The sensing element is a silver disk measuring 28 mm in diameter with a thickness of 7 mm that is
painted black on its radiation-receiving side. It has a hole from the periphery toward the center to allow
insertion of the bulb of a high-precision mercury-in-glass thermometer. To maintain good thermal contact
between the disk and the bulb, the hole is filled with a small amount of mercury. It is enclosed outside by
a heat-insulating wooden container. The stem of the thermometer is bent in a right angle outside the
wooden container and supported in a metallic protective tube. A cylinder with diaphragms inside is fitted
in the wooden container to let direct solar radiation fall onto the silver disk. There is a metallic-plate
shutter at the top end of the cylinder to block or allow the passage of solar radiation to the disk.