518 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



complete without reference to these terms, and Professor Thom- 

 son's definitions can hardly be improved. 



" Electric Density. — This term was introduced by Coulomb to 

 designate the quantity of electricity per unit of area in any part 

 of the surface of a conductor. He showed how to measure it, 

 though not in absolute measure, by his proof-plane. 



tc Resultant Electric Force. — The resultant force in air or other 

 insulating fluid in the neighbourhood of an electrified body is 

 the force which a unit of electricity concentrated at that point 

 would experience if it exercised no influence on the electric dis- 

 tributions in the neighbourhood. The resultant force at any 

 point in the air close to the surface of a conductor is perpendi- 

 cular to the surface, and equal to ^irp, if p designates the elec- 

 tric density of the surface in the neighbourhood. 



" Electric Pressure from the Surface of a Conductor balanced by 

 Air. — A thin metallic shell or liquid film, as for instance a soap- 

 bubble, if electrified, experiences a real mechanical force in a 

 direction perpendicular to the surface outwards, equal in amount 

 per unit of area to 27T/3 2 , p denoting, as before, the electric density 

 at the part of the surface considered. In the ease of a soap- 

 bubble its effect will be to cause a slight enlargement of the 

 bubble on electrification with either vitreous or resinous electri- 

 city, and a corresponding collapse on being perfectly discharged. 

 In every case we may consider it as constituting a deduction from 

 the amount of air-pressure which the body experiences when 

 unelectrified. The amount of deduction being different at dif- 

 ferent parts according to the square of the electric density, its 

 resultant action on the whole body disturbs its equilibrium, and 

 constitutes in fact the resultant electric force experienced by the 

 body." 



49. Tension. — The use of this word has been intentionally 

 avoided by us in this treatise, because the term has been some- 

 what loosely used by various writers, sometimes apparently ex- 

 pressing what we have called the density, and at others diminu- 

 tion of air-pressure. By the most accurate writers it has been 

 used in the sense of a magnitude proportional to potential or 

 difference of potentials, but without the conception of absolute 

 measurement, or without reference to the idea of work essential 

 in the conception of potential. We believe also that it has not 

 been generally, if ever, applied to that condition of an insulating 

 fluid in virtue of which each point has an electric potential, 

 although no sensible quantity of electricity be present at the 

 point. The expression " tension " might be used to designate 

 what we have termed the potential of a body. The tension be- 

 tween two points would then be equivalent to the electromotive 

 force between those points, or to their difference of potentials, 

 and would be measured in the same unit. 



