190 Mr. T. Harris on Distribution of Electric Force 



the results were similar. Thus the effects noticed do not 

 depend upon the position of the anode when it is beyond a 

 certain distance from the cathode nor upon the presence of 

 mercury vapour. 



Conclusion. 



In all cases there was a rapid increase of potential fall 

 across the discharge when the negative glow became faint 

 and the charges on the walls began to alter the character of 

 the discharge. With the electrodes far enough apart, the 

 length of the field corresponding to the dark space at first 

 increased rapidly with the decrease in pressure, but as soon 

 as the negative glow had faded from view and the visible 

 discharge became concentrated into the centre of the cathode, 

 the length of this field became very nearly independent of 

 the pressure. The length of the field at this stage depends 

 upon the diameter of the tube, being longer for wider tubes. 

 Throughout the rest of the discharge the force was too small 

 to be detectable, i. e. under a few volts per cm. 



With the electrodes close enough for the anode to con- 

 strict the dark space, the length of the field for the range of 

 pressures used was practically constant, and there was a 

 small region in front of the anode where the force was 

 extremely small. 



In the one case where the pressure was high enough for 

 the negative glow to have a well-defined boundary, the 

 curve appears approximately a straight line. Applying the 

 equation 



where X = the electric force along the axis, 

 % = distnnce along the axis, 



p = the density of the excess of positive over negative 

 electricity ; 



it is seen that in the dark space the positive electricity is in 

 excess, and that by a practically constant amount throughout 

 the space, which was shown more accurately by Aston. The 

 same holds roughly in the other cases as the cathode is 

 approached, but towards the anode end of the field not 

 much information can be obtained owing to the probable 

 distortion of the equipotential surfaces. Throughout the 

 rest of the discharge the positive and negative electricity are 

 present in equal quantities. 



The distortion of the equipotential surfaces does not affect 

 the measurements of the length of the fields of force, for the 



