46 Conductivities and Viscosities in Pure and in Mixed Solvents. 



we have but one seal and two metal strips. This follows from the fact 

 that the sensibility of the radiomicrometer is materially increased 

 when the resistance of the junction is made more nearly equal to that 

 of the wire loop. However, the advantages to be derived from the 

 compensating type of junction are well worth the sacrifice in sensibility. 



It is very important that the weight of the junction be kept as small 

 as possible, not only to lessen the weight of the suspended system, but 

 especially to reduce the heat capacity of the junction to a minimum. A 

 small heat capacity insures a quick-acting suspension, and one which 

 will more rapidly return to the zero-point. That the mass of material 

 in the junction is very small can be seen from the fact that the actual 

 weight of one of the completed compensating junctions was 2.9 mg. 



The quartz fiber 1 was obtained from molten quartz by means of a bow- 

 and-arrow arrangement. The total length of this fiber is nearly 30 cm. 

 A long fiber materially aids in eliminating vibrations. This is most 

 essential, especially when the work necessitates small deflections. The 

 period and sensibility of the radiomicrometer depend in a large measure 

 on the proper selection of the quartz fiber. 



The plane mirror m (fig. 6) is very thin and has an area of about 20 

 sq. mm. Directly in front of it is the lens I, having a focal length of 12 

 feet. By means of this arrangement the image of a lamp filament could 

 be sharply focused on a glass scale about 12 feet distant from the 

 mirror; and it is on this scale that the deflections as given by the radio- 

 micrometer are noted. 



The torsion-head stopper s, made of brass and tightly ground into the 

 glass tube T, serves the purpose of making adjustment. By means of 

 it the suspended system can be made to occupy any position in the 

 magnetic field, and the whole system can be raised or lowered without 

 danger of breaking the very fragile suspension. The torsion-head 

 stopper contains a brass rod r and the two screws s : and s n for making 

 the above adjustments. 



The glass tube T, in which the whole system is suspended, is about 

 45 cm. in length. To it is attached the lens I, previously referred to, 

 and the glass window p, through which the beam of light is directed 

 either to junction A or to junction B. 



The glass tube surrounding that portion of the suspended system to 

 which the thermo-j unction is attached was insulated from temperature 

 changes and air drafts by a layer of fine lead shot and wool fiber. To 

 prevent stray sources of light and air drafts from reaching the radio- 

 micrometer, it was inclosed in a wooden box covered with painted 

 canvas. 



That the image on the scale should remain steady at a focal distance 

 of 12 feet, it is necessary to shield the radiomicrometer very carefully 

 from the usual vibrations of the building. The long quartz fiber aided 



J Dr. C. W. Hewlett has very kindly supplied us with several satisfactory fibers. 



