122 VACUUM RADIOMICROMETER. 



The present study of the radiomicrometer was undertaken in con- 

 nection with the question of the feasibility of combining it with the 

 radiometer, thus developing more energy in the suspended system. 

 The chief difficulty encountered in this combination is to obtain systems 

 which are free from magnetic material. The diamagnetism also plays 

 an important part in this form of thermo-j unction, so that the radio- 

 meter effect is almost obliterated by the magnetic effect of the coil. 



In the radiomicrometer proper the Bi-Sb junction hangs vertically 

 at the axis of the system and hence is not so seriously affected by its 

 diamagnetism, and if the magnet is not too strong a fairly sensitive 

 instrument is obtainable. Since a window improves its steadiness, the 

 whole might as well be in a vacuum, which will eliminate heat conduc- 

 tion, as is well known for thermopiles. By so doing the sensitiveness 

 of a certain system, to be described presently, was increased by at 

 least 70 per cent. By using No. 40 copper wire and bits of Bi and Sb, 

 soldered with Wood's alloy, it is possible to reduce the weight to 10 mg., 

 which, as already mentioned, is a convenient weight for radiometers. 

 The period of such a system will be much less than that of the radi- 

 ometer of similar weight and time required for maximum deflection. 



Before describing the present instrument it will not be out of place 

 to mention some facts concerning tests of sensitiveness of instruments. 

 In previously described instruments no mention is made of the shield- 

 ing of the thermo-j unctions from radiation other than that which falls 

 directly upon the vane, so that the test of sensitiveness is not neces- 

 sarily a fair one, since reflection from the walls has an enormous effect 

 in increasing the sensitiveness. In fact the indicated sensitiveness may 

 be twice the true value. 



In the present instrument (fig. 92), which is a design (similar to 

 that of Boys) for a radiometer, radiomicrometer, or a combination of 

 the two, the receiving surface is directly behind the window, which is 

 covered with an adjustable slit the length of the vane. In this manner 

 no radiation reaches the walls of the instrument, which are black. 



The Bi for one part of the junction was obtained by melting the metal 

 between glass or iron plates, which were then pressed together. These 

 thin plates were then cut into strips from 0.15 to 0.2 mm. wide. The Sb 

 part of the junction was split from a large, well-crystallized piece of the 

 metal. These pieces could not be obtained quite so thin as the bismuth. 

 The Bi and Sb pieces were then soldered with Wood's alloy, in the form 

 of a V,to a thin sheet of copper. This is not so good as to have the pieces 

 more nearly parallel, since in the latter case the diamagnetic moment is 

 reduced. The dimensions of the Bi and Sb pieces were about 3.5 by 

 0.2 by o.i mm. The length of the copper loop of No. 40 wire was 4.5 



