678 BELL SYSTEM TECHNICAL JOURNAL 



field, perpendicular to the plane of the drawing, which causes them to 

 revolve so. This field limits the speeds of the electrons in the beam to 

 a certain range, which may be narrower than the range over which 

 the speeds of the electrons emerging from Z are spread. ^^ It has a 

 second purpose, which we shall see directly. The electrons are col- 

 lected in either of the chambers Ai and A<i,, alternate use of which ena- 

 bles the observer to vary the distance x figuring in equation (8). 

 Mostly, however, it is the density of the gas which is varied. 



If there is gas in the box, electrons which by striking molecules are 

 deflected even a little go to one of the partitions, and vanish from the 

 beam. So also do electrons which strike molecules and stick to them; 

 even were one to adhere to a particle of molecular size which happened 

 to be moving in the direction tangent to the beam, the resulting massive 

 ion would travel in a path with a different curvature, and fall against a 

 wall. Again, if an electron should suffer a loss of kinetic energy with- 

 out losing its freedom or its direction of motion, it too would be elimin- 

 ated from the beam; its path would be more curved after the impact 

 than before, and it would miss the slits. Again, if by an ionizing im- 

 pact a fresh electron should be ejected from a molecule, it would be 

 moving more slowly than those of the primary beam, and could not 

 stay with them. Finally, if an electron were bounced out of the beam 

 by an elastic impact with a molecule, it could not be bounced back in 

 in again by fewer than two additional collisions, and these it would 

 be most unlikely to make. Clearly, the quantity o- of which this ap- 

 paratus gives the value is very stringently defined ! 



Suppose now the magnetic field omitted, and the slits and chambers 

 of the device of Fig. 2 all arranged along one straight line. This gives 

 another well-known type of apparatus. The first of this kind was the 

 one which Mayer used, but I choose for representing here the one 

 designed by T. J. Jones (Fig. 3). The electron-beam, consisting of elec- 

 trons which emerge from the filament F and are accelerated by a rise 

 of potential between F and the screen A, is shaped by the sequence of 

 slits which is shown in the sketch, and enters the chamber B through 



12 Ordinarily the magnetic field is so adjusted, that the range of electron-speeds 

 aforesaid is centred at the value which is the most probable speed of the electrons. 

 The corpuscles which have come out of the source and have been accelerated to the 

 first slit have a certain distribution-in-speed with a maximum at some special value, 

 say iia; the range selected by the magnetic field has Uo as its center. However Ram- 

 sauer and Kollath found that they could not do this with electrons having less energy 

 than 0.45 equivalent volt; they then adjusted the field so as to select ranges of speed 

 lower than that which included the maximum, and were able to extend their experi- 

 ments as far down as to electrons of 0.16 equivalent volt. The use of the magnetic 

 field has the incidental advantage, that the values of electron-speed deduced from its 

 value and from the radii of the circular orbits are correct even if there are contact- 

 potential differences in the box — something which cannot be said for those deduced 

 from the voltage applied between box and filament. 



