20 Prof. Oliver Lodge on 



The following is my demonstration of the law of transfer- 

 ence and transformation : being inaeed the detailed statement 

 of the essential phenomena accompanying transfer of energy, 

 on the hypothesis of contact-action. 



A body* or other medium exerting force at any point, and 

 there moving in the sense of the force it is exerting, is in a 

 state of activity : it is passing energy on ahead. If its speed 

 continues constant, the force just spoken of cannot be the sole 

 force, the resultant force on it must be zero ; in that case it is 

 a mere transmitter, not itself active, only passing on what it 

 receives. But if it is itself active, i. e. parting with its own 

 energy, then its speed at the acting point (the place of appli- 

 cation of the force) must either decrease or increase. 



If its speed decreases, it must be parting with kinetic energy ; 



if its speed increases, it is parting with potential energy. 



Contrariwise : — - 



A body or other medium exerting force and being moved in 

 the sense opposed to that force is receiving energy. 



If it is not merely passing it on, in which case the resultant 

 force acting on it is zero, its velocity must vary. 



If the velocity increases it is gaining kinetic energy ; if its 

 velocity decreases it is gaining potential. 



Now the two bodies or things here spoken of are neces- 

 sarily existent in every case of activity ; one is the agent the 

 other the patient, one the emitter the other the receiver, one 

 the acter the other the reacter, one the driver the other the 

 driven ; and they are in contact while the activity lasts. 



Being in contact, their velocities along the line of stress 

 must increase or decrease together. 



If their common velocity is decreasing, then the driver loses 

 kinetic and the driven gains potential. 



If their common velocity is increasing, the driver loses po- 

 tential, the driven gains kinetic. 



This is my proof of the necessary concomitance of transfer 

 and transformation, and of the alternation of transformations. 

 But it will be well to illustrate the matter further. 



Let me first explain how I define potential or static energy 

 as used in the proposition criticised. I mean by it simply 

 the energy of a body under stress ; an elastic body which is 

 exerting force may always be said to have potential energy, 

 notwithstanding that wdien the force comes to be analysed it 



* Meaning by " a body " a thing of constant mass, an identical lump 

 of matter. A thing of variable mass, like a rain-drop or a railway-train 

 or a layer of moving gas, is gaining or losing matter as well as energy ; 

 and cannot be regarded as one simple body. 



