Radiations on Films containing Metallic Powders. 93 



Let ABCD represent the film, L a battery of one or two 

 voltaic cells, G a galvanometer, and K a key which can be 

 connected with the film by means of the wires s, w, the 



former clamped to the film at a point P, while the latter ends 

 in a stout platinum piece the end of which is a round knob 

 touching the film at a movable point Q. 



Now it is found that when the contacts are all made no 

 current passes, and we have to begin by making the platinum 

 knob touch the film very close to P — perhaps \ millim. 

 from P. Still no current passes, even when a spark of an 

 induction-coil is allowed to pass a few feet from the film. 

 But by touching either of the wires s, w with an electrified 

 body (that which I commonly use is a common gas-lighting 

 electrical machine), the resistance of the film and the contacts 

 P, Q is overcome, and a strong current is shown by the 

 galvanometer. We can now gradually increase the distance 

 between P and Q each time that Q is removed from P, the 

 wire s or w being touched by the electrified body ; and by 

 these successive steps the whole of the film is finally rendered 

 conducting. 



The special characteristic of such a film which I wish to 

 point out is this : if, while the film is conducting, the current 

 is stopped by breaking the contact at Q, the film will be 

 found to be a non-conductor if the contact is almost instantly 

 re-made at the same point Q ; while, if the contact is broken 

 anywhere else in the circuit, as at K, and then re-made, the 

 film will still be found to conduct. 



Again, if when the film has been rendered a conductor, the 

 circuit is broken at K, and left broken for (apparently) any 

 length of time, and then re-made, the film will be found to 

 be still a conductor. 



The breaking of the contact at Q is instantly fatal to the 

 conductivity of the film, if the film is not many days old ; 

 but, after some days, it will be found that on breaking the 

 contact at Q and then re -making it, the film remains a con- 

 ductor, unless we delay the re-making of the contact for half 



