Investigation of the Simple Coherer. 283 



moreover, the surfaces of the wires are not, as supposed, 

 perfect cylinders but, examined minutely, would obviously 

 appear most irregular. The calculations serve to show that 

 the conception of a bridge and its consequences are not by 

 any means incompatible with the data so far obtained. 



8. The Ionic Theory of Conduction. 



According to the recently developed theory, conduction 

 occurs in metals by much the same process as in electrolytes 

 and gases, i. e. by ionic movements. 



Under electric force atoms are split up into negative and 

 positive ions ; these, or at any rate the small negative ions, 

 are carriers of electricity, each having a unit charge. Any 

 atom receiving a negative ion maintains its neutrality by 

 afc once losing- a similar ion ; thus the charges are handed on 

 from atom to atom throughout the metal. If the current 

 became great per unit area of cross-section, we can imagine 

 that the majority of the atoms would be in action at any 

 instant, and that the commotion among them would be very 

 great. Prof. J. J. Thomson * recently suggested that an 

 ion may attain such speed as to shatter an atom on meeting 

 it, producing from it many ions. 



Another view of conduction is that held by Mr. Suther- 

 land f, who considers the shattering of the atoms an un- 

 necessary assumption ; he supposes that electricity exists in 

 neutrons which split under electric force into positive and 

 negative electrons ; these have an existence independent of 

 the atoms and pass the charges along on some Grothuss-chain 

 system. 



Adopting these views, imagine the current passing through 

 the simple coherer. In the wires themselves, the agitation of 

 atoms (or neutrons) would be moderate- ; but at the place of 

 contact, where the area of cross-section is so restricted, the 

 commotion would be intensified in inverse proportion to the 

 area (say 10 9 times). A loosening of particles and formation 

 of a " bridge" would be most likely to occur, with sudden drop 

 in. resistance of the contact. The particles of the bridge 

 being deposited or laid in this peculiar way would be likely 

 to have some " orientation " or arrangement for maximum 

 conduction. When the radiation stops, the bridge is com- 



* Phil. Mag. xlvii. 1899. 



t Phil. Mag. Sept. 1900. 



Other writings on this subject are: — J. J. Thomson : A paper read 

 before the Paris Congress on Physics, 1900; Uiecke, Wied. Ann. 1.x. vi 

 p. 353. 



U2 



