Vacuum Solar Radiation Thermometer
In order that the heat absorbed by the blackened bulb of the solar radiation thermometer may not in part be carried off by the currents of air which would come into contact with it, the instrument has been improved by Messrs. Negretti and Zambra into the vacuum solar radiation thermometer
, as illustrated by fig. 64.
sts of a blackened-bulb radiation thermometer, enclosed in a glass tube and globe, from which all air is exhausted. Thus protected from the loss of heat which would ensue if the bulb were exposed, its indications are from 20Â° to 30Â° higher than when placed side by side with a similar instrument with the bulb exposed to the passing air. At times when the air has been in rapid motion, the difference between the reading of a thermometer giving the true temperature of the air in the shade, and an ordinary solar radiation thermometer, has been 20Â° only, whilst the difference between the air temperature and the reading of a radiation thermometer in vacuo has been as large as 50Â°. It is also found that the readings are almost identical at distances from the earth varying from six inches to eighteen inches. By the use of this improvement, it is hoped that the amounts of solar radiation at different places may be rendered comparable; hitherto they have not been so; the results found at different places cannot be compared, as the bulbs of the thermometers are under very different circumstances as to exposure and currents of air. Important results are anticipated from this arrangement. The observations at different places are expected to present more agreement. Observers would do well to note carefully the effect of any remarkable degree of intensity in the solar heat upon particular plants, crops, fruit or other trees.
Terrestrial Radiation Thermometer is an alcohol minimum thermometer, with the graduations etched upon the stem, and protected by a glass shield, as shown in figure 65, instead of being mounted on a frame. The bulb is transparent; that is to say, the spirit is not coloured.
In use, it should be placed with its bulb fully exposed to the sky, resting on grass, the stem being supported by little forks of wood. The precautions required with this thermometer are similar to those for ordinary spirit thermometers, explained at page 76.