54 



NA TURE 



[November 12, 1908 



a relation from which the magnetic force is eliminated. 

 To ensure that the tangent to the wire is horizontal when 

 s = o, the following method is used. P is a chisel-edge 

 carried by a screw and placed about i mm. in front of 

 the fixed end of the wire ; this is adjusted so that when 

 the magnetic field is not on, the wire just touches the 

 edge : this can be ascertained by making the contact with 

 the wire complete an electric circuit in which a bell is 

 placed. When the magnetic field is put on the w'ire is 

 pulled off from the edge, and the tangent at s = o is no 

 longer horizontal ; it can, however, be brought horizontal 

 by raising or lowering the pulley D until the wire is again 

 in contact with P, which can be ascertained again by the 

 ringing of the bell. Then y, is the vertical distance 

 between the point where the wire now crosses the edge of 

 the scale and the point where it crossed it before the mag- 

 netic field was put on. Since y, y,, «', and T can easily 

 be measured, equation (3) gives us the value of e!mv, while 

 the deflection under the electric force gives the value of 

 e/niv,. 



If y is the vertical displacement of the patch of phos- 

 phorescent light on the screen produced by the magnetic 

 field, X the horizontal displacement due to the electrostatic 

 field, we see that 



where A and B are constants depending on the position of 

 the screen and the magnitudes of the electric and magnetic 

 forces. These quantities can be calculated by means of the 

 equations just given. 

 Since 



-'' = lz. 

 .r A ' 



X A e' 



We see that if the pencil is made up of rays having a 

 constant velocity, but having all values of e/m up to a 

 maximum value, the spot of light will be spread out by 

 the magnetic and electric fields into a straight line extend- 

 ing a finite distance from the origin. While if it is made 

 up of two sets of rays, one having the velocity v, the other 

 the velocity "',, the spot will be drawn out into two straight 

 lines as in Fig. 4. 



If efm is constant and the velocities have all values up 

 to a maximum, the spot of light will be spread out into 

 a portion of a parabola as indicated in Fig. 5. 



We shall later on give examples of each of these cases. 



The discharge was produced by means of a large induc- 

 tion coil, giving a spark of about 50 cm. in air, with a 

 vibrating make and break apparatus. Many tubes were 

 used in the course of the investigation ; the dimensions of 

 these varied slightly. The distance of the screen from the 

 hole from which the rays emerged was about q cm., the 

 length of the parallel plates about 3 cm., and the distance 

 between them 0-3 cm. 



Propt'iiics of the Positive Kays mlien the Pressure is not 

 exceedingly low. 

 The appearance of the phosphorescent patch after deflec- 

 tion in the electric and magnetic fields depends greatly 

 upon the pressure of the gas. I will begin by considering 

 the case when the pressure is comparatively high, say of 

 the order of 1/50 mm. At these pressures, though the 

 walls of the tube in front of the kathode were covered 

 with bright phosphorescence and the dark space extended 



right up to the walls of the tube, and was several centi- 

 metres thick, traces of the positive column could be de- 

 tected in the neighbourhood of the anode. I will first take 

 the case where the tube was filled with air. Special pre- 

 cautions were taken to free the air from hydrogen ; it was 

 carefully dried, and a subsidiary discharge-tube, having a 

 kathode made of the liquid alloy of sodium and potassium, 

 was fused on to the main tube. When the discharge 

 passes from such a kathode it absorbs hydrogen. The 

 discharge was sent through this tube at the lowest pressure 

 at which enough light was produced in the gas to give 

 a visible spectrum, until the hydrogen lines disappeared 

 and the only lines visible were those of nitrogen and 

 mercury vapour. This pressure was a little higher than 

 that used for the investigation of the positive rays, but a 

 pump or two was sufficient to bring the pressure down to 

 this value. The appearance of the phosphorescence on the 

 screen when the rays were deflected by magnetic and 

 electric forces separately and conjointly is shown in Fig. b. 



The deflection under magnetic force alone is indicated 

 by vertical shading, under electric force alone by horizontal 

 shading, and under the two combined by cross shading. 



The spot of phosphorescence is drawn out into a band 

 on either side of its original position. The upper portion, 

 which is very much the brighter, is deflected in the direc- 

 tion which indicates that the phosphorescence is produced 

 by rays having a positive charge ; the lower portion (in- 

 dicated by dots in the figure), which though faint is quite 

 perceptible on the willemite screen, is deflected as if the 

 rays carried a negative charge. The length of the lower 

 portion is somewhat shorter than that of the upper one, 

 but is quite comparable with it. The intensity of the 

 luminosity in the upper portion is at these pressures quite 

 continuous ; no abrupt variations such as would show 

 themselves as bright patches could be detected, although, 

 as will be seen later on, these make their appearance at 

 lower pressures. Considering for the present the upper 

 portion, the straightness of the edges shows that the 

 velocity of the rays is approximately constant, 

 while the values of e/m range from zero at 

 the undeflected portion to the value approxi- 

 mately equal to lo* at the top of the deflected 

 band. This value of e'm is equal to that for 

 a charged hydrogen atom, and, moreover, 

 there was no specially great luminosity in the 

 positions corresponding to c/m = io','i4 and 

 lo'/iG, the values for rays carried by nitrogen 

 or oxygen atoms, though these places were 

 carefully scrutinised. As hydrogen when pre- 

 sent as an impurity in the tube has a ten- ;'. .. 

 dency to accumulate near the kathode, the ';■! 

 ^following experiment was tried to see whether ■,■. 

 the Kanalstrahlen were produced from traces v^.'j 

 of hydrogen in the tube. The discharge wvns 

 sent through the tube in the opposite direc- p^^ ^^ 

 tion, i.e. so that the perforated electrode was 

 the anode, the electric and magnetic fields 

 being kept on. When the discharge passed in this way 

 there was, of course, no luminosity on the screen ; on 

 reversing the coil again, so that the perforated electrode 

 was the kathode, the luminosity flashed out instantly, pre- 

 senting exactly the same appearance as it had done when 

 the tube had been running for some time with the per- 

 forated electrode as kathode. 



The fact that a spot of light produced by the undeflected 

 positive rays is under the action of electric and magnetic 

 forces drawn out into a continuous band was observed by 

 W. Wien, who was the first to measure the deflection of 

 the positive rays under electric and magnetic forces. The 

 values of e/m obtained from the deflections of various parts 

 of this band range continuously from zero, the value corre- 

 sponding to the undeflected portion, to lo', the v.Mue 

 corresponding to those most deflected. Wien explained 

 this by the hypothesis that the charged particles which 

 make up the positive rays act as nuclei, round which 

 molecules of the gas through w'hich the rays pass condense, 

 so that very complex systems made up of a very large 

 number of niolecules get mixed up with the p.-irticles form- 

 ing the positive rays, and that it is these heavy and 

 cumbrous systems which give rise to that part of the 

 luminosity which is only slightly deflected. I think that 



NO. 20.^7, A'Ol.. 



