The instrument uses a single antimony-bismuth thermo-electric junction soldered to a thin, blackened copper plate. The copper plate is suspended from the top of the brass cylinder by a quartz fiber. The two ends of the thermo-junction are soldered to the ends of a slender loop of light copper wire, which hangs between the poles of a permanent magnet. The magnet poles can be seen at the top of the glass-faced portion of the instrument..

   When thermal radiation falls on the blackened copper disk, the thermo-electric junction produces a current in the copper loop. This feels a magnetic torque, and the suspended system twists. A very light mirror is attached to the bottom of the quartz fiber to allow the rotation to be followed. The funnel rotates, and allows radiation to be directed toward the disk. 

   Boys formed the fine quartz fiber by attaching one end of a rod of quartz to an arrow, and the other to a solid object, melting the middle portion of rod of quartz in a hot flame, and shooting off the arrow; this method he described in 1887. 

   Using this instrument with a 16 inch reflecting telescope, Boys was able to observe thermal radiation from a candle at a distance of up to three miles.