334 



PHENOMENA, ATOMS, AND MOLECULES 



between the pairs of leads to each section was less than 50 ohms. Since the 

 greater part of each annular fold received no platinum in this evaporation 

 process, these folds now acted as insulation between the three electrodes, 

 Ci, Co and C2. In addition these folds could later be conveniently heated by 



(a) 



(b) (c) 



Fig. 6. Preparation of the experimental tube, (a) Outer glass mantle with platinum 



filament for coating walls, (b) Frame on which the filament structure is mounted and 



inserted in outer tube, (c) Frame turned through 90°. (d) The completed tube. 



a few turns of resistance wire to prevent electrical leaks which otherwise 

 often occurred at the higher caesium pressures. 



(2) Preparation of flic filament structure 



Since the filaments A (2 mil) and 5 (5 mil) must be accurately and 

 uniformly spaced, they were first mounted on a frame as shown in Fig. 6b. 

 This frame consisted of two pairs of glass seals (with tungsten leads) 

 separated by a rigid glass rod. Fig. 6c shows the frame turned through 90°. 

 The filament structure was adjusted and aligned entirely on this frame. 

 The total length of each filament was 30 cm while the length of the central 

 part in cylinder Co, was 3 cm. To obtain this length of 30 cm more con- 

 veniently, the ends of each filament were wound into tight spirals, (S), 

 the 2 mil filament being wound on a 3 mil mandrel and the 5 mil on a 5 mil 

 mandrel, leaving a straight length of 10 cm between the spirals. By trial 

 the tension springs (T) were selected and stretched so that when heated 

 the filaments remained taut and did not shift laterally. 



(3) Complete assembly 



Now the platinum coating filament was removed at Xi and the tube 



