Sine Galvanometer 



379 



horizontal, were then located precisely and cut out on a Brown and Sharpe universal 

 milling machine. The spool was then given the thermal treatment with paraffin already 

 described, after which the remainder of the machine work was done. 



For the final turning of the cylinder B, in which the grooves were to be cut, and the 

 cylinder A, by which the spool was to be mounted on the turn table, the spool was 

 mounted on a specially constructed mandrel, a diagram of which is given in Figure 13. 



Fia. 13. Horizontal Section of Mandrel used in Precise Turning of Spool. 



The cylinders A and B were turned with a fragment of diamond which happened to be 

 on hand. An attempt was made to do all but the final part of the work of cutting the 

 spiral grooves with a tool of hardened steel ; but the marble threads broke out so badly, 

 in spite of great care, that this work had to be done over again with a special diamond 

 tool. This diamond tool, obtained for the final work on the grooves, had an angle of 84, 

 and has given excellent satisfaction. 



To turn the groove for one of the coils, the spool on the mandrel was so adjusted 

 in the lathe that the spiral would have the exact relation to the spool called for by the 

 design, and the groove cut to about one-half its final depth. Then the groove of the same 

 spiral near the other end of the spool was cut to about half its final depth. The two 

 grooves, part of the same spiral with 2 mm. pitch, were then gradually cut down almost 

 to their final depth. Then the mandrel, with the dog and the spool, was turned through 

 exactly 180 with respect to the face-plate, and the grooves for the second spiral were cut 

 in the same manner almost to their final depth. All the spirals are right-handed. 



The last cut of the two grooves forming each spiral was taken with the same setting 

 of the tool and with the screw in continuous operation. For the second spiral, the dog 

 and mandrel had, of course, to be turned, as above, through 180, and the tool had to 

 be reset. 



When the grooves, which extended over somewhat larger areas than were to be 

 occupied by the wires, had been cut, the spool was mounted on a Brown and Sharpe 

 universal milling machine and the eight radial cylindrical holes opposite EE, Figure 14, 

 drilled precisely at the centers of the proper grooves in the axial plane which was to 

 be horizontal in the completed instrument. The holes were drilled with a diameter 

 1.17 mm. considerably larger than that of the wire, as the use of a drill with the small 

 diameter of the wire would have endangered the precision and would have introduced 

 the serious risk of the drill's being broken off inside the marble. The inner half of each 

 hole was then counterbored and threaded to receive a threaded brass plug 2.38 mm. in 



