Ions produced by Rontgen Rays. 71 



was sometimes necessary to interrupt a current of 25 

 amperes at the key S. Although a large adjustable con- 

 denser was used for suppressing the spark, considerable 

 difficulty was experienced at first in getting uniform con- 

 ditions in the induction-coil at the instant when the break 

 occurred in the primary circuit. This difficulty was over- 

 come 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 mode- 

 rate 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 case 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 bearing 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 II (referring again to fig. 1) is fastened by 

 a catch, Z, at a position of maximum displacement from the 

 vertical. The keys are then all closed, bringing the elec- 

 trodes A and B to potential zero and closing the primary of 

 the induction-coil which operates the X-rays 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 re- 

 combine 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 electrodes to the 

 plate B. The earthing key is now opened and the key D 



