The Campbell-Stokes Heliograph sunshine recorder from Tiree Met Station.
Courtesy of Mr Ray Sharp
The Campbell-Stokes sunshine recorder was invented in 1853 by John Francis Campbell, the editor of ‘Popular Tales of the West Highlands’, and modified by Sir George Gabriel Stokes in 1879. It consists of a glass sphere, about ten centimetres in diameter, mounted on a metal stand.
Manufactured to Met Office specification, the glass sphere focuses the rays of the sun to an intense spot which chars a mark on a curved graduated card mounted concentrically with the sphere. As the earth rotates, the position of the spot moves across the card. The card is held in place by grooves, of which there are three overlapping sets, to allow for the height of the sun during different seasons of the year.
Its main advantage is its simplicity and ease of use. However, the results require interpretation by an observer and may differ from one person to another.