44 S. J. Plimpton — Recombination of Ion's 



coil at the instant when the break occurred in the primary 

 circuit. This difficulty was overcome by the use of a specially 

 constructed key for breaking the large primary current. 



In designing keys for purposes of this sort, attention has 

 usually been centered on getting a rapid motion at the instant 

 of break. It was found possible to use very moderate motion 

 provided a firm uniform pressure was maintained between the 

 contacts up to the instant of breaking and separation of the 

 contacts then effected without vibration or chattering. The 

 most successful break was constructed as follows : 



A rigid bar armature of T section was mounted in conical 

 bearings at one end and carried a platinum contact at the 

 other. The complementary platinum contact was mounted 

 rigidly on the same heavy base with the bearing. The key 

 was closed firmly by a permanent horse-shoe magnet. The 

 bearings were kept in such adjustment that the magnet could 

 just overcome the friction at a particular position. The bear- 

 ing screws were locked in place by another pair of set screws. 

 A looseness in these bearings, which was not sensible to the 

 touch, would vary the electrometer readings from five to ten 

 per cent. 



When this key was used with a current strong enough to 

 properly excite the bulb, the flashes of X-rays produced almost 

 perfectly uniform readings of the electrometer. 



The apparatus as a whole is operated in the following 

 manner : 



The pendulum "R, referring again to fig. 1, is fastened by a 

 catch Z at a position of maximum displacement from the ver- 

 tical. The keys are then all closed, bringing the electrodes A 

 and B to potential zero and closing the primary of the induc- 

 tion coil which operates the X-ray tube. The key D is then 

 opened and the pendulum quickly released from the catch Z. 

 The pendulum automatically opens the key S, giving rise to a 

 flash of rays which ionizes the gas between the electrodes in 

 the chamber K. Since both electrodes are at zero potential, 

 the ions continue to recombine until the pendulum has opened 

 the key F, when a potential is applied to the electrode A which 

 drives over the ions of one sign remaining between the elec- 

 trodes to the plate B. The earthing key is now opened and 

 the key D closed so that the charge received on B may be 

 measured by the electrometer. 



. By taking a series of readings in this way with the key F at 

 different positions along the path of the pendulum, the rela- 

 tive numbers of ions remaining in the gas after definite inter- 

 vals of time was obtained directly. Throughout the experi- 

 ment, readings were frequently repeated with the key F in 

 such a position that the field was applied before the flash 



