1896-97.] Conductive Effect in Air by Rontgen Rays. 415 
Feb. 5. 11.30 a.m. 
Rays-zero. 
+ 23 ’5 sc. divs. from metallic zero 
+ 25-0 
+ 23-0 
+ 23-0 
3 3 
33 
3 3 
3 3 
3 3 
3 ) 
3 3 
J 3 
33 
Distance between 
surfaces. 
1*2 cms. 
. 2*2 „ 
. 3-8 „ 
• 6'0 „ 
We next removed the oxidised copper plate, and substituted a 
polished zinc disc. With it we obtained the following results : — 
Rays-zero. 
- 82 sc. divs. from metallic zero 
-79 
-81 
-90 
-90 
3 3 
3 3 
3 3 
33 
3 3 
3 •> 
3 3 
33 
33 
3 3 
3 3 
3 3 
Distance between 
surfaces. 
. 1 cm. 
. 1-5 „ 
. 3-0 „ 
. 7*0 „ 
• 7*5 „ 
The steady reading of the rays-zero was very nearly reached in 
each case in about 15 secs., but the observation was continued for 
one or two minutes till we found the reading steady. 
Thus we see that, as previously found by Mr Erskine Murray, 
the rays-zero is independent, or nearly independent, of the distance 
between the opposed metallic surfaces.] 
§ 23. Towards realising the case of an insulated metal surrounded 
by metal of identical surface-quality connected to sheaths, we 
covered over the oxidised copper with tinfoil. The tinfoil wall 
facing it was very rough, and not so well polished. The insulated 
tinfoil was 4 cms. distant from the end of the box to which its 
surface was parallel. 
When the ultra-violet light fell on the insulated metal alone 
through a slit, the ultra- violet-light-zero was + 53 scale divisions 
from the metallic zero. A charge given to it, whether positive or 
negative, was discharged slowly. After making these experiments, 
we again observed the difference of zeros, and found that now the 
ultra-violet light reading was at the end of the first four minutes 
+ 2 scale divisions from the metallic zero ; at the end of the next 
four minutes it was - 8 scale divisions from it. 
When the ultra-violet light fell on the disinsulated metal and 
not on the insulated, the insulated when charged retained its 
charge. 
