578 



Professor Sir J. J. Thomson 



[April 2, 



with the Wehnelt cathode made this method of investigation particu- 

 larly suited for these investigations. The first method used to mea- 

 sure the variations in the electric force along the discbarge was to find 

 the variation in the difi^erence of potential between two platinum wires 

 1 mm. apart as the wires were moved from the cathode to the anode. 

 Several devices were used for this purpose : in some the platinum 

 wires (surrounded up to about a millimetre from their tips with glass 

 rods) were carried on a sort of railroad and moved from cathode to 

 anode. The electrodes in the discharge- tube in this case were fixed. 

 The measurements of the potential-differences made by this method 

 at low pressures gave the very remarkable result that just on the 

 cathode side of the bright part of a striation the electric force was 

 negative (i.e. that the force on a positive charge was in the direction 

 from cathode to anode) : on crossing over the bright boundary to the 

 anode side the electric force at once became positive, and rose to 



Fig. .1 



Fig. 



M-Xi^ 



2,.. Bright fiirt ofolruUoon. 



a high value. It soon, however, began to diminish, and went on 

 diminishing up to the cathode side of the bright part of the next 

 striation on the anode side. The distribution of the electric force in 

 the striation is represented in Fig. 1, and the corresponding distribu- 

 tion of positive and negative electricity in Fig. 2, the ordinates repre- 

 senting the distribution of the electrification both as to magnitude 

 and sign. Thus if these measurements of the electric field can be 

 relied on we have intense negative electrification at the bright head 

 of a striation (by head is meant the side next the cathode), and a weak 

 positive electrification through the rest of the field. The transition 

 from positive to negative force was very abrupt and well marked, so 

 much so indeed that the position of the platinum wires in the stria- 

 tion could be ascertained with great accuracy without looking at the 

 discharge, by observing the deflexions of the electrometer by which 

 the potential-difference between the wires was measured. 



