This is an example of a standard portable silver disk pyrheliometer, typical of those built and used in the early twentieth century to measure the energy received by the earth from the sun, called the "solar constant." It is built around an insulated mercury bulb thermometer whose reservoir is encased in a blackened silver disk set inside a wooden cylindrical chamber at the bottom of the instrument, and whose stem is encased in a silver cylindrical rod at the side of the main chamber. The level of the thermometer mercury column is read visually.
This instrument was likely built in the shops of the Smithsonian Astrophysical Observatory, based upon a design refined by Charles Greeley Abbot. It was found in the model shops at North Capitol Street and bears no inventory marks.
Transferred from the Astro-physical Observatory of the Smithsonian Institution.