Callender sunshine recorder by Cambridge Instruments, 1913

This form of electrical sunshine recorder was devised by Professor H. L. Callender in 1900 and operated with his Callender Recorder which employed a modification of the Wheatstone Bridge.

The instrument is essentially a differential platinum thermometer consisting of two resistance coils, one black and the other bright, which form the two arms of a Wheatstone Bridge. The blackened coils absorb heat while the bright ones do not. In practice the instrument was connected to the recorder which gave a continuous trace of the net output. The instruments were individually calibrated and given a factor by means of which the total amount of heat actually received could be calculated and expressed centimeter per second.