146 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XIII.) 



in the same media if confined in tubes of glass, shell-lac, sulphur, or other such sub- 

 stances, though they be excellent insulating dielectrics ; nor can they be expected ; 

 for in such cases, the polar forces, instead of being all dispersed amongst fluid par- 

 ticles, which tend to move under their influence, are now associated in many parts 

 with particles that, notwithstanding their tendency to motion, are constrained to re- 

 main quiescent. 



1576. The varied circumstances under which, with conductors diff'erently formed 

 and constituted, currents can occur, all illustrate the same simplicity of production. 

 A hall, if the intensity be raised sufficiently on its surface, and that intensity be 

 greatest on a part consistent with the production of a current of air up to and off" 

 from it, will produce the effect like a point (1537.) ; such is the case whenever the 

 glow occurs upon a ball, the current being essential to that phenomenon. If as large 

 a sphere as can well be employed with the production of glow be used, the glow will 

 appear at the place where the current leaves the ball, and that will be the part 

 directly opposite to the connection of the ball and rod which supports it; but by in- 

 creasing the tension elsewhere, so as to raise it above the tension upon that spot, 

 which can easily be effected inductively, then the place of the glow and tlie direction 

 of the current will also change, and pass to that spot which for the time is most fa- 

 vourable for their production (1591.). 



1577. For instance, approaching the hand towards the ball will tend to cause 

 brush (1539.), but by increasing the supply of electricity the condition of glow may 

 be preserved ; then on moving the hand about from side to side the position of the 

 glow will very evidently move with it. 



1578. A point brought towards a glowing ball would at twelve or fourteen inches 

 distance make the glow break into brush, but when still nearer glow was reproduced, 

 probably dependent upon the discharge of wind or air passing from the point to the 

 ball, and this glow was very obedient to the motion of the point, following it in every 

 direction. 



1579. Even a current of wind could affect the place of the glow ; for a varnished 

 glass tube being directed sideways towards the ball, air was sometimes blown through 

 it at the ball, and sometimes not. In the former case, the place of the glow was 

 changed a little, as if it were blown away by the current, and this is just the result 

 which might have been anticipated. All these effects illustrate beautifully the ge- 

 neral causes and relations, both of the glow and the current of air accompanying it 

 (1574). 



1580. Flame facilitates the production of a current in the dielectric surrounding 

 it. Thus, if a ball which would not occasion a current in the air have a flame, whether 

 large or small, formed on its surface, the current is produced with the greatest ease; 

 but not the least difficulty can occur in comprehending the effective action of the 

 flame in this case, if its relation, as part of the surrounding dielectric, to the electrified 

 ball, be but for a moment considered (1375. 1380.). 



