The B23/S280 is a power measuring device or bolometer. It consists of a heating element coupled to a negative temperature coefficient element via a thermally efficient insulator.
Basically a resistor close coupled to a thermister and sealed in a vacuum. The connections to the active region are made by very thin wires to minimise the heat loss through conduction.
The device can be calibrated with a DC voltage and then be used to measure the power of an irregular waveform such as a radio RF envelope.
The load is 100 Ω and a current of 1 mA changes the output resistance from 1550 - 1520 Ω.
These devices are for the measurement of very small powers.
The active component with the insulator clearly seen in the centre.
Thanks to Edin Lelic for the images and measured data.
The envelope is mm in diameter, and excluding the base pins is mm tall.
Updated October 01, 2013.