at the Electrodes in Vacuum-Tube Discharge. 617 



Apparatus. 



A diagram of the discharge-tube (diam. 4 cm.) is given: in 

 fig. 1. In order to force the discharge be- 

 tween the opposing faces of the circular disk Fi#- 1- 

 electrodes (diam. 18 mm.), each was inlaid in 

 guttapercha with its stem sheathed in glass 

 tubing. One electrode, as also the exploring 

 wire used to obtain the potential of the 

 gas, was movable along the axis of the 

 discharge-tube, each being carried by an air- 

 tight IJ-tube passing in from the outside 

 through a barometer column. Through these 

 U-tubes the necessary electrical connexions 

 were carried. This arrangement allowed the 

 difference of potential between the electrodes 

 and any position in the gas along the axis of 

 the tube to be measured. The discharge- 

 tube was jointed in the middle, thus allowing 

 the electrodes to be changed and the explor- 

 ing wire to be adjusted. 



Potentials were measured by means of a 

 Thomson quadrant electrometer — those of the- 

 magnitude of the cathode-drop being mea- 

 sured by charging the quadrants by a standard 

 cell and the needle with the potential sought. 

 The current was furnished by a battery of 

 600 accumulators, regulated by a resistance 

 consisting of a solution of cadmium iodide in 

 amylic alcohol, and measured by a Weston 

 millimetre (scale-divisions in tenths). Ob- 

 servations were confined to atmospheric 

 nitrogen, purified and dried by the usual means. This 

 was stored in an absolutely air-tight drying-chamber and 

 drawn at will. The pressure of the gas was observed on a 

 mercury-manometer by means of a cathetometer. 



Variation in the Drop at the Electrodes with Distance Apart. 



An effect, at first thought surprising, is obtained when the 

 drop at the anode is observed at various distances from the 

 cathode. As the anode is made to approach the cathode it 

 graduall}- consumes the positive column, while the dimensions 

 of the negative glow and the Faradny dark space remain 

 constant. Proceeding through the Faraday dark space there 

 is simply a thick luminous disk on the anode ; the appearance 

 is then that of the so-called dark discharge. Arriving in the 



