338 Professor Oliver Lodge [June 1, 



the insulating film, except perhaps at one or two atoms.* If the film 

 is not permitted to conduct at all, it is not very sensitive ; the most 

 sensitive condition is obtained when an infinitesimal current passes, 

 strong enough just to show on a moderate galvanometer. 



Now let the slightest surging occur, say by reason of a sphere 

 being charged and discharged at a distance of forty yards ; the film 

 at once breaks down — perhaps not completely, that is a question of 

 intensity — but permanently. As I imagine, more molecules get 

 within each other's range, incipient cohesion sets in, and the mo- 

 mentary electric quiver acts somewhat like a flux. It is a singular 

 variety of electric welding. A stronger stimulus enables more 

 molecules to hold on, the process is surprisingly metrical; and, as 

 far as I roughly know at present, the change of resistance is propor- 

 tional to the energy of the electric radiation, from a source of given 

 frequency. 



It is to be specially noted that a battery current is not needed 

 to effect the cohesion, only to demonstrate it. The battery can be 

 applied after the spark has occurred, and the resistance will be found 

 changed as much as if the battery had been on all the time. 



The incipient cohesion electrically caused can be mechanically 

 destroyed. Sound vibrations or any other feeble mechanical dis- 

 turbances, such as scratches or taps, are well adapted to restore the 

 contact to its original high-resistance sensitive condition. The more 

 feeble the electrical disturbance the slighter is the corresponding 

 mechanical stimulus needed for restoration. When working with the 

 radiating sphere, Fig. 19, at a distance of forty yards out of window, 

 I could not for this reason shout to my assistant, to cause him to 

 press the key of the coil and make a spark, but I showed him a 

 duster instead, this being a silent signal which had no disturbing 

 effect on the coherer or tube of filings. I mention 40 yards, because 

 that was one of the first outdoor experiments; but I should think 

 that something more like half a mile was nearer the limit of sensi- 

 tiveness. However, this is a rash statement not at present verified. 

 At 40 or 60 yards the exciting spark cculd be distinctly heard, and 

 it was interesting to watch the spot of light begin its long excursion 

 and actually travel a distance of 2 inches or 3 inches before the sound 

 arrived. This experiment proved definitely enough that the efficient 

 cause travelled quicker than sound, and disposed completely of any 

 sceptical doubts as to sound-waves being, perhaps, the real cause of 

 the phenomenon. 



Invariably, when the receiver is in good condition, sound or other 

 mechanical disturbance acts one way, viz. in the direction of in- 

 creasing resistance, while electrical radiation or jerks act the other 

 way, decreasing it. While getting the receiver into condition, or 

 when it is getting out of order, vibrations and sometimes electric 

 discharges act irregularly ; and an occasional good shaking does the 



* See Phil. Mag. Jan. 1894, p. 94. 



