THE RADIOMICROMETER. 1 53 



In the course of the discussion it will be noticed, as was previously 

 shown in a general way, that each instrument has some quality which 

 makes it useful for a certain kind of work. For measuring very narrow 

 emission lines and determining dispersion curves the bolometer is no 

 doubt the best adapted. For measurements requiring a larger receiving 

 surface, the linear thermopile is the more sensitive and the more precise. 

 It has the further advantage that there is no permanent current. On the 

 contrary, the bolometer has a current which heats the bolometer strip above 

 the temperature of the surrounding air. This causes air currents which 

 make the zero unstable. This is not true of the thermopile. Less is 

 known concerning the radiometer, which rivals the bolometer and the 

 thermopile in sensitiveness. Furthermore, the radiometer is not subject to 

 magnetic perturbations. Its window limits its usefulness to the region of 

 the spectrum up to 20 /<. The fact that it is not portable is a very minor 

 objection. 



I. THE MICRORADIOMETER. 



Since we are concerned with radiation meters of the greatest sensitive- 

 ness, the ingenious device of Weber, 1 called the microradiometer, deserves 

 passing notice. The instrument is not unlike a combination of a differ- 

 ential air thermometer and Wheatstone bridge. Two arms of the bridge 

 consist of a thin glass tube containing a bit of mercury at the center with 

 a solution of zinc sulphate at the ends, into which dip platinum electrodes. 

 The ends of the bulbs are covered with rock-salt windows. If radiant 

 energy is allowed to enter one of these bulbs, the air expands and pushes 

 the liquids toward the opposite bulb. This will change the relative lengths 

 of the column of mercury and of the solution between the platinum termi- 

 nals, which means a change in resistance in the bridge arm, and a conse- 

 quent deflection of the galvanometer. The instrument was stated to be 

 sensitive to a temperature change of 0.00001 , and while it is not adapted 

 to spectrum radiation measurements, it might be used in total radiation 

 work where an elaborate installation is not convenient. By making the 

 receiving bulb of opaque non-conductive material and by covering the 

 inside with lampblack, or platinum black, this would be as complete an 

 absorber ("black-body") as the thermopile or bolometer. Its efficiency 

 would, of course, depend upon the gas inclosed. 



II. THE RADIOMICROMETER. 



The radiomicrometer is essentially a moving coil galvanometer, of a 

 single loop of wire, with a thermo-junction at one end. This instrument 

 was invented independently by d'Arsonval 2 and by Boys. 3 The former 



1 Weber: Archiv. Sci. Phys. et Mat. (3), 18, p. 347, 1887. 



2 d'Arsonval: Soc. Franc, de Phys., pp. 30 and 77, 1886. 



3 Boys: Proc. Roy. Soc, 42, p. 189, 1887. 



