Investigation of the Simple Coherer. 285 



D. ran Gulik * observed that i£ two free platinum wires 

 were separated by as much as 00004 cm., then, when 

 radiation reaches them, a spark pierces the intervening di- 

 electric, and particles pass through the hole thus made, 

 forming a bridge of particles from wire to wire. [I have 

 found a maximum striking distance of 0*000012 cm., but my 

 surfaces were rigidly fixed.] 



Suridorph f, using an iron coherer, found a string of 

 particles formed from one contact-surface to another. On 

 observing that heating destroys coherence, and that cooling 

 does not do so, he inferred that the chain breaks up under 

 end pressure but not under tension. [But suppose orientation 

 to occur, then heating loosens the particles and orientation 

 would be destroyed, whereas cooling would have a contrary 

 effect.] 



Branhj % observed small surface-resistance in copper, zinc, 

 brass, silver, german-silver, and large resistance in aluminium, 

 iron, lead, bismuth. [The difference between the two sets is 

 certainly not in readiness to form oxide ; it may depend on 

 power of condensing air on the surface.] 



Tommasina § and Harden [[ showed that when coherence 

 occurs a chain of particles is formed by the orientation of 

 small fragments under the influence of the electric field. 



Gutlie <$" Trowbridge^. — The experimental method used 

 has some points of resemblance to the aphometer method 

 described above ; one contact of two metals was used, the 

 pressure of the surfaces being controlled by a screw. One 

 important result was that the high initial resistance was not 

 produced until some hours had elapsed after cleaning. 



They also found that the potential-difference between the 

 ends of the contact rises to a definite maximum when current 

 increases. This is a curious and striking result. It is 

 evident that when the maximum potential-difference has been 

 obtained, any increase in current is attended by a pro- 

 portional decrease in resistance of the contact. 



The authors suggest that there is a layer of condensed, 

 badly-conducting gas, possibly water-vapour. A current 

 then produces ions in this layer, and resistance falls. One 

 would like to try, as a test of this conception, whether co- 

 herence occurs between platinum wires which have been 



* Wied. Ann. 1898. 

 t Wied. Ann. July 1899. 

 % Cumptes Rendus, 1898, p. 219. 

 § Cumptes Rendus, July 1899. 

 || Electrotechn. Zeitschr. April 1900. 

 1J Phys. Rev. July 1900, p. 22. 



