This consists of a tube with a glass bulb at either end; one is clear and the other is blackened (Figure 1). The tube is partly filled with ether and therefore both bulbs contain a mixture of air and ether vapour. When infrared radiation falls on the apparatus more is absorbed by the blackened bulb than by the shiny one, and the pressure inside this bulb rises and pushes the ether along the tube.
A vane mounted on a
vertical pivot is enclosed in a glass bulb filled with air at low
pressure. One side of each part of the vane is blackened and the other
is silvered (Figure 2). When infrared radiation falls on the radiometer
the black surfaces absorb more energy than the shiny ones and so become
hotter. The air molecules hitting one of these blackened surfaces will
gain energy and rebound with an increased velocity so pushing the vane
round. (There is a constant temperature gradient across the vane). The
black surfaces are the trailing surfaces in this case. This piece of
apparatus has almost become an executive toy and can be bought in a
number of gift shops.
If the pressure of air in the bulb is
reduced nearly to that of a vacuum the vane will begin to rotate in the
opposite direction. This is because of the actual pressure of radiation
on the shiny surfaces; the quanta of radiation rebound strongly from
these surfaces so pushing the vane round. The shiny surfaces are now the
trailing surfaces.
This instrument was invented by Langley in 1881. It is simply a blackened strip of platinum and the radiation falling on it is measured by the resulting change in the resistance of the strip. This is measured by connecting the strip into one arm of a Wheatstone bridge.
The principle of the thermocouple was used by Nobili and
Melloni in 1830 to measure the intensity of radiation.
A series of
thermocouple junctions were connected in series, so that the final
e.m.f. generated was much larger than that due to one junction (Figure
3). They called this arrangement a
thermopile.
The diagram in the large blue circle is an enlarged view of detecting surface showing the junctions between the two metals. In reality of course it would be blackened to receive the maximum amount of radiation.
The junctions onto which radiation is to fall are blackened. The
thermocouple has been used to measure the radiation from the planets and
hence their temperatures may be found.
This consists of a telescope that has a lamp filament in the focal plane of the eyepiece. Radiation from the source is focused on to the filament. The lamp filament and the source are viewed through a red filter and the temperature of the filament is adjusted by altering the current through it until the filament disappears. The instrument is calibrated by comparison with a source of known temperature.