Bimetallic actinograph, M.O. Mark III, by Casella, 1955

This solar radiation measuring instrument was designed by M Robitzsch in Germany in about 1915. At first the instrument worked in the principle of two bimetallic rings placed one above the other. By the year 1929, the mechanical design had been changed to an arrangement of two horizontal bimetallic strips, placed side by side, and further developments took place.

The instrument now had a receiving surface, under the glass dome, constisting of two parallel rectangular white-coated strips of bimetal between which is placed a similar strip of bimetal coated black. At one end the three strips are connected to a common rod. While the other ends of the white strips are connected to the frame of the instrument, the other end of the black strip is connected to a simple lever mechanism and its movements are recorded on a chart placed on a normal clock drum.

These movements are proportional to the temperature difference between the strips, and thus radiation intensity.