1891.] liquid electrodes in vacuum tubes. 233 



exhaustion proceeded. It seemed when longest to reach as far as 

 1 cm. above the cathode, becoming less distinct as it expanded. 

 At the lowest pressures reached, however, while the 4 or 5 mm. of 

 the tube immediately above the cathode were perhaps somewhat 

 darker than the average, the luminosity had certainly attained a 

 maximum within not more than 7 or 8 mm. of the Hg. surface. 

 The stage at which the Faraday space became indistinct depended 

 to some extent on the nature of the break in the induction coil. 

 With a rapid break and quiet spark the luminosity fell off and 

 the Faraday space remained longer distinct. 



Electrodes Hg. surface and Pt. wire. 



With electrodes so different in form the variations in the 

 phenomena cannot be entirely ascribed to difference of material. 

 According to Goldstein ' an alteration in the size of the anode 

 does not affect the striae in a stratified discharge, but a dimi- 

 nution in the size of the cathode while leaving unaltered the 

 length of the individual striae increases the distance of the head 

 of the positive discharge from the cathode. E. Wiedemann a 

 found that the relative facility of discharge between points and 

 between plates changes with the pressure. Thus at pressures 

 over 1 mm. he found the discharge to pass between points and 

 not between plates at the same distance apart, whereas at lower 

 pressures it passed most easily between the plates. Lehmann 3 

 lays down some apparently general laws as to the effects of 

 making blunter a cathode, but he does not always seem con- 

 sistent on this point, and I have some doubts as to how far he 

 bases his views on experiment. So many secondary influences 

 are at work, such as the size of the tube and the distance of 

 the electrodes, especially when comparable with their transverse 

 dimensions or with the length of Crookes' space, that one would 

 hardly expect a 'priori any simple general law to apply. 



At the lower pressures the platinum wire rapidly became 

 red hot and the deposit on the tube around it became very 

 thick, so that it was impossible to see anything of the Crookes' 

 space. If it did exist it must have been considerably less than 

 the Crookes' space in the Hg. tube when that was last seen 

 distinctly. The comparison instituted was thus between the 

 distances from the cathodes of the further extremity of the 

 Faraday space, i.e. the head of the positive discharge. The 

 following are some of the data obtained, the simultaneous lengths 

 of the Crookes' space at the Hg. cathode being given when 

 noted : 



i. 12, 1881, p. 275. 



i. 20, 1883, pp. 795—7. 



22, 1884, pp. 320—1. 



i Wied. Ann. 12, 1881, p. 275. 



- Wied. Ann. 20, 1883, pp. 795—7. 



3 Molekularplnjiik, Bel. n. pp. 277—8, and Wied. Ann. 2 



