Strain along the Lines of Force. 677 



considerable theoretical importance was attached to this 

 property of electricity. That they did so regard it as one of 

 the corner-stones of their theory is clearly shown by their own 

 statements. Thus Faraday says (Par. 1297) : — "The direct 

 inductive force, which may be conceived to be exerted in 

 lines between the two limiting and charged conducting 

 surfaces, is accompanied by a lateral or transverse force 

 equivalent to a dilatation or a repulsion of these representa- 

 tive lines ; " and (Par. 1224) : — " The attractive force which 

 exists amongst the particles of the dielectric in the direction 

 of the induction is accompanied by a repulsion or a diverging 

 force in the transverse direction." Maxwell ('Electricity 

 and Magnetism/ vol. i. p. 165) writes even more expli- 

 citly : — " The hypothesis that a state of stress of this kind 

 exists in a fluid dielectric, such as air or turpentine, may at 

 first sight appear at variance with the established principle 

 that at any point in a fluid the pressures are equal. But in 

 the deduction of this principle from a consideration of the 

 mobility and the equilibrium of the parts of the fluid, it is 

 taken for granted that no action such as that which we here 

 suppose to take place along the lines of force exists in the 

 fluid. The state of stress which we have been studying is 

 perfectly consistent with mobility and equilibrium of the 

 fluid, for we have seen that, if any portion of the fluid is 

 devoid of electric charge, it experiences no resultant force 

 from the stresses on its surface, however intense these may be. 

 It is only when a portion of the fluid becomes charged that 

 its equilibrium is disturbed by the stresses on its surface, and 

 we know that in this case it actually tends to move/' 



As is well known, he measures this stress by the famous 



KV 2 



formula p = ^— -y 2 , where p is the numerical value of the 



tension, K the specific inductive capacity of the dielectric, and 

 Y/d the fall of potential per unit length in the dielectric. 



In support of this theory and equation a number of in- 

 vestigators have obtained an expansion both in the volume of 

 an electrified glass thermometer and an elongation of a glass 

 tube electrified radially, and have apparently found these 

 deformations agree with the theoretical formula. When the 

 mechanical is substituted for the electrical pressure, by the 



formula p = F/S = (jl t , we obtain — . ==- = - — . The lef t- 

 l I V o7r/jj 



hand member is the expansion per unit length of the tube 

 under unit electrical conditions at right angles to the lines 

 of force. By taking the average of Can tone's results we get 



