378 



Special Reports 



-15cm. 

 -20 cm. 



4 



ently not hitherto pointed out, that it makes the mean intensity along any straight line 

 normal to and bisected by an axial plane through or half way between the terminals of 

 the double spiral zero, as follows from considerations of symmetry and Maxwell's 

 relation between the current and the integral of the magnetic intensity in a closed path 

 around it. If this mean intensity in the horizontal direction were not zero, no error would 

 be introduced except a possible minute error due to a change in the angle between the 

 axis of the magnet and the surface of the mirror which it might bring about as might 

 also the variation of the component of the horizontal intensity of the Earth's field 

 parallel to the magnet's axis brought about by its motion. 



It was originally planned to wind one pair 

 of spirals left-handed and the other right- 

 handed, but both were wound in the same 

 direction on account of the increased facility 

 and precision with which this could be done. 

 In view of the remarks made in the last para- 

 graph there seems no advantage to be gained 

 by the more troublesome arrangement. 



9. Through the kindness of Messrs. S. 

 Klaber and Co. of New York City, two pieces 

 of Carrara marble were selected in Italy by a 

 representative of the firm and worked up there 

 into rough spools of the approximate dimen- 

 sions needed. When both had been turned in 

 the instrument shop sufficiently to make a 

 more satisfactory examination possible, it was 

 difficult to say that one was superior to the 

 other. Neither piece was entirely homogenous, 

 but both were free from pronounced veins, and 

 the black specks of iron pyrites, said often 

 to be present in Carrara marble, were almost 

 entirely missing. One of the pieces was chosen 

 and was carefully machined to nearly its final 

 form. It was then heated slowly in a gas fur- 

 nace to the approximate temperature of boiling 

 paraffin, 250 F., and was kept at this tem- 

 perature for forty hours. It was then inserted 

 into a mass of pure paraffin previously heated 



to the temperature 230 F., and the vessel containing both was then put in the furnace 

 and kept there for another forty hours at temperatures between 230 F. and 240 F. 

 The cylinder was then quickly removed from the paraffin and cooled slowly to room tem- 

 perature in the furnace over a period of about 30 hours. 



The large holes (67 mm. diameter), with axis vertical in the mounted instrument, 

 Figures A and B, provide for the insertion of the magnetometer, as well as for holding the 

 end standards used in the measurement of the diameters. The other and smaller holes 

 (20 mm. in diameter), with axes in the mean plane of the central cylinder, also serve the 

 latter purpose, the two in the central horizontal plane, Figures A and B, serving in addition 

 as sight holes. Any of the smaller holes may also be used to hold a thermometer. The 

 holes in the horizontal and vertical axial planes were drilled before the final turning of 

 the grooves, etc., the latter pair serving as guides in doing the machine work for the 

 precise location of the spool and coils. 



The spool was first carefully tinned mi the face-plate of the lathe to the approximate 

 dimensions indicated in Figure 12. Both the large holes, and the 20 mm. holes with axes 





D 



^8 



Fio. 12. 



A / B' 



-Horizontal Section of Spool. 



