16 Prof. Oliver Lodge on 



Energy is only really conserved under conditions of uni- 

 versal contact-action. 



Of course, the believer in action at a distance is not at a 

 loss : in order to retain his fiction he has invented another 

 unreality, which he calls possible energy. He says (truly 

 enough for many purposes) that when a stone is raised from 

 the earth a great deal more work is done on the stone than on 

 the earth ; and hence that, although at every instant the stone 

 and earth have equal opposite momenta, the energy is nearly all 

 possessed by the raised stone. But since the energy of an 

 inert mass is by no means apparent, since it only has the 

 power of gaining actual energy when let drop, its energy 

 when inert and merely elevated is called possible or potential. 

 Or sometimes, rather more accurately but much more vaguely, 

 energy is said to belong to the configuration of earth and stone *. 



Now this idea of potential energy is convenient as a 

 mathematical expression, and used in its proper signification it 

 is an essential reality ; but used in the sense above quoted, a 

 very common mode of using it, it is a mere receptacle for 

 stowing away any portion of energy which it is not convenient 

 for the moment to attend to ; and I defy anyone to realize it 

 as a thing. 



Yet I myself constantly employ the term potential or 

 static energy, and assert that every activity not only transfers 

 energy from one body to another, but also transforms it from 

 kinetic to potential, or vice versa. 



Yes, but I also assert that the transformation can only 

 accompany transference, and that transference cannot occur 

 without transformation. Whereas^ on the ordinary view, the 

 energy of the raised stone is supposed to gradually transform 

 itself into kinetic as the stone drops, but to remain in the 

 stone all the time. 



I say that the energy was no more in the stone when 

 merely elevated than it is in a strung arrow or the bullet of 

 a cartridge. The energy is in the bow, or in the powder, and 

 is rightly said to be in a static, or strain, or potential form. 

 It can transfer itself to the projectile, and simultaneously 

 transform itself into kinetic, at the pull of a trigger |. 



* Cf. Phil Mag. June 1881, p. 532, and June 1885, xix. p. 484. 



t On page 141, Prof. MacGregor makes a statement in which I entirely 

 fail to catch his meaning. It runs as folio ws : — " When Prof. Lodge 

 states that ' a bullet fired upwards gradually transfers its undissipated 

 energy to the gravitation medium, transforming it at the same time into 

 potential,' he seems to me to assume that the bullet is rigid and that the 

 medium is without inertia." Surely my critic does not consider that 

 either plasticity in a projectile or inertia in a gravitating medium is 

 essentially involved in determining the height attained by a body 

 thrown up. 





