Radiations on Films containing Metallic Poivders. 91 



accompanied by the rumbling noises which are heard in a 

 telephonic circuit which contains a microphone. 



Now the most striking peculiarity of such a column is that, 

 while it is in the non-conducting state, if a spark is allowed 

 to pass anywhere in its neighbourhood between the knobs of 

 a Leyden jar or the poles of an induction-coil, the waves sent 

 out from the spark at once render the column a conductor. 



This experiment was reproduced at the meeting of the 

 Physical Society on October 27 by Mr. Croft, and it at once 

 struck me as bearing a strong resemblance to the effects 

 produced by electromagnetic waves on photoelectric impul- 

 sion-cells. These cells I have already fully described (see 

 1 Philosophical Magazine,' March 1891) ; and in addition to 

 what I showed with regard to them, I may here add that the 

 effects of the sparks of an induction-coil on an impulsion-cell 

 were observed when the cell (enclosed in a dark box) was 

 over 140 feet distant from the coil and from everything 

 connected with it. Moreover, it was found that, in most 

 cases, the power of electromagnetic radiations to change 

 the state of the cell from one of insensitiveness to one of 

 sensitiveness to light depends on the length of the electro- 

 magnetic waves ; for, by adding capacity (by means of Leyden 

 jars) to the sparking circuit, the waves ceased to affect the 

 cell, and, on removing this capacity, their ability to effect 

 the change was restored. 



I have always held that this action of the cell is due to a 

 rearrangement of the molecules on the sensitized surface of 

 the plate in the cell by the electrical disturbances produced 

 in the wires connected wdth its poles, these disturbances being 

 due to the radiations emanating from the sparking circuit ; 

 and I found that by completely enclosing the cell, together 

 with its terminal wires, in a metal box, the ability of the 

 radiations to act on the cell was destroyed. 



After having seen the experiment shown by Mr. Croft, I 

 filled several glass tubes with the filings of copper, tin, zinc, 

 bismuth, antimony, &c, and found M. Branly's result in 

 every case. It seems to me that if the filings are extremely 

 fine (powders, in fact) the result is much more difficult to 

 obtain. Powders used in this way are very great insulators, 

 and they must be strongly pressed together by the corks 

 which close the glass tubes in order to conduct ; but then it 

 becomes difficult to alter their conductivity by mechanical 

 disturbances. On the other hand, a tube filled with very 

 coarse iron filings was found to conduct under all circum- 

 stances ; so that, apparently, the filings must be neither very 

 fine nor very coarse. 



