204 
MR. J. ZELENY ON THE VELOCITY OF THE IONS 
strength of rays used. For the much weaker radiation and the greater distance here 
used the correction does not perhaps exceed 1 per cent. For gases other than air 
the effect has not been determined, and has been assumed to be no greater than with 
air. 
10. J. Perrin* has shown that when the Ilontgen rays impinge upon a metal 
surface the ionization in the gas near it is increased by an amount depending upon 
the nature of the metal and upon the state of its surface. M. G. SagnacI and 
P. Langevin have shown since that this is due to a secondary radiation started at 
the metal surface by the Rontgen rays. It is possible that the ions so produced are 
of a different nature from those produced by the direct rays, but in the absence of 
any evidence to that effect the much more probable case is assumed that the two 
kinds are identical. 
The effect of the secondary rays, therefore, is to produce an uneven distribution of 
the ions in the space exposed to the direct rays, and also to widen the ionized area 
near the metal surfaces. This makes more difficult the accurate determination of the 
potential A in equation (9), the tendency being to get it too large. J. Perrin found 
that the surface effect was by far the least for aluminium, what he calls the coefficient 
being ‘0 for aluminium in air as compared to '9 for gold in air. The effect is also 
very much dependent upon the cleanliness of the surface. It is thus seen that in the 
apparatus used this effect was made as small as possible by using unpolished 
aluminium as the material for those parts of the cylinders upon which the rays 
impinged. That the secondary rays did not produce an appreciable amount of 
ionization at a short distance to the side of the beam of the direct rays was shown 
by passing these rays near to the insulated juncture in the inner cylinder while the 
gas in the tubes was at rest. No conductivity was observed to that part of the inner 
cylinder which was not exposed to the direct rays. 
Further experiments tried for the effect of the secondary rays by coating the inside 
of the aluminium cylinder on the apparatus with tin-foil will be described later among 
the observations for dry air. 
11. W. C. PbxTGExj has shown that the air itself where it is exposed to the rays 
acts as a source of a weak secondary radiation. The writer is not aware of any 
experiments showing any conductivity produced by this radiation, but the experiment 
referred to in the last section, where a beam of rays near the juncture of the inner 
cylinder produced no appreciable conductivity on the other side, shows that in these 
experiments the effect may be disregarded. 
12. When D (fig. 3), the part of the inner cylinder joined to the electrometer, 
takes up a charge in the progress of an observation, the electric field in the vicinity 
of the juncture becomes slightly distorted, tending to lessen the number of ions 
* J. Perrin, ‘Comptes Rendus,’ vol. 124, p. 455. 
t M. G. Sagnac, ‘Journal de Physique,’ 1899, p. 65. 
X W. C. Rontgen, ‘ Wied. Ann.,’ vol. 64, p. 18. 
