322 Mr Aston, Distribution of intensity 



Behaviour during change of pressure. 



The pressure in a freshly set up bulb always increases with 

 running owing to the liberation of gas by heat etc. so that the 

 changes due to gradual alteration of pressure can be observed 

 most conveniently by exhausting highly, starting the coil and 

 watching the events on the screen. Thus using a concave cathode 

 of about 8 cms. radius of curvature set just in the neck of the 

 discharge bulb the following sequence of events was observed. 

 At very low pressures with a potential of about 50,000 volts the 

 parabolas are very faint but correspond to the general type, the 

 primary streak a^ and spot h-^ being much brighter than their 

 satellites (doubtless due to few collisions). As the pressure rises 

 the discharge becomes curiously unsteady the spots on the screen 

 become much fainter and change with flickering into the pure 

 atomic type (Fig, 1), 6i having practically disappeared. This form 

 of discharge which is evidently abnormal lasts for a certain time 

 depending on the rate of increase of pressure. Then with absolute 

 suddenness h-^ flashes out intensely bright and with it appear at 

 the same instant its satellites a^ and a^. At the same time the 

 current through the bulb increases, the discharge settles down and 

 the negative glow makes its appearance. As far as it was possible 

 to judge the satellites a^, and a^ are of equal brightness and generally 

 much brighter than the negative atomic satellite «!. 



The appearance of the discharge bulb while the pure atomic 

 type is shown on the screen is difficult to describe but quite 

 characteristic and different from the general. Near its critical 

 upper limit of pressure it was found possible to effect the change 

 to the general type by bringing a magnet near the cathode and 

 so disturbing the discharge. On removing the magnet the discharge 

 at once reverted to the atomic type. This form of controlled 

 change from the one to the other gave an excellent opportunity 

 of testing the invariable association between the primary spots 

 and their appropriate satellites. 



Possible cause of disappearance of primary molecular rays. 



It is unlikely that change of pressure is itself the determining 

 factor in the disappearance of the molecular type. This seems to 

 be due to some disturbance in the discharge by the cathode stream 

 (not caused by the diffuse one given by a convex cathode) which 

 makes the formation of the negative glow impossible. 



The facts so far may be brought into line fairly well by the 

 somewhat speculative assumption that molecular rays can only 

 originate freely in parts of the discharge where the electric force 

 is very small, e.g. the negative glow, ionisation by more violent 



