Campbell-Stokes type sunshine recorder.

Glass sphere diameter 11cm held in a line 45degrees to the horizontal by brass screws on either end. Screws are attached to a black metal half ring, width 3cm, thickness 8mm. On the outside of the half ring is a brass scale sticking out perpendicular to the ring. The scale goes from 0 to 80 from the middle to the bottom end with divisions marked 1degree apart. The top half of the scale appears never to have been marked. On the inside of the ring is another ring running across (so they make a cross where they meet). This second ring is also made of black metal, diameter 15cm, width 8cm, thickness 5mm. It is this ring that holds the paper scale behind the glass sphere. The base is metal painted black and is 165mm x 225mm x 35mm, and is attached by a black metal piece that clamps the scale ring.

Ball diameter 4" (10.2 cm)

Sunshine recorders are positioned on a local meridian (north-south line) and record the number of hours of sunshine. Each day, a new card strip is placed into the grooves behind the glass sphere. As the Sun crosses the sky, the sphere acts as a magnifying lens, burning a line into the card that’s specially designed to scorch rather than burst into flames. The strip varies in length and position according to the seasons.

The idea was first devised by John Francis Campbell in 1853 and was later developed by the physicist Sir George Gabriel Stokes in 1879. Although simple to use and easy to maintain, the instruments are susceptible to variable results and need human intervention to replace the card strip each day, hence modern meteorological observatories rely on electronic sensors instead.