The duration of sunshine is measured by the?

The duration of sunshine is measured by the Campbell-Stokes recorder. The Campbell-Stokes recorder is a simple and low-cost device that measures the amount of sunshine duration in a day. The device consists of a glass sphere that acts as a magnifying lens and focuses the sun's rays onto a card, which is mounted on a recording drum. As the sun moves across the sky, the card is burnt by the focused rays of sunlight, leaving a trace or burn mark on the card. The length of the burn marks left on the card corresponds to the duration of sunshine for that particular day. To use the Campbell-Stokes recorder, it needs to be placed in an open area, preferably on a flat and level surface, and away from any obstructions that may cast shadows. The recorder is oriented in a way that the glass sphere faces the sun, and the recording drum rotates with the movement of the sun. Overall, the Campbell-Stokes recorder is an effective and straightforward tool for measuring the duration of sunshine, and it has been used for many years by meteorologists and climatologists around the world.