Prof. Thomspn on the Dynamical Theory of Heat. 381 



passive conducting are first considered. On the other hand, the 

 electromotive force acting in the portion from which the energy 

 proceeds is not itself determined by such tests, but is equal to 

 the whole electromotive force of the souj'ces contained in it, di- 

 minished by the reaction of the force which is measured in the 

 manner just explained. The same tests applied to any two 

 points whatever of a complete conducting circuit, however the 

 sources of energy are distributed through it, show simply the 

 electromotive force acting and reacting between the two parts 

 into which the circuit might be separated by breaking it at these 

 points. In some cases, for instance some cases of thermo-electric 

 action which we shall have to consider*, these tests would give a 

 zero indication to whatever two points of a circuit through which 

 a current is actually passing they are applied, and would there- 

 fore show that there is no electric action and reaction between 

 different parts of the circuit, but that each part contains intrin- 

 sically the electromotive force required to sustain the current 

 through it at the existing rate. An actual test of the electromo- 

 tive force of sources contained in any part of a linear conductor 

 is defined, with especial reference to the circumstances of thermo- 

 electricity, in the following statement : — 



144. Def. The actual intrinsic electromotive force of any 

 part of a linear conducting circuit is the difference of potential 

 which it produces in two insulated conductors of a standard 

 metal at one temperature, when its extremities are connected 

 with them by conducting arcs of the same metal, and insulated 

 from the remainder of the circuit. 



The electromotive force so defined may be detei'mined, either 

 by determining by some electrostatical method the difference of 

 potentials in the two conductors of standard metal mentioned in 

 the definition, or by measuring the strength of the current pro- 

 duced in a conducting arc of the standard metal of infinitely 

 greater resistance than the given conducting arc, applied to con- 

 nect its extremities when insulated from the remainder of its 

 own circuit. 



145. With reference to the distribution of electromotive force 

 through a solid, the following definitions are laid down : — 



Def. 1. The intrinsic electromotive force of a linear con- 

 ductor at any point is the actual intrinsic electromotive force in 

 an infinitely small arc through this point divided by its length. 



Def 2. The efiicicnt electromotive force at any point of a 

 linear conducting circuit is the sum of the actual intrinsic elec- 

 tromotive force in an infinitely small arc, and the electromotive 

 force produced by the remainder of the circuit on its extremities, 

 divided by its length. 



* For one of these see § 1<)7 below. 



