Determining sunshine duration 130 years ago meant polishing the old crystal ball and scorching holes in strips of card, while it sounds mystical the Campbell-Stokes recorder was cutting edge for its time and the lack of moving parts made it a reliable and legitimate method of measuring and recording sunshine duration, albeit a little inaccurately.
The Campbell-Stokes recorder was originally devised by John Francis Campbell in 1853 and later modified in 1879 by Sir George Gabriel Stokes. Campbell devised the idea of a glass sphere filled with water to act as a lens to focus sunlight and scorch a wooden surface, originally the wall of the sphere’s wooden container. Later, Campbell improved the design by changing the water filled sphere to a solid glass sphere. While novel, the basic design had limited use as the repeated scorch marks on the same bowl could not provide a clear measurement of the sunshine received.
While several people, including Campbell himself, experimented with the idea of using a replaceable strip of material to more accurately measure daily sunshine, it was George Gabriel Stokes who ran with the idea of removeable material strips and produced the design which is commonly seen today. By suspending the ball in metal clamps and making a metal ring with grooves in to firmly hold the card, Stokes improved the design in to one which produced measurable and accurate results. In fact modern designs have deviated little from the original, however standardisation has been introduced by the World Meteorological Organisation (WMO) to ensure worldwide measurements are comparable.
Measuring sunshine duration – how it works
When sunshine is present the glass ball would focus the rays and burn a hole through a strip of photosensitive card. As the sun moves across the sky a corresponding hole would be made in the card. Should there be no sunshine then no hole would be produced, low intensity sunshine (potentially at sunrise and sunset or partial clouding) may only produce scorching to the card.
To factor in the changing position of the sun throughout the year, different shaped strips of card are used, typically 3 shapes are available.
At the end of each day the card would be removed and the duration of sunshine calculated by measuring the length of the holes made in the card, tools are available to improve the accuracy of the reading.
While ingenious for its time, the Campbell-Stokes recorder has several draw backs. For a start, sunset and sunrise holes were not always well formed and on days when the sunshine was patchy a level of interpretation by the person reading the card was required, these could cause great discrepancies in the sunshine duration. Other factors like rain damaging the card and snow/frost covering the crystal added further inaccuracies.
New and improved
As the demand for more complete and accurate data grew as did the requirement for an instrument which could produce the results.
For a more detailed history of the Campbell-Stokes recorder we would refer you to the Royal Metrological Society’s website: