radiometer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A device that measures the intensity of radiant energy, consisting of a partially evacuated glass bulb containing lightweight vertical vanes, each blackened on one side, suspended radially about a central vertical axis to permit their revolution about the axis as a result of incident radiation.
2. n. An instrument that detects electromagnetic radiation.

Century Dictionary and Cyclopedia

1. n. An old instrument for measuring angles; the cross-staff. The end of the staff was held to the eye, and the crosspiece was shifted until it just covered the angle to be measured, when the latter was read off on the longitudinal staff.
2. n. An instrument which serves to transform radiant energy into mechanical work. It consists of four crossed arms of very fine glass, supported in the center by a needle-point, and having at the extreme ends thin vertical disks or squares of pith, blackened on one side. When placed in a glass vessel nearly exhausted of air, and exposed to rays of light or heat, the blackened surfaces absorb the radiant energy and become heated, the molecules of the air remaining in the vessel striking against them gain from them greater velocity and there results an increased pressure, causing a more or less rapid revolution of the arms. By varying the conditions as to degree of exhaustion, size of bulb, etc., a number of experiments are performed with the radiometer which serve to illustrate the mechanical effects of the rapidly moving molecules of a gas.
3. n. An instrument for the detection and measurement of small amounts of radiant energy. An adaptation of Crookes's form of radiometer to delicate heat-measurements has more recently been devised by E. F. Nichols. The Nichols radiometer has two vanes at the ends of a short cross-arm, suspended by a quartz fiber in vacuo. In order to protect the instrument from the effects of stray heat, the containing-vessel is usually of metal, with a window of transparent rock-salt or fiuorite for the admission of the radiation to be measured, and a second window of glass through which the angular movement of a light mirror attached to the suspended parts may be observed. The construction of the radiometer varies considerably according to the work for which it is designed. The form of instrument constructed for the measurement of the heat of the fixed stars is shown in Fig. 1. The vanes D, D and the mirror M, the plane of which is perpendicular to that of the vanes, are mounted upon a fine rod of drawn glass, AB, the upper end of which is attached to a very fine quartz fiber, 32 millimeters long, BC. The vanes are disks of thin mica coated with lampblack and about 2 millimeters in diameter. The distance between the centers of the vanes is 4.5 millimeters. The mirror, which is 3 × 2 millimeters, consists of a silvered portion of thin microscope-cover glass. The apparatus is essentially a torsion balance, the mass (about 6 milligrams) and moment of inertia of which are exceedingly small. When radiation passing through the fluorite window F (Fig. 2) falls upon one of the vanes, the vane is repelled, and the balance revolves through an angle such that the torsional elasticity of the quartz fiber precisely counteracts the torque due to radiation. The deflection thus produced is measured by observing the angular movement of a beam of light reaching the mirror M through the window W (indicated in the diagram by the dotted circle). The sensitiveness of this form of radiometer and of the corresponding forms used in the exploration of the spectrum is greater than that of any other device known to the physicist. It was found possible, for example, to measure the heat received from Arcturus, which is approximately equal to the one hundred-millionth part of that from a candle at a distance of one meter, and even the heat from Vega, which is only half as bright as Arcturus. The sensitiveness of the radiometer varies with the pressure of the gas in which the vanes are suspended, reaching a maximum at a pressure of about .05 millimeters. It also depends upon the distance between the vanes and the fluorite window. The best effect is produced when this window is within two or three millimeters of the vanes, as shown in Fig. 2.

Wiktionary

1. n. A device that measures radiant energy.

GNU Webster's 1913

1. n. (Naut.) A forestaff.
2. n. (Physics) An instrument designed for measuring the mechanical effect of radiant energy.

WordNet 3.0

1. n. meter to detect and measure radiant energy (electromagnetic or acoustic)