-1859] WRITINGS OF JOSEPH HENRY. ool 



also remove the free electricity farther from the centre of 

 attraction of the redundant matter, the tendency to restore 

 the normal condition is much lessened, and a new quantity 

 will be repelled into the lower end from the upper, and thus 

 produce at that end a greater intensity of excitement. If 

 we increase indefinitely the length of the conductor, (or what 

 amounts to the same thing) if we connect the lower end of 

 it by means of a metallic wire or other conductor with the 

 earth or elongate it till it touches the earth, then we shall 

 have the maximum of effect. The neutral point will descend 

 to the earth, while the conductor, thoughout its entire length, 

 will be charged negativeh'. 



The effects which we have described are those which would 

 take place if we supposed the electricity in the globe suffered 

 no change in its distribution on account of the induction ; 

 but this cannot be the case, since in the action of one body 

 on another — an equal re-action must be produced, hence the 

 un-saturated matter in will re-act on the free electricity in 

 the globe, and draw down into its lower side a portion of 

 that which before existed in the upper side, and thus render 

 the lower side more intensely redundant than before. This 

 additional quantity of free electricity in the lower side will 

 tend to increase the amount of un-saturated matter in the 

 upper part of the conductor. The maximum effect will be 

 produced, as we have before stated, when the lower end of 

 the conductor is brought in contact with the earth, which 

 may be considered as a conductor of infinite capacity. In 

 this condition the self-repulsion of the atoms of the fluid in 

 the lower part of the globe, and the attraction of the un- 

 saturated matter in the upper end of the conductor, may 

 become so great as to cause a rupture of the intervening air 

 and a transfer of the redundant electricity in the form of 

 a spark from the upper to the lower body. 



If instead of the metallic conductor we substitute a rod 

 of shellac or glass of the same length and diameter under 

 the same conditions, no spark (or but a very feeble one) will 

 be produced. The natural electricity cannot be driven 

 down on account of the non-conducting character of the 



