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Popular Science Monthly/Volume 7/October 1875/Correspondence

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 describes an experiment intended to demonstrate the proposition that electricity of high tension travels through the substance of a conductor independently of its superficies.

Without questioning the general truth of this proposition, I would call attention to one or two flaws in the author's demonstration.

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 deduces his theorem.

This experiment seems to me unsatisfactory, 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 argument will hold if the effect be ascribed mainly to heat, since but little, comparatively, 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 electricity was at rest [static), the gold-leaf was quite capable of receiving as heavy a charge as the most powerful machine could impart."

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 attraction exercised by the electricity induced upon the exterior. The gold-leaf, in connection 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.
Springfield, Mass., August 17, 1875.
 

To the above Prof. Phin replies as follows:

"1. The first objection is to the experiment 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 mechanical 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 delicate strip of gold-leaf, though the passage of a spark, even without the aid of a Leyden jar, will destroy a strip three-eighths of an inch wide."