Campbell–Stokes Recorder: A Simple Device That Measures Sunshine

This crystal ball located outside Darwin Airport Meteorological Office, in Darwin, Australia, doesn’t look into the future, but provides invaluable information about the past. This antique technology, called a Campbell-Stokes sunshine recorder, is one of the simplest meteorological device that’s still in use today. It measures the intensity of sunshine on any given day the same way a thermometer measures temperature or a barometer measures air pressure.

The device consist of a solid glass sphere, typically about 4 inches in diameter, that concentrates the sun's rays to an intense spot on a calibrated paper, resulting in a burn. As the sun blazes across the sky, the hot image of the sun traces a scorching path on the paper. The intensity and the position of the burn indicates the time and the strength of the sunshine.

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A Campbell–Stokes sunshine recorder outside Darwin Airport Meteorological Office, in Darwin, Australia. Photo credit: Bidgee/Wikimedia

The sunshine recorder was invented by no scientist, but a renowned Scottish author and scholar, John Francis Campbell, in 1853. At that time, it was common knowledge that certain transparent glass balls, that were commonly used as paper weights, behaved like “burning glass” leaving smoking holes over desks and writing paper placed near windows. Similar unexpected combustion occurred near glasses of water, bottles, a knot in a window pane, or a bowl of goldfish. Although this behavior of glass took many people by surprise, the use of mirrors and lenses to concentrate sun’s beam has been around since the days of ancient Greek. During the siege of Syracuse in the 2nd century BC, Archimedes famously set enemy ships on fire by bouncing sun’s rays off a parabolic mirror.

Exactly what inspired an author of Celtic folklore to dabble in meteorology is a mystery, but John Francis Campbell was determined to build a device that could record the intensity of sunlight over the course of a day. Campbell couldn't get hold of a glass sphere, but he did found a hollow glass globe which he filled with water and turned it into a lens. He mounted the globe a few inches over an wooden bowl so that a concentrated beam of sunlight fell on the wood. As the sun moved across the sky, the beam scorched a path across the wood. Whenever the sun was hindered by clouds, the line would break. The length of the break indicated for how long the sun remained covered, and the position of the break on the wood indicated the time of the day. Campbell also discovered that the hotter the sun was, the deeper was the burn.

Campbell’s device was so simple and effective that it was promptly adopted by meteorologist. In 1879, Irish physicist replaced the wooden frame with metal and added removable paper cards to record the burns. The Campbell–Stokes Sunshine Recorder has remained essentially unchanged since then. The device has been the standard instrument for recording sunshine in many parts of the world for more than a century, which makes them a good source of long term and reliable data. Many old burn cards that were sitting in drawers in observatories and universities for decades are now being used by researchers to study and compare the amount of solar radiation reaching the Earth, and even the cloud type and thickness in the past.

Although these antique instruments are now replaced by electronic sensors, you can still find functioning Campbell–Stokes Recorder at many meteorological offices and observatories around the world.

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Close up of a summer sunshine card for the Campbell-Stokes recorder. The amount of sunshine is recorded in 10th's of an hour. Photo credit: CambridgeBayWeather/Wikimedia

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