This consists of a small box containing an iron disk thinly coated with the rare element selenium. When electromagnetic radiation in the form of light waves falls on this surface it sets up a potential difference between the iron and the selenium, which in turn causes a minute electric current to flow through the sensitive microammeter connected between them. The magnitude of this current is proportional to the intensity of the light, and it can be used as a measure. For use with miners' hand lamps the cell is mounted in a box, the microammeter being housed in the top or side. The lamp to be tested is placed in the box on a turntable, and the candlepower is read off directly since the instrument is already calibrated.