4:66 Hutcliins and Robinson — Concerning Or voices Tubes. 



strokes it is observed that no more air is obtained. The purnp 

 contains air, however, condensed upon the glass walls. To 

 remove this A is put in position 2, and the mercury raised 

 until a drop passes B. B is then shut and the mercury 

 dropped until only a drop remains above A. A is then shut 

 and the movable mercury tank dropped to its lowest point, 

 when A is put in position 1. Pumping now goes on as before 

 only with B shut, and the tank is raised only a third as high 

 as before. After four or live strokes it is well to pass the 

 mercury again above B. 



If the highest possible degree of exhaustion is desired this 

 process can be repeated between B and C, but this is not neces- 

 sary in exhausting a Crookes tube. 



As soon as the stage of pumping with B shut is reached, the 

 tube which is being exhausted must be strongly heated, mov- 

 ing the lamp flame over every part of it, and after two or three 

 strokes more the current from the coil is turned into the tube. 

 By the combined action of the heat and current the occluded 

 air is driven from the glass and exhaustion proceeds rapidly. 

 It should not occupy over twenty or thirty minutes for a mod- 

 erate sized tube. 



Allowing the tube to cool, if short sparks can be drawn from 

 the bulb and there is little or nothing to be seen in it except 

 green light, the exhaustion is complete. There is danger of 

 carrying it too far, for the vacuum very much increases during 

 the first hour that the tube is used ; but of these matters a 

 little experience is the best teacher. 



Searles Science Laboratory, Bowdoin College, May 6, 1896. 



