illustrated by the Behaviour of Electric Vacuum Valves. 3 



the passage of the current is rendered impossible, unless 

 some supplementary — some tortuous or indirect route is pro- 

 vided, whereby a stream o£ positive carriers can reach the 

 cathode after travelling through a region in which the 

 bombardment is less vigorous. 



The general direction of the whole bombardment depends 

 on the shape and aspect of the cathode. From all flat sur- 

 faces it must be intense and in nearly parallel lines ; from a 

 concave surface it is well known to concentrate to a focus ; 

 from a convex surface it is divergent, and therefore feebler ; 

 and from a very convex surface, like an edge or still more 

 a point, it is distributed in so many directions that part of 

 the opposite procession of heavy atoms has a chance of 

 eluding the projectiles. 



Consequently to any edge or point of the cathode positive 

 charges may be seen streaming in; evidence of the stream 

 beino- a local manifestation of the colour due to the chemical 

 nature of the gas, owing to a certain percentage of the atoms 

 being struck as they come along in the teeth of the diffuse 

 bombardment. It would appear that uncharged atoms, even 

 if struck, do not glow perceptibly. Perhaps because they 

 are all driven out of the field — the region of the dark space — • 

 at once. 



If the cathode contains a hollow space into which the 

 positive carriers can find their way, this space becomes filled 

 with the chemical glow, — i. e. the glow characteristic of the 

 chemical nature of the gas, — which is presumably caused by 

 radiation from its positively charged atoms when they are 

 struck by the flying electrons. 



If the cathode is a long cylinder or wire, the lateral 

 radiation is sufficiently diffuse to allow some obstructed access 

 to its curved surface ; and if the wire is coiled up into a 

 spiral, access is much easier, for then the space in the interior 

 of the spiral becomes practically a hollow space into which 

 some of the positive carriers contrive to enter and rest till 

 they have lost their charges. 



But if the cathode is made spherical and quite smooth, it 

 must be extremely difficult for the positive carriers to find 

 any means of access. Still more difficult will it be if the 

 cathode is a small flat surface at the mouth of a narrow tube 

 so arranged as to furnish the only means of approach. 



A cathode shaped like a tetrahedron or triangular pyramid 

 with its edges not completely closed (made by folding over 

 the corners of an equilateral triangle and leaving millimetre 

 chinks) is an instructive shape. To each point of the 

 pyramid the positive stream is seen going, because thence 



B 2 



