746 



THE POPULAR SCIENCE MONTHLY 



CORRESPONDENCE 



THE FORM OF LIGHTNING-RODS. 



To the Editor of the Popular Science Monthly : 



IN a paper in The Popular Science 

 Monthly for August, entitled " The 

 Form of Lightning-Rods," Prof. Phin de- 

 scribes an experiment intended to demon- 

 strate the proposition that electricity of 

 high tension travels through the substance 

 of a conductor independently of its super- 

 ficies. 



Without questioning the general truth 

 of this proposition, I would call attention 

 to one or two flaws in the author's demon- 

 stration. 



He cites the fact that a moderate charge 

 shatters a strip of gold-leaf, while a stronger 

 one fails to affect a wire having less surface 

 and a greater section. From this he de- 

 duces his theorem. 



This experiment seems to me unsatis- 

 factory, for the reason that a disruptive 

 force may be supposed to be exerted in both 

 cases, but that the superior strength of the 

 wire enables it to resist what destroys the 

 frail gold-leaf. Of course, the same argu- 

 ment will hold if the effect be ascribed 

 mainly to heat, since but little, compara- 

 tively, would suffice to fuse the gold-leaf or 

 even to dissipate it as gas. 



Prof. Phin, referring to this experiment, 

 says : " Here we see that, while the electri- 

 city was at rest [static), the gold-leaf was 

 quite capable of receiving as heavy a charge 

 as the most powerful machine could im- 

 part." 



It doubtless was " capable of receiving 

 the most powerful charge," but the fact is 

 not proved by the experiment, for, in the 

 position in which the gold-leaf was placed, 

 viz., on the knob of the jar, it was not 

 charged at all ! 



The charge must have been collected 

 upon the inner coating, through the attrac- 

 tion exercised by the electricity induced 

 upon the exterior. The gold-leaf, in con- 

 nection with the inside of the jar, was then, 

 properly speaking, no more charged than 

 were objects connected with the outside, 

 e. g., the table, and, to a certain extent, 

 every other object on the surface of the 

 planet. L. H. Andrews. 



SpRiNorrELD, Mass., August 17, 1875. 



To the above Prof. Phin replies as 

 follows : 



" 1. The first objection is to the experi- 

 ment in which a gold wire is shown to be 

 capable of carrying off a discharge which 

 destroys a strip of gold-leaf presenting a 

 far greater surface. Whether we attribute 

 the destroying power to heat or to mechan- 

 ical force, it is a fact that the thin gold-leaf 

 is destroyed while the stouter wire remains 

 uninjured, and this is all that is necessary 

 to be known so far as lightning-rods are 

 concerned. 



" 2. The second objection is, that the 

 gold-leaf in contact with the knob of the jar 

 is not ' charged.' Of course, if the gold-leaf 

 is not charged, the same remark applies to 

 the knob itself; how, then, does it happen 

 that, under such circumstances, the knob 

 will powerfully attract or repel (according 

 to circumstances) a pith-ball brought near 

 it ? Probably the most intense charge of 

 static electricity could be imparted directly 

 from the prime conductor. This we have 

 often done, without injuring the most deli- 

 cate strip of gold-leaf, though the passage 

 of a spark, even without the aid of a Ley- 

 den jar, will destroy a strip three-eighths 

 of an inch wide." 



EDITOR'S TABLE. 



EXPENSIVENESS OF SCIENTIFIC 

 EDUCATION. 



AS we have often said, and as will 

 probably have to be many times 

 repeated and explained before its full 

 meaning is generally appreciated, there 



are two kinds of education, scientific 

 and non-scientific; or one which brings 

 the mind to bear upon actual things, 

 and another which occupies It with 

 their symbols. One turns the intellect 

 directly upon Nature, and aims to train 



