92 Prof. Minchin on the Action of Electromagnetic 



As in the case of the impulsion-cells, the effect of the 

 electromagnetic disturbances is cut off by completely en- 

 closing the tubes and their * terminal wires in a metal box. If 

 the terminal wires are allowed to project outside the box — 

 whether they are connected with the battery and galvano- 

 meter or not — the tubes are affected, which clearly shows 

 that the result is due to the electrical disturbances set up in 

 the leading wires of the tube. 



In order to establish a closer connexion with the impulsion- 

 cells, it seemed to me that films containing metallic particles 

 almost in mathematical contact, but yet free to undergo very 

 minute displacements of rotation, should replace the tubes of 

 filings. Accordingly, 1 have formed such films by im- 

 bedding fine metallic powders in layers of gelatine and of 

 collodion. The powders which I employ in these films are 

 very much finer than the filings which I have used in the 

 glass tubes. 



The media, or bases, in which these powders are embedded 

 are (so far as I have gone) gelatine and collodion. 



To form a gelatine film proceed thus : — Pour a very thin 

 layer of dissolved gelatine over a glass or ebonite plate ; let 

 this layer get almost quite dry ; if it gets quite dry, hold it 

 over a beaker of boiling water until it absorbs a sufficient 

 quantity of steam to render it very slightly plastic : take a 

 test-tube with fine metallic powder immersed in alcohol ; 

 shake this up well, and rapidly pour some of it over the 

 gelatine surface, securing, to all appearance, both continuity 

 and uniformity of the metallic layer. The alcohol soon 

 evaporates, leaving the metallic layer embedded in the gela- 

 tine, but not wholly sunk below it. 



In the case of a collodion film, the metallic powder is shaken 

 up in a test-tube and then rapidly poured over a glass plate. 

 After some time the film dries and peels off the plate. The 

 surface of the film which was in contact with the glass is 

 the metallic surface sought ; the other surface of the film 

 probably contains no metallic particles, and is simply a con- 

 tinuous surface of dry collodion which is an absolute non- 

 conductor. 



If the layer of gelatine above described is too thick, it will 

 probably happen that its upper surface (that remote from the 

 glass or ebonite plate) does not contain metallic particles, and 

 is a non-conductor. In this case, if the film is peeled off the 

 plate there remains on the plate a metallic surface which will 

 answer our purpose. 



Such a film may now replace the tube of filings in 

 M. Branly's experiment. 



