54 SCIENCE PROGRESS 



reservoir and just behind this the very fine capillary tube which 

 allows the gas to leak slowly into the discharge bulb shown 

 on the right of the large Du Bois electromagnet. In a cor- 

 responding position on the left of the latter is the "camera" 

 made of glass tube partially covered with paper ; this contains 

 the plate-holder and supports at the top the glass "winch" by 

 which the plate is raised or lowered. Behind the magnet may 

 be seen the Gaede pump and the induction coil. Attached 

 to the camera is the large Dewar charcoal bulb, which is 

 cooled by immersion in the vessel of liquid air ; the latter stands 

 on the table, together with an accurate ammeter for measur- 

 ing the current flowing through the magnet and a red 

 photographic lamp for use during the removal of the plate 

 when the exposure is ended. 



The endeavour may now be made to explain, as briefly 



Fig. 3. 



and simply as possible, how by subjecting the moving charged 

 particle to an electric and a magnetic field, each at right 

 angles to its path, both the velocity and the mass of the 

 particle may be deduced. 



Let A (fig. 3) be such a particle of mass m t carrying a posi- 

 tive charge of electricity e and moving with velocity v in the 

 direction A B. If this particle be not influenced by electric or 

 magnetic forces, it will obey the ordinary laws of motion and 

 move in a straight line, striking a distant screen at a point B. 

 If, however, we cause it to pass through an electric field of 

 strength X between the plates P P, it will be deflected away 

 from the positive and towards the negative plate in the plane 

 of the paper and finally strike the screen at some other point C, 

 the displacement B C = x being given by the equation : 



x = k, — J 



If now the electric field be cut off and P P' made the poles 

 of a magnet of field strength H, the moving particle will be 



