The Campbell–Stokes recorder (sometimes called a Stokes sphere) is a type of sunshine recorder. It was invented by John Francis Campbell in 1853 and modified in 1879 by Sir George Gabriel Stokes. The original design by Campbell consisted of a glass sphere set into a wooden bowl with the sun burning a trace on the bowl. Stokes's refinement was to make the housing out of metal and to have a card holder set behind the sphere. The unit is designed to record the hours of bright sunshine which will burn a hole through the card. This basic unit is still in use today with very little change. It is widely used outside the United States (the Marvin sunshine recorder is generally the instrument used by the U.S. National Weather Service). Technology The crystal ball is typically 10 cm (4 inches) in diameter, and is designed to focus the rays from the sun onto a card mounted at the back and is set on a stand. The card is held in place by grooves of which there are three overlapping sets, to allow for the altitude of the sun during different seasons of the year. The recording of each day goes onto one card. In the northern hemisphere the winter cards are used from 15 October to 29 February, the equinox cards from 1 March to 11 April and 3 September to 14 October. The summer cards are therefore used from 12 April to 2 September. Each card is marked as to the hour, with local noon being in the centre, and is read in tenths. In the northern hemisphere, the unit is set in a stand facing south to enable the maximum amount of sun to be recorded. It is important to place the unit in an area where the sun will not be blocked by buildings, trees or flagpoles. A modification to the standard unit for polar regions is the addition of a second, north facing, sphere and card, to record the sunlight during the summer when it remains in the sky for 24 hours. Advantages The major advantage of this type of recorder is its simplicity and ease of use. There are no moving parts and it thus requires very little maintenance. The unit can be used anywhere in the world with little or no modification to the design. Electrically operated pyranometers can do the same work, but