458 



NATURE 



{Sept. 27, 1877 



be very appropriately so termed by some physicists as bfins; E. 

 of piitential. or rather E. of difTerence of ]iofential, or E. which 

 is the complement of potential. Tliis is evidently sometimes in 

 the minds of our teachers : indeed, Clerk Maxwell directly tells 

 usi that this is one sense in which the title is suitable ; he calls 

 it a "very felicitous expression," because it has the two differ- 

 ently applicable meanings we have mentioni;d. 



7. Here, then, is our next gravamen. These two characters 

 of this type of E. are quite heterogeneous and unconnected. 

 Now a sitnple name can only refer to one character of a person 

 or thing ; and if it happens, by accident, that the werd that 

 constitutes the name has two quite different meanings or refer- 

 ences, and that both are applicable to the person or thing, if we 

 mentally ajiply them both to that individual, we are guilty 

 of a sort of punning or verbal skylarking ; as if for instance we 

 should call Mr. Smith an upright man on account both of his 

 erect carriage and of his moral probity. This is bad ennnuh ; 

 but further, the two characters which might be implied in the 

 name "potential E." are not merely heterogenrous ; they are 

 mutually incompatible ; they cannot be put together into th- 

 same complex idea, at least by ordinary mortals. Surely there 

 is no occasion to .stop to prove this. 



It is evident that the majority of our teachers feel this and the 

 preceding inconveniences themselves. And I confess that I am 

 now going to bring against them a more serious accusation than 

 that of merely using unsuitable language. There is a most 

 singular and apparently significant omission to be noticed in 

 nearly all the manuals, referring to this sutiject, into which I 

 have looked ; we have already alluded to this. 



8. But we must t.ike this opportunity of numbering it as the 

 eighth of our gravamina. It is this, while teaching us about the 

 different classes of E. and telling us that one is called " potential, ' 

 they abstain from telling us why it is so called ! This omission 

 is so remarkable, iu itself, as occurring in bonks intended to 

 impart instruction, and so unlike the ordinary behaviour of our 

 doctors, that there must be some very particular rea-nn for it. A 

 single person might make this omission by pure accidental inad- 

 vertence ; but when a number ol persons do so it cannot be thus 

 accounted for. There is no explanation but this — that they 

 perceive the botherations connected with this confounded, I 

 mean confounding, name, "potential E.," and rather than 

 acknowledge how matters stand, and own themselves to blame, 

 they try to slur the thing over by giving no meaning of the name 

 at all. Rankine, indeed," just alludes to the la-t meaning of 

 "potential E.," which refers to its connection with potential 

 (function) ; but that is all ; not a word about the incongruity 

 between it and his own original meaning, just as if none existed 

 Clerk Maxwell, however, as we have just seen, boldly takes the 

 bull by the horns, and tries to make both himself and us believe 

 that he is delighted with this Janus-like name and with the com- 

 pounding of its two incompatible meanings. It so happens that 

 the writings of that distinguished physicist contain as striking an 

 illustration as could be conceived of the inconvenience of the 

 ambiguity of this "very felicitous expression." We have already 

 mentioned it, but with a different object in view. Having told 

 us in one place that "potential E." is "the E. which the system 

 has not in actual possession," he also tells us elsewhere 'that " the 

 leaden weight of a clock when it is wound up has potenial E. , 

 which it loses as it descends." The weight set--to and works 

 with E., whieh it has not in possession, Imt only has the power 

 to acquire, and which it loses the power of acquiring ! ! ■* In the 

 first statement he was thinking of the first meaning of potential 

 E. and in the second statement of the other meaning. 



It mi|:ht be said that if we discard the first meaning of 

 "potential E." on account of its intrinsic wiongness, we shall, 

 at the same time, aliolish this last difficulty, which arises from 

 its relation with the second meaning, and that this second mean- 

 ing, which is admitte jly good m tlsiij\ will then have nothing 

 against it. But in the first place the associations of the name 

 "potential E." with the first meaning are too strong to be easily 

 got rid of. It would be all but impossible to retain the word 

 and confine it strictly to the second meaning. 



9. But besides, " potential E." in this second meaning, though 



* " Heat," p 91: " Matter and Motion," p. 81 : — " Potential E —A very 

 felicitous expiession, since it not only sigi.ifies ihe F. which the system trjs 

 not in actual possession, but only has the power to acqu re, but it also indi- 

 cates its connection with what has been called (on other grounds) the Poten- 

 ti.ll Function, ' 



5 Phil. Mug., Febni.iry, 1SO7. 



3 "Theory 01 Heat," p 281. 



'' We have already seen above, that the weight never acquires more 

 than a quite insensible amount of "actual ¥..." so called. 



good in itself, has inconveniences inrlependent of this when 

 applied as I believe it universally is, to E. conceived of as exist- 

 ing in tlie body moved ; for potential (function) does not appertain 

 to the body moved, but entirely to \\ig force concerned. 



C. — Pot/ntial E., as meaning " Energy of Potency. " 



As to the third meaning of "potential E. ," it has been 



said (and indeed Rankine may have had this in view in ore 



place ') that it need not be taken to imply anythiuj; more than 



E. of potency or power without reference to Potential (function) 



10. But according to this, "potential E." would mean the 

 power of doing work which consists in power ; and it would be 

 as great a tautology as " umbrageous shades." 



11. And again, if it be the special distinclion of one class of 

 E. that it is E. of potency it necessarily follows that the other 

 class, observe the so-called acual E , is i?. of inipcleiicy I 



12. And besides, there is the same incompatibility between 

 this meaning, C, and meaning A, as there is between B and A.'* 



As to the whereabouts of Potential Energy. 

 We shall now pass from the perplexities connected with this 

 unlucky name, "potential E." to consider the behaviour of our 

 teachers towards the thing itself. It will conduce to clearness 

 to drop this name now, since our objections are no longer directed 

 against it, and adopt another very common one for the same 

 thing, viz., " E. of position." 



13. The E. of position is usually regarded and spoken of as 

 belonging to, or being in the body in question which may be 

 about to move and acquire E. of motion. This puzzles poor P. 

 terriMy ; not only on account of the difficulty of grasping the 

 thing mentally and of putting any clear meaning into it. but also 

 because the doctors, both individually and collectively, often 

 display such curious inconsistency respecting it. 



But before proceeding to consider directly the undesirableness 

 of this way of viewing E. of position, let us observe that it is the 

 cause of all the above perplexities, which, indeed, seems to be 

 sufficient olijection to it ; and let us endeavour to find out why 

 the doctors should have had recourse thereto. 



The physicists having determined, for the reasons below, 

 to talk of this E of position as being in the body, and that body 

 being just the same (and, when regarded as attractifrf or repelled', 

 as is u-ually done, equally inert) in whatever position it stands, 

 it becomes necessary to provide for this by a little ingenious 

 dodge; for such the phrase "potential E.," as now generally 

 used, really is. " Potential E." plays the same part as a con- 

 juror's empty case or shape, which is made to represent some 

 s-ilid object which is really lying elsewhere, or is perhaps actually 

 doing dutv there. Our physical prestidigitateurs tell poor P. 

 that this E. is "in the boiy," that the body " /;ai it," that 

 the body " possesses it," with other simi'ar expressions. But 

 what is it that they are presenting to him all the time? "Potential 

 E.," which sounds to him very fine, and which he thinks must 

 be something very serviceable, but which is in reality only an 

 empty shape, for it is " E. which the body /;(U not in actual 

 possession " ! ! They have adopted the precise inverse of the 

 famous device employed by Ulysses when he told Polyphemus 

 that his name was OCtis. We have mentioned and reckoned 

 this grievance already, on its own account ; we have returned to 

 it now only to show how the present one necessitates the use of 

 this delusive name "potential E." 



Why then is it that our teachers (save the mark !) wish thus to 

 make-believe that they have got their E. of position in the body ? 

 The principal reason is this — They have to keep straight with 

 the me'aphysicims. In these da\s it is generally perceived that 

 we should, as much as possible, avoid treating force as an 

 objective something When eneigy does not come prominently 

 foiward into discussion they can use the same forms of expression 

 about force as their grandfathers did, though intending them 

 only as such. The term "force" is " very use'ul," in that " it 

 enables us to abbreviate statements which would otherwise be 

 long and tedious ; " and no harm is done by using it when the 

 necessary reservation as to its being only a convenient mode of 

 speech is known to underlie all the statements and discussion. 

 But when Energy, which mut be taken as real and objective in 

 some sense, is the subject of their t.alk, they become extra 

 cautious, and, fearing to put this objective affair into non-objec- 



' " Encyclop. Brit " (1857), vol. xiv.. article " Mechanics." 



= In Nicholl's '■ Cyclop, of Phys. Sciences" potential E. is said to mean 



E. of a power or force, but it is ea!.y to see that this does iiot mend the 



matter at all. As a distinguisliing title it implies that a moving body has 



no force nor power . no "power of performing work, ' that is to say no E. 



