Nichols radiometer
Encyclopedia
A Nichols radiometer was the apparatus used by Ernest Fox Nichols
and Gordon Ferrie Hull
in 1901 for the measurement of radiation pressure
. It consisted of a pair of small silvered glass mirror
s suspended in the manner of a torsion balance by a fine quartz
fibre within an enclosure in which the air pressure could be regulated. The torsion head to which the fiber was attached could be turned from the outside by means of a magnet
. A beam of light
was directed first on one mirror and then on the other, and the opposite deflections observed with mirror and scale. By turning the mirror system around so as to receive the light on the unsilvered side, the influence of the air in the enclosure could be ascertained. This influence was found to be of almost negligible value at an air pressure of about 16 mmHg
(2.1 kPa). The radiant energy of the incident beam was deduced from its heating effect upon a small blackened silver disk, which was found to be more reliable than the bolometer
when it was first used. With this apparatus the experimenters were able to obtain an agreement between observed and computed radiation pressures within about 0.6 %. The original apparatus is at the Smithsonian Institution
.
This apparatus is sometimes confused with the Crookes radiometer
of 1873, in which vanes turn in a partial vacuum under the influence of low pressure gas molecules and not directly by the photons themselves.
Ernest Fox Nichols
Ernest Fox Nichols was a U.S. educator and physicist. He was born in Leavenworth County, Kansas, and received his undergraduate degree from Kansas State University in 1888. After working for a year in the Chemistry Department at Kansas State, he matriculated to graduate school at Cornell...
and Gordon Ferrie Hull
Gordon Ferrie Hull
Gordon Ferrie Hull was a Canadian / American teacher, mathematician and physicist, especially known for the experimental detection of the radiation pressure exerted by light which he achieved in 1903....
in 1901 for the measurement of radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
. It consisted of a pair of small silvered glass mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
s suspended in the manner of a torsion balance by a fine quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
fibre within an enclosure in which the air pressure could be regulated. The torsion head to which the fiber was attached could be turned from the outside by means of a magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
. A beam of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
was directed first on one mirror and then on the other, and the opposite deflections observed with mirror and scale. By turning the mirror system around so as to receive the light on the unsilvered side, the influence of the air in the enclosure could be ascertained. This influence was found to be of almost negligible value at an air pressure of about 16 mmHg
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
(2.1 kPa). The radiant energy of the incident beam was deduced from its heating effect upon a small blackened silver disk, which was found to be more reliable than the bolometer
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
when it was first used. With this apparatus the experimenters were able to obtain an agreement between observed and computed radiation pressures within about 0.6 %. The original apparatus is at the Smithsonian Institution
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...
.
This apparatus is sometimes confused with the Crookes radiometer
Crookes radiometer
The Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement...
of 1873, in which vanes turn in a partial vacuum under the influence of low pressure gas molecules and not directly by the photons themselves.