Mr. G. J. Stoney on the Experiment of Mahomet's Coffin. 189 



is drawn to the nearer end, precisely equal quantities of both 

 kinds being simultaneously evolved. Now, if the strip be far off, 

 in which case the positive electricity of the knob will act with 

 very nearly equal effect upon the two ends of the strip, positive 

 electricity will escape from the sharp end more freely than nega- 

 tive from the blunter end. The strip will therefore become ne- 

 gatively charged and be attracted. On the other hand, if the 

 strip be very near the knob, the difference between the forces 

 with which the electricity of the knob acts upon the nearer and 

 further ends will become so great that the negative electricity 

 will be made to flow inwards, in spite of the bluntness of the 

 inner end, more copiously than positive electricity escapes out- 

 wards. Here, then, the strip would become positively charged 

 and be repelled. There is therefore an intermediate distance at 

 which neither the repulsion nor the attraction preponderates; and 

 at this distance the strip in the first instance places itself, or at 

 least would place itself if it were devoid of weight. 



The gold-leaf may be regarded as acted on by three forces : — 

 the relief of atmospheric pressure due to the attraction of the 

 electricity of the knob upon the negative electricity of its nearer 

 end (this is a force directed towards the knob) ; the similar relief 

 of atmospheric pressure due to the repulsion of its positive elec- 

 tricity, which is a force acting outwards from the knob ; and the 

 weight of the gold acting downwards. If the two former forces 

 lay in a right line, the weight of the gold-leaf could not be sup- 

 ported. We must therefore inquire into the disposition of the 

 lines of force in the neighbourhood of the knob of a charged jar. 



This may be done by first considering the form of the equi- 

 potential surfaces*, to which, as is known, the lines of force are 



* If an electrified body were brought near the knob of a charged jar, it 

 would by induction alter the distribution of electricity in the jar. But we 

 may conceive a Leyden jar to be first charged, and that then the coatings, 

 stem, and knob become perfect non-conductors. In this hypothetical case 

 the approach of other electrified bodies would not alter the charge in the 

 jar. Let us suppose that the charge in the jar is positive, and that a par- 

 ticle charged with one unit of positive electricity is taken from a position 

 in which it is not acted on by any electrical forces, and brought up to a 

 point P in the neighbourhood of the knob. The repulsion of the charge 

 in the jar of course opposes this motion ; and it will therefore require the 

 expenditure of a certain amount of mechanical energy, or, as I should prefer 

 to call it, labour, to overcome the electrical repulsion. This labour may 

 be expressed in foot-pounds, kilogrammetres, or other more convenient 

 units. The number which expresses the amount of labour that we must 

 apply in order to carry the particle up to the point P is what is called the 

 potential at that point of the electricity in the jar. 



If all the points be found which require the same expenditure of labour 

 as P does to carry the unit particle of electricity from a neutral position up 

 to them, they will all lie on a surface which is called the equipotential sur- 

 face passing through P. 



