1 8 Recent Advances in Electricity. 



another in the non-conducting portion. The best modern 

 measurements have shown that the capacity is about nine 

 times as great when the insulator is glass as when it is air. 

 Faraday rejected as unthinkable the notion of direct attraction 

 and repulsion between two things at a distance from one 

 another, and maintained that they could only act upon one 

 another through the co-operation of the intervening medium. 

 Apparent attraction might be produced by a diminution of the 

 pressure of the medium against the bodies on their near sides, 

 and apparent repulsion by an increase of this pressure. He 

 made great use of the conception of lines of force extending out 

 all round an electrified conductor through the surrounding 

 medium and connecting it with conductors at a distance which 

 are charged with the opposite kind of electricity. These hues 

 are not usually straight, but curved, and at every point in their 

 length the resultant force is in the direction of a tangent. 

 According to Faraday, these lines of force act like stretched 

 strings which are tendmg to pull their two ends nearer 

 together, but, unlike strings, each line repels its neighbours. 

 Or, to state the view more precisely, if we divide up the whole 

 non-conducting material into filaments, running in the 

 direction of the lines of force, each filament is in a condition 

 of longitudinal tension and is at the same time in a condition 

 of lateral compression. These filaments are called tubes of 

 force. 



Clerk Maxwell, the great prophet of the modern school of 

 electrical thought, adopts this view in every particular, and 

 makes it mathematically precise. According to him, if you 

 select any point at random in the air or other insulator 

 through which the lines of force run, the tension at this point 

 in the direction of the line of force is exactly equal to the 

 pressure at the same point in every perpendicular direction. 

 This tension (and its equal pressure) is not, however, the same 

 at all points in the medium, but is greatest where the electrical 

 force, as usually understood, is greatest, being, in fact, propor- 

 tional to the square of the electrical force. If we travel in 



