AN'D MODEM PHYSICS. 181 



effects may bo in part electrical, in part mechanical ; 

 the electrical state of the body is in general changed, 

 while in addition, mechanical forces tending to move 

 the body are set up. Experiment must teach us how 

 the electrical state depends on the electric force, and 

 what is the connection between this electric force and 

 the magnetic forces which may, under certain circum- 

 stances, be observed. Now, in specifying the electric 

 and magnetic conditions of the system, various other 

 quantities, in addition to the electric force, will have 

 to be introduced ; the first step is to formulate the 

 necessary quantities, and to determine the relations 

 between them and the electric force. 



Consider now a wire connecting the two poles of 

 an electric battery in its simplest form, a piece of 

 zinc and a piece of copper in a vessel of dilute acid 

 electric force is produced at each point of the wire. 

 Let us suppose this force known ; an electric current 

 depending on the material and the size of the wire 

 flows along it, its value can be determined at each 

 point of the wire in terms of the electric force by 

 Ohm's law. If wo take cither this current or the 

 electric force as known, wo can determine by known 

 laws the electric and magnetic conditions elsewhere. 

 If we suppose the wire to be straight and very long, 

 then, so long as the current is steady and we neglect 

 the small effect duo to the electrostatic charge on the 

 wire, there is no electric force outside the wire. There 

 is, however, magnetic force, and it is found that the 

 lines of magnetic forco are circles round the wire. It 

 is found also that the* work dono in travelling onco 

 completely round the wire against the magnetic forco 



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