Investigation of the Simjrfe Coherer. 271 



the contact resistance drops to a low value, 4 co to 25 to 

 according to conditions. On one occasion consecutive values 

 of this resistance, in ohms, were 5, 5, h, 6*5, 7, 6'5, 6. 

 This is good agreement considering the nature of the case. 



From the foregoing statements, it will be seen that we have 

 a contact of two wires crossing at right angles. This contact 

 I have called for convenience a Simple Coherer. The wires 

 can be pulled asunder or pressed together with any desired 

 small force which can be measured ; they can be like or 

 unlike and of any material. Through this contact any desired 

 current can be passed, the resistance of the contact at any 

 moment being observed. The problem is to ascertain the 

 nature, if any, of the coherence produced, under these varying 

 conditions : — 



(1) When direct currents pass through the contact ; 



(2) When radiation falls on the contact from an outside 



source, e.g. a Ruhmkorff coil. 



4. P seudo-coherence. 



In order to simplify the issue, I first tried whether any 

 sticking o£ the contact- wires (well scraped) occurred when they 

 were brought together, no current passing. It was found that 

 sticking certainly did occur in some cases ; and as there is no 

 electrical action and the gravitational attraction of the wires 

 for one another would be of a much lower order, the adhesion 

 observed must be due to some surface action or to water or 

 some impurity on the surface. 



Now, filing and scraping the wires w T ould remove dirt 

 or oxide and expose pure metal, but water condensed from 

 the air would not be so easily removed, or, if removed, would 

 at once begin to condense again if the air were sufficiently 

 damp. 



Trial showed that, unless the air is dry, water is always 

 present on the surfaces of the wires, however well scraped ; 

 it is at once removed by rubbing with hot dry blotting- 

 paper (a streak being left by most surfaces on the paper), 

 and it can be immediately produced again, with its adhering 

 qualities, if the observer breathes gently near the contact. 

 It is this film of water, so easily formed from the air, which 

 produces the sticking above referred to, and which may be 

 called P seudo-coherence* ; when the surfaces touch, water 

 is drawn up from all sides into anticlastic surfaces in 

 opposition, and, though these surfaces are small, the force of 

 surface-tension acting may be considerable. 



* See effect mentioned on p. 265. 



