52 SCIENCE PROGRESS 



the influence of the enormous electric field, they attain corre- 

 spondingly high velocities and those which fall axially upon 

 the cathode pass through the narrow " canal-ray " tube and 

 emerge as a fine beam of "canal-rays." 



The charged particles travelling in a definite direction, at a 

 high velocity, are subjected to the analysing influence of electric 

 and magnetic forces by causing the beam to pass between the 

 pieces of soft iron P P' which are placed between the poles 

 MM' of a powerful electromagnet. P and P' constitute the pole 

 pieces of the magnet but are electrically insulated from it by thin 

 sheets of mica TV A 7 ' and so can be raised to any desired electrical 

 potential difference by means of the leads shown in the figure. 

 As the rays pass between the faces of PP', they are subjected 

 to the influence of electric and magnetic forces simultaneously 

 and after they have been analysed, in a manner to be described 

 later on, they enter the " camera " G and finally impinge upon 

 the fluorescent screen or photographic plate H. In order that 

 the stray magnetic field may not interfere with the main discharge 

 in A } shields of soft iron, //, are interposed between the magnet 

 and the bulb. 



Fluorescent screens made of powdered Willemite were used 

 in all the earlier experiments but as these only show the impact 

 of the rays very faintly in a dark room and give no permanent 

 record, they are unsuitable for the purpose of accurate measure- 

 ments ; a notable advance in technique was made by the use 

 of photographic plates. When exposed to a beam of positive 

 rays, the surface of such a plate undergoes a chemical change of 

 a nature somewhat similar to that caused by actinic light and 

 may be developed in the ordinary way, a more or less intense 

 deposit of silver being formed wherever it has been struck by 

 the rays. The plates which have been found to give the best 

 results are the well-known Sovereign brand made by the Im- 

 perial Plate Co. The most convenient way of exposing the 

 plate is to use a device which the writer has used previously in 

 other experiments requiring accurate movement of an object in 

 a high vacuum. It is roughly indicated in the accompanying 

 figure (fig. 2), which shows the complete camera. The photo- 

 graphic plate is placed in a light frame supported by a silk 

 thread ; the frame can be wound up and down by means of a 

 winch the axle of which works in an air-tight, ground joint. 

 While the pressure, etc., is being adjusted, the plate is kept at 



