Page:Popular Science Monthly Volume 86.djvu/440

436 Because of its high magnetic permeability, iron is used to shield the suspended system of magnets from variations in the magnetic field, such as produced by passing street cars, etc. This galvanometer is very sensitive, so that it responds to a current of less than one ten-billionth

of an ampere, and in observing the heating effect produced by different stars, measurements could be made in four to five seconds.

This outfit consisting of an ironclad galvanometer and two receptacles containing the thermocouples was constructed in Washington, D. C, and carried to Mt. Hamilton, Calif., a distance of over 3,200 miles without serious mishap (one thermocouple was broken in climbing the mountain). A vacuum pump was shipped, but it was not unpacked. The slight vapors given off from the cement and stop-cock grease were removed from these receptables by occasionally heating the calcium by means of a small alcohol blast lamp. From this it is evident that one of the principal achievements was in demonstrating that with an equipment of several thermoelements, mounted in evacuated receptacles, one can go to the remotest station for radiation measurements, without taking an expensive or cumbersome vacuum pump.

After observing for several nights it was found that red stars emitted far more radiation than do the blue ones having the same photometric brightness, and attention was given mainly to the solution of this question. Accordingly stars were selected having the same visual magnitude, but differing in color; and which were close together in right ascension and zenith distance, in order to obtain the measurements of the ways traversing the same air mass. In passing through our atmosphere, the radiations from the red stars suffer a greater absorption than do the blue stars. For this reason it was desirable to eliminate, as much as was possible, the effect of air mass.

As the work progressed, it became a rather instinctive feeling to