Signedon circular mark: JULIEN FRIEZ BALTIMORE BMD USA BEFORE OBSERVATORY
FunctionThis is the detector of a device that registers the times that the sun has been shining over an interval of time. It functions as an interruptor which closes a circuit when the amount of heat coming from sunshine is above a certain threshold. During its use, it would be connected by a cable to a recording instrument (which could be kept indoors or protected from the elements).
Following is a detailed description from the Glossary of Meteorology, published by the American Meteorological Society [Second Edition, 2000], and available online "http://amsglossary.allenpress.com/glossary" target="_blank">here:
"[The Marvin sunshine recorder] consists of two bulbs, one of which is blackened, that communicate through a glass tube of small diameter. The tube is partially filled with mercury and contains two electrical contacts. When the instrument is exposed to sunshine, the air in the blackened bulb is warmed more than that in the clear bulb. The warmed air expands and forces the mercury through the connecting tube to a point where the electrical contacts are shorted by the mercury. This completes the electrical circuit to the pen on the chronograph. The Marvin sunshine recorder is equally sensitive to the direct rays of the sun and to diffuse radiation from the sky (the heat from the latter at midday in overcast may be more than that from direct sunshine in the early morning); thus, the instrument is not without ambiguities. It is standard equipment at National Weather Service stations."
ProvenanceFrom the Blue Hill Meteorological Observatory in Milton, MA.