PHYSICS AND PHILOSOPHY 317 



but that coil must have lowered the efficiency 

 of the shop by several per cent. The wire for 

 the secondary coil was about as small as a hair 

 and insulated with fine silk thread. To wind it 

 with perfect smoothness on the coil cylinder I 

 fixed the latter between the centers of a screw- 

 cutting lathe and fed to it the wire through a 

 guide attached to the tool post, having adjusted 

 the lathe as if to cut one hundred threads to the 

 inch. After starting the lathe my hand never 

 left the pulley shifter nor my eye the wire until 

 a full layer had been wound, excepting when the 

 overriding of the wire compelled me to turn 

 back. The insulated wire was then as even as the 

 thread on a spool of silk and after being painted 

 with a rubber solution, wound with gutta percha 

 tissue and coated with shellac the feed was re- 

 versed and the winding of the second layer pro- 

 ceeded, this time from left to right if the first 

 had been the other wav. 



But after each winding and before its insula- 

 tion it was imperative to test the conductivity of 

 the coil since a single break in the wire, which 



