Sine Galvanometer 



385 



temperature coefficients of the marble spool in different radial directions for each 

 Helmholtz coil. As was to be expected after the preliminary thermal treatment given 

 the marble, the temperature variation was found to be reversible. 



The measurements at the lower temperatures were made in four azimuths differing 

 successively by 45 on a single spiral near the center of each of the two coils numbered 

 1 and 4 comprising a Helmholtz pair. The mean diametrical coefficient for the marble 

 adjacent to coils 1 and 2 was found to be + 8.1 X 10" 6 per degree C; that for the marble 

 adjacent to coils 3 and 4, 4- 6.7 X 1CT 8 per degree C. Earlier researches 1 have shown 

 that marble may have expansion coefficients differing greatly in magnitude from point 

 to point, and in different directions, and even differing in sign. In the case here con- 

 sidered the measured expansion varied from 0.001 mm. to 0.008 mm. for coil 1, and from 

 0.010 to 0.014 mm. for coil 4. 



In Figure 15 the overall diameter of each of the four spirals reduced to 23 C. is 

 exhibited as a function of the azimuth of the lower micrometer, the azimuth of the lower 

 ends of two of the coils being taken as zero, that of the other two as jr. 



2ir 



4ir 



Sir 



S-rr 



12 TT 



10 rr 

 ANGLES 

 Fio. 15. Diameters of the Spirals. 



16 TT 



18 TT 



207T 



15. To determine the diameter of the wire a length of about a meter was cut off 

 from the wire to be used. Then a coil was wound, then another length of about a meter 

 cut off, then another coil wound, e-tc. The mean diameter of the wire in a coil was assumed 

 to be the mean diameter of the two lengths at its ends. Equally spaced measurements of 

 two diameters at right angles were taken for each length. The mean diameter of the 

 wire for each coil was 0.576 mm., with an average departure of less than 0.001 mm., and 

 with no difference apparent between measurements made in directions at right angles 

 to one another. 



1 See Souder anrl Hindert, Scientific Papers, Bureau of Standard", No. 352, Dec. 1919. 



