384 Prof. C. T. Knipp and Mr. L. A. Welo on a 



aluminium with a small segment cut away. On the under 

 side of this disk a shallow box is built. One end of the box 

 is slightly undercut on the inside so that the photographic 

 plate can be wedged in, and a wedge-shaped clamp actuated 

 by a spring in the wall at the other end secures the plate in 

 position, not only from falling out but presses it up against 

 the disk, so that its position is never in doubt. The cover is 

 a thin aluminium plate, hinged at a side, bent over at the 

 edges to prevent light from entering edgewise, and is 

 backed by a spring so that it will stay closed. The lid 

 is opened by the winch shown at J 4 , which winds a string 

 hooked to the handle on the lid. The discharge-tube is 

 closed at the top by a piece of plate-glass, sealed on with 

 half-and-half (beeswax and resin) wax. 



The platinum strip on which the lime is placed is heated 

 electrically. To do away with troublesome winding of the 

 wires as the tube is turned, a hard rubber plate containing 

 two rings of mercury was arranged, immediately beneath 

 the brass pipe bearing. The rings are connected to the 

 terminals of a battery by binding-posts leading up under 

 the rings through the rubber. From the rings, wires lead 

 up the inside of the brass pipe but outside the exhaust stem, 

 through the brass pipe above the wooden frame, and thence 

 to sealed-in platinum terminals set in tubes perpendicular 

 to the plane of the drawing and indicated by the dotted 

 circle at D. Yfires on the inside of the cathode-mounting 

 lead to the cathode itself. One terminal of the heating 

 circuit leads to the negative end of a high potential battery, 

 while the positive end of the high potential battery is con- 

 nected to the sealed-in platinum wire below J l5 as has 

 already been stated. 



The coil of six turns, wound so close that the current may 

 be considered a circular one, is wound on a built-up and 

 accurately turned wooden core. It is pivoted on the hori- 

 zontal support, shown above the discharge-tube, so that it 

 hangs vertically, but is prevented from turning about a 

 vertical axis by an adjustable clamp (not shown) on the 

 support. A pointer on the coil which moves over a gradu- 

 ated circle on the support gives the orientation. A pointer 

 on the brass supporting-pipe and a graduated circle on the 

 wooden frame gives, it might be added, the orientation of 

 the tube itself. When it is desired to remove the cover from 

 the discharge-tube, the coil is lifted off and the support 

 swung back about the pivot P. 



When making a determination, the pump is generally run 

 for over an hour, the charcoal bulb being gently heated during 

 the while to drive off gas absorbed at ordinary temperatures. 



