Campbell stoke’s sunshine recorder

This instrument, for measuring the amount of bright sunshine, has been in use since 1876. The solid glass sphere focuses the sun’s rays to an intense (moving) spot that burns a trace on a card placed in the spherical mount behind the sphere. Once the recorder has been precisely aligned for it’s location no further maintenance is needed apart from keeping the sphere clean. Special cards are used in the recorder: the vertical lines indicate the hours and the card is burnt away by the focussed sunlight, leaving a white trace. To allow for the seasonal change in the elevation of the sun, different cards are used during different seasons