302 On Lightning Conductors to Powder Magazines. [No. 99. 



course, a favorite illustration of his as it occurs in more 

 than one part of his published writings. Practically, it is 

 but of little consequence whether the conductor be active or 

 passive ; but of all the substances excited at the moment — en- 

 gaged in the vast induction we have described — of the cloud, 

 the air, the earth, and the things on its surface— the lightning 

 rod is that in which the induction is the most powerful, and 

 towards which the explosion is therefore the most likely to 

 occur. Call it passive, if Mr. Daniell so pleases, but the electric 

 fluid is more active in it than any where else. The discharge 

 takes place ; — the first instalment, or the head of the column 

 rushing to the point of the conductor, heats the air through 

 which it passes — rarifies it, and diminishes the resistance to the 

 outpouring of the rest of the electric accumulation. The excess, 

 unable in a unit of time to pass over the bar, rushes to sur- 

 rounding objects.* Did it occur to Mr. Daniell that no pru- 

 dent man builds his house by preference on the bank of a 

 mountain water-course ? The water-course is doubtless passive, 

 and it will quietly and silently carry off the stream of an ordinary 



* The Board are requested to consider Mr. Faraday s opinion on this 

 point : — 



" The fact however is, that disruptive discharge is favorable to itself. It 

 is at the outset a case of tottering equilibrium, and if time be an element in 

 the discharge, in however minute a proportion, then the commencement of the 

 act at any point favors its continuance and increase, and portions of power 

 will be discharged by a course which otherwise they would not have taken. 



" The mere heating and expansion of the air itself by the first portion of elec- 

 tricity which passes, must have a great influence in producing this result. 



" As to the result itself, we see its effect in every electric spark, for it is not 

 the whole quantity which passes that determines the discharge, but merely 

 that small portion of force which brings the deciding molecule up to 

 its maximum tension ; then when its forces are subverted, and discharge 

 begins, all the rest passes by the same course from the influence of the favor- 

 ing circumstances just referred to, and whether it be the electricity on a 

 square inch or a thousand square inches of charged class, the discharge is 

 complete. Hereafter we shall find the influences of this effect in the for- 

 mation of brushes, and it is not impossible that we may trace it producing 

 the jagged spark, and forked lightning." Faraday's Experimental Researches, 

 p. 451. para. 14, 17—20. 



