499 
IMPARTP]!) TO A VACUUM BY HOT COAH)UCTORS. 
good conductr»r of electricity. The results show that in the case of an incandescent 
lamp, heated to the highest temperature it will stand, the specific conductivity of the 
surrounding space is comparable with that of the filament. 
In the case of a hot conductor the current across tlie intervening space to the 
electrode will, of course, only go in one direction. The current whefi the hot metal 
is charged positively, and the electrode put to eartli, is vanishingly small in comparison 
with the current wdien the wire is charged negatively. 
I he lemainder of the present paper is divided ipi as f( )llow's i— 
A. —llieoreiicfd [)ivesfigation. 
1. Calculation of the saturation current. 
II. Equilibrium of corpuscles near a hot plane of infinite area. 
B. — Experimental In rei^ligafion. 
I. Experiments with platinum. 
II- ,, carbon. 
Ill- ,, sodium. 
C. — Conclusion. 
A. —-Theoretical Investication. 
I. Cedcvlation of the Satuixttion Current 
♦ 
§ f. The apjilication of the kinetic theory of gases to the equilibrium of the free 
negative electrons or corpuscles inside a metal scarcely needs justification here, since 
it has already been made use of by Professor Drude.^' It may, howmver, he 
permissible to point out some results which show that the similarity lietween a 
coipuscle in a metal and a molecule in a gas under ordinary conditions is very close 
indeed. Professor IhomsonI has showm, from the change of resistance of bismuth in 
a magnetic field, that the mean free path of a corpuscle in that metal has the value 
10 ^ centim. ; while a series of experiments by Mr. Patterson| indicate that for 
lilatinum, gold, tin, silver, copjier, zinc, cadmium, mercury, and carbon the mean free 
path has values wdiich lie between 5-9 X 10~^ and 4T X centim. The mean 
free })ath for a nitrogen molecule in air under standard conditions is 10~^ centim. ; 
so that the mean free path of a corpuscle in bismuth is the same as that of a molecule 
111 air at j^fl^ an atmosphere pressure, whereas for other metals the mean free 
path IS the same as tliat in air at about 10 atmospheres pressure. The free time is, 
* ‘Dp.ude’s Aniialen,’ vol. I, p. .572, &c. 
f ‘Rapports preseiites an Cong-res International de Physique,’ Parts, 1900, vol. 0, p. l;ps. 
t ‘Phil. Mag.’ (6), vol. 3, p. 655. 
3 S 2 
