274 



Mr. P. E. Shaw : 



5. Direct- Current Coherence. 



Born *, and Guthe and Trowbridge f , have pointed out that 

 the initial high resistance of a coherer is due apparently to a 

 film of condensed gases on the surfaces. I also found this ; 

 for if the wires, just cleaned, be brought into contact a cur- 

 rent, though an uncertain one, passes ; the voltmeter-readings 

 nutter, and, after a few contact " makes " and "breaks," the 

 index comes to restj showing that resistance is practically 

 infinite. Now this is quite unlike true coherence, which is 

 accompanied by perfect steadiness, for any length of time, 

 in ammeter and voltmeter. When, either by direct current 

 or alternating discharge, true coherence is effected, stopping 

 or reversing the current will not produce any change, and 

 in twelve hours' time the surfaces will still be found to hold 

 perfectly. Table V. is an illustration of the point ; after 

 two readings there is no coherence, and the voltmeter-reading 

 goes to its full amount. 



Table V. 



E.T. is the column showing resistance to overcome torsion. 



K.S. 



sticking. 



Remarks. 



Current Direct. 

 ,, Reversed. 



R.T. 



R.S. 



Voltmeter. 



Contacts cleaned 

 with emery-paper 

 1 and wiped. 



No current. 

 CD. 



2720 

 2720 



2700 



2620 



0-45 





CD. 



2720 



2620 



0-50 





CD. 



2700 



2680 



1-40 





CD. 



2700 



2680 



1-40 





O.E. 



2700 



2680 



1-39 





C.E. 



2700 



2680 



1-39 



6. Coherence Proper. 



Having cleared up some obscurities, we have now to deal 

 with coherence as usually understood ; i. e., due to vibrations 

 from some discharge falling on the contact. It may be 

 stated at once that coherence occurs at a contact when 

 radiation falls on it even when no independent current is 

 passing through the contact, and even when the contact- 



* Dorn, Wied. Ann. lxvi. 



f Guthe and Trowbridge, Phys. Bev., Aug. 1900, p. 22. 



