44 Conductivities and Viscosities in Pure and in Mixed Solvents. 



permanent magnet at the bottom. Plate 2, figure 2, is a side view of 

 the radiomicrometer. A larger view of the lens and grating is shown 

 in plate 3, figure 1 ; an end view of the spectroscope in plate 3, figure 2. 



THE RADIOMICROMETER. 



The methods employed in the construction of the radiomicrometer 

 and especially of the thermo- junctions, we owe to Professor A. H. Pfund. 1 

 By means of these methods we were able to build a very satisfactory 

 instrument. 



The radiomicrometer previously constructed by Guy 2 was found to 

 be unsatisfactory for use with the grating spectroscope. Its sensibility, 

 according to a recent test, was 2 per square millimeter of exposed vane, 

 candle and scale being at a meter's distance. The full period of this 

 instrument was, however, very short, being only 8 seconds. His radio- 

 micrometer could have been made about as sensitive as the one con- 

 structed for this work, by using a longer and finer quartz fiber and 

 making the full period about 20 seconds. However, as this instrument 

 was not equipped with a compensating junction, and as its drift due to 

 this cause, had proved so troublesome in the work of the previous year, 

 it was decided to construct a new radiomicrometer. 



Briefly, the radiomicrometer consists of a thermo-electric junction 

 attached to a loop of non-magnetic wire. The whole system is sus- 

 pended by a quartz fiber in a glass tube. A strong magnetic field 

 surrounds that portion of the tube inclosing the loop of wire. Radiant 

 energy falling upon the blackened junction is converted into electrical 

 energy. In proportion to the amount of energy received by the 

 junction, the suspended system turns through a definite angle in the 

 magnetic field, the loop tending to set itself at right angles to the lines 

 of force. The deflection or turn is given by means of a mirror attached 

 to the suspended system. 



The essential parts of the radiomicrometer are shown in figure 6, 

 which represents the type of compensating junction that was con- 

 structed and used. It is fastened to the loop of copper wire w at c 

 and c', as shown in figure 6. To support the loop of wire small glass 

 rods are placed at h and h'. A light mirror is attached to h at m. 

 A quartz fiber, /, is fastened to the end of the glass rod h, and to the 

 brass stopper S. The whole system is then set in the glass tube T, 

 having two windows I and p. The lens I focuses the reflected light 

 from the mirror m, on a glass scale. At p is inserted a plane-glass 

 window 1 mm. thick. The beam of light which is to be measured 

 passes through this window before falling on the junction. The mag- 

 netic field is placed between I and p. 



MPhys. Rev., 34, 228 (1912); Phys. Zeit., 13, 870 (1912). 

 "Carnegie Inst. Wash. Pub. No. 190, 30 (1913). 



