232 Aug. Bighi on the Production of Electrical 



nexion with the earth. One of the sides of the box is made 

 in chief part of a large lead plate provided with a circular 

 window, in front of which on the inside and at a small distance 

 is that part of a Crookes' tube which is struck by the kathodic 

 rays. The window may be closed either by a large lead plate 

 or by a thin plate of aluminium. In either case, when the 

 apparatus in the box are at work, no electrical force pro- 

 ceeding from the internal charge of the case is manifested on 

 the outside. 



When conductors, electrified or not, are placed in front of 

 the window closed with aluminium, I observed the following 

 phenomena : — 



4. If a conductor charged negatively is placed in front of 

 the window, no sooner is the Crookes' tube at work than the 

 charge rapidly disperses. 



If the conductor is uncharged, it becomes positively elec- 

 trified ; and in the same conditions the final charge is dif- 

 ferent according to the nature of the substance : thus, with 

 gas-graphite it is greater than with copper, and with copper 

 greater than with zinc. 



With the latter metal the deflexion obtained is negative 

 when the distance between it and the aluminium is small 

 enough. This arises from the difference of potential of contact; 

 but at a greater distance the positive charge produced by the 

 X-rays preponderates. 



In fine, if a photo-electrical couple is placed in front of the 

 window, a deflexion of the electrometer is obtained, as if 

 with ultra-violet radiations. I have not yet examined whether 

 this deflexion is exactly equal to that which the latter radia- 

 tions would produce. 



Hence the IL-rays have, in common with the ultra-violet 

 ones, the property of dispersing the negative charge, and of 

 giving rise to positive charges in unelectrified bodies. 



The electrical action of the X-rays diminishes, as was to be 

 expected, if the bodies on which they act are moved away from 

 the window from which the rays proceed. 



If a board of pine-wood, or a thick plate of aluminium or 

 of glass, or the hand, is placed so as entirely to cover the 

 aperture, it more or less diminishes the effect produced by the 

 X-rays, but in general does not obscure them completely. A 

 piece of glass- mirror less than a centimetre in thickness 

 absorbs more than pine-wood 6 centim. in thickness. 



5. Ultra-violet radiations produce no appreciable action on 

 bodies positively electrified, for if in any case they seem to 

 disperse positive electricity, it may be ascertained that the 

 effect observed was due to the dispersion of negative electricity 

 by the surrounding bodies. 



