294 Mr. P. E. Shaw : 



tried. A bridge at once forms, if the distance is less than 

 this amount. There are two ways in which the surfaces 

 might come into contact, when set at this small distance 

 apart : (a) by a shattering of the surface particles under 

 great electric stress, a bridge of particles being laid across ; 

 (b) by the diaphragm bending until it touched the other 

 contact. Taking Lord Rayleigh's measurement* that the 

 ratio force/displacement of an ordinary diaphragm = 10 7 C.G.S., 

 then the displacement 10~ 5 cm. would only require about 

 100 dynes. If this force came into action on the diaphragm, 

 it would meet the other contact without the help of (a). 



While allowing that (b) may influence the result, it can 

 with certainty be said that (a) does occur, for repeatedly there 

 was evidence during the experiments of a quantity of loose 

 particles or of some easily-crushed bridge which formed 

 during coherence. It seems again that it must be a thin 

 bridge and not loose particles, for the surface always made a 

 sharp contact of a distinct sort. 



The evidence of van Gulik on this " striking " distance has 

 already been quoted. 



(2) A pronounced rending apart of the surfaces, observed 

 as a " snap " in the telephone, was found after coherence 

 with the more fusible copper-iron and fuse-metal, but not 

 with carbon, platinum, and iridio-platinum. 



(3) The action of platinum (Table XIII.) is remarkable. 

 It calls for some explanation. The only one seems to be that 

 orientation occurs due to coherence which enables the surfaces 

 to hold one another after discharge but not before; also there 

 is a remarkable fatigue and recovery involved. 



(4) A measure was found of the distance of incomplete 

 contact of hard carbon. It is a very variable quantity due 

 to the soft, friable nature of its surfaces as com j tared with 

 metal surfaces. The extremes were 5 x 10~~ 6 cm. to 

 50xl0 -6 cm.; i.e., for complete contact the movement 

 necessary was sometimes as much as the fir>t number and 

 never more than the last. This property alone would indicate 

 carbon as the material par excellence for microphone contacts, 

 no metal used in any way resembling it in this respect. 

 In working with carbon coherers it is well to bear this action 

 in mind. 



12. Summary of Results. 



The Electric Micrometer has proved of service in studying 

 the Simple Coherer, for by it we obtain some idea of the 



* a 



Quantitative Theory of the Telephone," Phil. Mag. xxxviii. 1894.. 



