuracy in temperature control of +0-005ยฐC in his cryostats. A resistance thermometer, included in a bridge circuit, is used as the temperature pick-up. The out-of-balance signal, arising from overheating or undercooling, is amplified and rectified and controls the current through a heater. The absence of a phase-sensitive stage is an inconvenience, making necessary an additional control on the cryostat temperature. Wilson and Stone 9 developed this system in 1957, introducing a phase-sensitive stage. The heater was incorporated as the anode load of the output power stage. The magnitude of the current through it depends on the relative phases of the voltage applied to the bridge and of the out-of-balance signal. Vasil'ev ' used this circuit, connecting it with the heating current control, thereby increasing still further the accuracy of temperature regulation. The present authors have used Vasil'ev's circuit as the basis for constructing a precision laboratory cryostat for studying the electrical and elastic properties of crystals in the region of polymorphic phase transitions.