# Pyrheliometer

## Definitions

• Webster's Revised Unabridged Dictionary
• n Pyrheliometer (Physics) An instrument for measuring the direct heating effect of the sun's rays.
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Century Dictionary and Cyclopedia
• n pyrheliometer An instrument devised by M. Pouillet for measuring the intensity of the heat of the sun. It consists of a shallow cylindrical vessel of thin silver or copper, containing water or mercury in which a thermometer is plunged. The upper surface of the vessel is covered with lampblack, so as to make it absorb as much heat as possible, and the vessel is attached to a support in such a way that the upper surface can always be made to receive the rays of the sun perpendicularly. The actual amount of heat absorbed by the instrument is calculated by ordinary calorimetrical means: the area of the exposed blackened surface is known, and the amount of water or mercury which has been raised through a certain number of thermometric degrees is known, and thus the absolute heating effect of the sun, acting upon a given area under the conditions of the experiment, can be readily found. Also pyroheliometer.
• n pyrheliometer In the earlier forms given to this instrument, such as that of Pouillet (see pyrheliometer), the heat of the sun's rays was determined by the rise of temperature of water or mercury contained in a vessel which exposed to sunlight a known area of surface. In modern practice electrical methods have been employed which are of far greater sensitiveness, accuracy, and convenience. The compensated pyrheliometer of Ångström, which belongs to the latter class of instruments, consists of two very thin metal strips (AB) of equal size, mounted a few millimeters apart. The strips are blackened on the face which is to be exposed to radiation. To the back of the strips are attached the two junctions of a thermo-element, in circuit with which is a delicate galvanometer by means of which their temperatures may be compared. If one of the strips, as A, is exposed to radiation while the other is screened, the rise in temperature of the former will produce an electric current in the circuit containing thermo-junctions and cause a deflection of the galvanometer. If, now, current from a battery, C, is sent through the shaded strip, this also will be heated and by adjustment of a variable resistance, R, the heating effect of the current in B may be made precisely to counterbalance the heating effect of radiation in A. When the current from the battery is just sufficient for this purpose the galvanometer deflection is reduced to zero. If i is the intensity current necessary to produce a balance, r the resistance of the strip through which it flows per centimeter of length, b its width, and a the absorbing power of the surface, the intensity of the radiation upon the exposed strip will be given by the equation The temperature of the two strips being the same, the losses by radiation, convection, and conductivity are equal and these troublesome corrections are avoided. Because of the small thermal capacity of the instrument its final temperature is reached in a few seconds. The metal of the strips is usually platinum, but manganin, for which the coefficient of resistance with temperature is very small, is to be preferred. The frame which carries the strips is mounted in a tube which likewise contains the screen for protecting the unexposed strip. This screen can be shifted so as to expose first one and then the other strip to the snn's rays. By means of the switch S the compensating current can be sent through either strip at will.
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Chambers's Twentieth Century Dictionary
• n Pyrheliometer pir-hē-li-om′e-tėr an instrument for measuring the intensity of the sun
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## Etymology

Webster's Revised Unabridged Dictionary
Gr. fire + sun + -meter,
Chambers's Twentieth Century Dictionary
Gr. pyr, fire, hēlios, sun, metron, measure.