They comprise a tiny NTC thermistor of nominally 10kΩ resistance at 20°C, surrounded by a small coil of wire with ~100 ohm resistance, mounted in an evacuated glass tube. As current is passed through the coil, the thermistor heats up and its resistance drops. The coil is electrically insulated from the thermistor. A maximum current of 20mA through the coil causes thermistor resistance to drop to 50 ohms.
Applications include automatic gain control, wien-bridge oscillator amplitude stabilisation, remotely controlled variable resistors, etc.
Suitable for signal power measurement up to 40mW, this style of indirectly heated device seems also to have been known as a "thermal converter", though my references mention that these are considered obsolete devices and no longer manufactured.
To quote from the datasheet:
Type B are indirectly heated, bead-type thermistors.
The thermistor element consists of a small bead of resistance material, about 0.02 in, (0.5mm) diameter, which is integrally formed on two parallel platinum wires that are both carried right through the bead and, in assembly, connection is made to both ends of the wire. This gives the assembly greater rigidity and increases the reliability of the thermistor.
The heater surrounding the element is a rigid coil of a special design which ensures maximum concentration of heat on the resistance bead. The resistance of the heater (which has a negliible temperature coefficient) is 100 ohms.
The thermistor sub-assembly comprises the bead, an insulating cement and the heater coil. Careful check-testing of bead and heater, both before and after assembly, maintains a high standard of reproducibility between thermistors of a type. The assembly is sealed in a bulb, evacuated and gettered and thus provides a high vacuum thermistor.
Standard Telephones and Cables LimitedComponents Group, Semiconductor Division (Transistors),Footscray, Sidcup, KentTel : Footscray 3333November 1963
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