J^ay 25, 1876] 



NA TORE 



69 



these forces immediately equilibriate themselves at the expense 

 of calling into play other forces of the same or of opposite 

 kind amongst the molecules of the dielectric. These forces give 

 rise to the diminishing potentials as they are equilibriated over 

 greater and greater surfaces. When another conductor is brought 

 nto the neighbourhood, since throughout it the electrical forces 

 are in equilibrium amongst themselves, the various molecular 

 forces are as before manifested only at the surface, and they are 

 necessarily negative where the conductor obtrudes into regions 

 of higher positive potential than its own mean, and positive 

 where it lies in the regions of lower positive potential. But not 

 only this, the molecular forces which keep the electrical forces in 

 the dielectric in equilibrium cannot thus simply be pushed, as 

 it were, backwards and forwards, but must fall into equilibrium 

 in their own way — in other words there is a redistribution of 

 electiicity both on the inductor and inducer, which can only be 

 determined by properly drawing the equipotential surfaces cor- 

 lesponding to the new arrangement (if possible). The state of 

 stress of the particles of the dielectric surrounding any small 

 conductor is not affected by its total motion of translation, except 

 that as it is moved from the other conductors it is redistributed 

 on the surface. 



If now we draw a series of equipotential surfaces, that parti- 

 cular one which corresponds to the potential of the conductor 

 will divide ir, as Prof. Clerk Maxwell says, into two parts, on 

 one of which is negative electricity, and on the other positive, in 

 other words the state ot stress of the particles outside the con- 

 ductor is of one kind on one side, and of the opposite kind on 

 the other. Now comes my first question. If this is the case 

 how can it be said that there is either more positive electricity 

 on the inducer nearest the inductor as Prof. Clerk Maxwell says, 

 or less as Prof. Volpicelli says, than at the other end, when in 

 fact there is none, but the force is negative ? No doubt we can 

 take for mathematical purposes a negative quantity as the sum 

 of two others, one positive and the other negative and greater, 

 but can the existence of the positive quantity be called a "fact" 

 in consequence ? 



There is a way, however, in which we might be inclined to 

 say that the positive electricity is least nearest the positive in- 

 ductor, but this looked at in the same way as before, raises a 

 second question. If we make a small conductor touch any part 

 of the induced conductor, and then try it in the usual way, we 

 might say that the spot on which we touched it when the small 

 conductor was most electrified had the greatest amount of elec- 

 tricity upon it, and might determine its kind. But before doing 

 this we ought to ask what will be the effect of bringing the new 

 conductor into the neighbourhood, and this depends on its shape 

 and size. The equipotential surfaces will all be altered, and the 

 alteration may be such that the one belonging to the first induced 

 conductor may leave the new one entirely on the positive or 

 entirely on the negative side, or may divide it into two like the 

 first induced conductor. In connecting with the earth we make 

 the new conductor so large that the old one is all on the negative 

 side ; and the fact that by breaking contact we can keep the old 

 conductor charged with negative eleclricity shows that we may 

 take any smaller part from the wholly negative side and it will 

 also show the same electricity, as in inductive machines. If the 

 new conductor be so shaped or so large that it cuts thtough the 

 neutral equipotential surface, on removing it only the balance of 

 the forces called into play will te left to be equilibriated by the 

 molecular forces, and that balance may be positive though the 

 contact was on the negative side of the former neutral surface. 

 In this way only could a finite conductor take positive electricity 

 from the negative side, but in this case it is due to induction on 

 the new conductor as temporarily forming part of the old, and 

 not to the original induction on the first conductor. What 

 experimental proof, then, is there, or can there be, if these prin- 

 ciples are true, that there is any positive electricity neaiest the 

 positive inductor before the distribution is disturbed by tco long 

 or large a conductor being brought into the field? and how, 

 therefore, is Melloni's theory true? 



Also, might not a point if properly placed on the negative 

 side, cut through the neutral equipotential surface and so dis- 

 charge positive electricity ? 



I should be glad to know, from a good authority, that we rr.ay 

 thus explain these phenomena by a reference to lorce alone and 

 not to hypothetical fluids, and without meddling with such use- 

 ful, perhaps, but unmechanical ideas as " bound " and " free." 



J. F. Blake 



Dynamometers and Units of Force 



In Nature (vol. xiv., p. 29) Prof. Barrett says "it would 

 be interesting to know on w hat grounds Prof. Plennessy bases 

 his emphatic and reiterated assertion." The assertion referred 

 to is contained in my former communication (Nature, vol. xiii., 

 p. 466). The grounds on which it is based are as follows : — In 

 order to accurately measure units of force according to the 

 C. G. S. system, spring balances which could be depended upon 

 to the D^y of a gramme or ^ of a grain nearly would be required. 

 In mechanics the forces to be compared and measured usually 

 amount to several kilogrammes, and powerful spring dynamo- 

 meters are most suitable for their estimation. Dynamometers 

 such as those alluded to as being sent for exhibition from the 

 College of Science to South Kensington are of this kind. By 

 experiment I have found them unfit for the estimation of small 

 units of force. I should be much interested in seeing Prof. 

 Barrett or Dr. Ball measuring a C. G. S. unit or ^^ of a 

 gramme by the aid of one of these dynamometers. It should 

 be remembered that in this discussion I all through refer to these 

 dynamometers and others of a similar kind employed in me- 

 chanics. I was already aware of the belief expressed by Sir 

 William Thomson and Prof. Tait, that spring balances, "?/ 

 carefully constructed," would rival or even surpass the ordinary 

 balance. While thus referring to the possible perfection of the 

 spring balance with the qualifying particle "//," they justly 

 remark that the pendulum is the most delicate of all instruments 

 for the measurement of force. A pendulum will probably 

 always furnish the best means for measuring force in absolu!e 

 measure, whether by large or small units ; and I entertain strong 

 doubts as to whether the spring balance can ever supersede the 

 beam balance for accurate determinations of weight. In no de- 

 partment of experimental inquiry are such minute quantities 

 weighed, and nowhere is greater accuracy in determining differ- 

 ences of weight required than in chemical analysis, and chemists 

 almost universally employ the beam balance in preference to the 

 spring balance in their most delicate analytical researches. 



In my former communication I mentioned that the dynamo- 

 meters alluded to could not be depended on within the tenth of 

 a kilogramme. In saying this I have spoken of them in the 

 most favourable terms, for the larger one can scarcely be de- 

 pended upon within the fifth of a kilogramme. 



Prof. Barrett quotes a statement as " occurring in Prof. Hen- 

 nessy's own syllabus," which implies that I had adopted and 

 used the C. G. S. system. The words quoted belong to a syl- 

 labus WTitten by Dr. Ball for the session 1874-75. I entered on 

 my duties after the commencement of that session, and my name 

 was attached to new editions of the syllabus instead of the 

 rame of its author, while the part of the syllabus relating to 

 naechanics remained untouched. I had been always under the 

 impression that Prof. Barrett was perfectly aware that I was not 

 the author of this syllabus, and although technically it might be 

 regarded as the syllabus of applied mathematics in the College 

 until a new one could be prepared and published with the sanc- 

 tion of the Science and Art Department, it stems scarcely 

 correci in a scientific discussion to quote it as expressive of the 

 views of a person who was well known not to be its author. 



Prof. Barrett, in his first letter, laid much stress on the intro- 

 duction of spring dynamometers into Dr. Ball's courses on 

 mechanics for the estimation of force in absolute measure ; as if 

 such an employment of these instruments was entirely new. It 

 is but just to observe that dynamometers of the same kind, and 

 graduated in the same way, have been long since employed in 

 other courses of mechanics, and such instruments are figured 

 and described in some of the most common elementary books 

 used in the colleges of Europe. With reference to the dynami- 

 cal units which I prefer to employ in my courses of mechanics, 

 Prof. Barrett uses the phrase, "a mixed system of kilogram- 

 meters and foot-pounds." I never mix the two kinds of units. 

 I keep them perfectly distinct. I employ both, because in the 

 practical applications of mechanics, students may be called upon 

 to apply one or the other. As far as I have been able to ascer- 

 tain, these are the units in most general use among engineers 

 throughout the world ; and I should as soon expect mechani- 

 cians to adopt the C. G. S. system as to hear that bankers 

 adopted our smallest coin as the unit of account instead of the 

 sovereign, and to see the j: rices of stocks in the money market 

 no longer quoted in pounds but in farth ngs. 



Royal College of Science for Ireland 



Henry Hennessy 



