Recent Advances in Electricity, 19 



imagination along one filament from end to end, the force 

 varies inversely as the cross section of the filament ; hence the 

 tension and pressure vary inversely as the square of the section. 

 In the case of a liquid, or a gas, like the atmosphere, we must 

 understand by tension a diminution of pressure, so that in a 

 space pervaded by electrical force it is no longer true that a 

 gas or liquid exerts equal pressure in all directions at a point ; 

 on the contrary, it exerts minimum pressure in the direction 

 of a line of electrical force, and maximum pressure in all 

 directions at right angles to this. Such a distribution of 

 mutual forces can be shown to be consistent with itself, and 

 consistent with thej equihbrium of the fluid. Any one who 

 wishes for satisfaction on this point will find a very compact 

 and intelligible proof in Prof. J. J. Thomson's recent book, 

 " Elements of the Mathematical Theory of Electricity and 

 Magnetism." He supposes a filament, or " tube of force," cut 

 across by two sections near together, and shows that the 

 forces which act upon the whole surface of the intercepted 

 portion of the filament form an equilibrating system. The 

 energy of a charged leyden jar, or of any system of charged 

 conductors separated by an insulating medium, is to be 

 regarded as of the same kind as the energy of a compressed or 

 extended spring, and as residing not in the conductors, but in 

 the intervening medium ; and calculation shows that, when 

 systematic units are employed, the numerical value of the 

 energy, per unit volume, at any point of the medium, is the 

 same as the numerical value of the tension and pressure at 

 this point. This also is proved very neatly and clearly in J. 

 J, Thomson's book. Some kind of elastic yielding must be 

 supposed to accompany the tension and pressure, otherwise no 

 work would be done and no energy stored up. In one and the 

 same substance the yielding is proportional to the force ; but 

 in different substances it is very different for the same force, 

 being proportional to the " specific inductive capacity." The 

 yielding is not a distortion of the substance as a whole, but 

 of its separate molecules. Its exact nature we do not know, 



