186 Mr. T. Harris on Distribution of Electric Force 



of gas and voltage across the discharge were noted, and the 

 length of the equivalent spark-gap found for the potential 

 across the secondary tube. The position of the electrodes of 

 the main discharge was found from a scale fixed to the tube. 

 These electrodes were raised until tlte ben m from the secondary 

 tube passed close to the anode, and the reading on a scale on 

 the willemite screen was taken. The electrodes were lowered 

 cm. by cm. until the beam of rays passed as close to the 

 cathode as its velocity permitted, the spot of phosphorescence 

 being at the extreme end of the scale. 

 The effects of varying 



(1) the gas pressure, 



(2) the distances between the electrodes for aluminium 



and nickel cathode, 



(3) diameter of the discharge-tubes, 

 were determined. 



Results. 



The results are shown in the curves, in which electric 

 force is plotted against distance from electrode. Fig. 4 is for 

 a tube 2*7 cm. in diameter with aluminium electrodes 12 cm. 

 apart. Starting with a fairly high pressure when the con- 

 ditions are such that the discharge consists simply of a well- 

 defined dark space with negative glow extending right up 

 to the anode, the curve A is obtained. The point q is the 

 edge of the dark space; right through the negative glow 

 the force was too small to measure. Aston found, under 

 similar circumstances, a linear curve with a discontinuity at 

 the edge of the dark space. Here there is no discontinuity, 

 but this may be explained by the fact that the boundary of 

 the dark space was slightly concave to the cathode, instead 

 of a plane parallel. 



As the pressure is lowered the appearance of the discharge 

 varies rapidly. The dark space begins to extend towards 

 the anode, and the negative glow gradually becomes fainter 

 until the boundary between it and the dark space becomes 

 practically indefinable. The discharge at the cathode takes 

 on the appearance of a broad beam no longer filling the whole 

 tube, but tending to concentrate in the central portion of ihe 

 anode. Green phosphorescence begins to appear upon the 

 walls of the tube about the anode. This state of the tube is 

 represented by curve B. If the pressure is still decreased the 

 negative glow fades completely away, the broad band at the 



