ABSTRACT
In the early part of last December I called upon a friend, who showed me an improved form of induction-coil, which, from the peculiarity of its construction, was capable of giving much more brilliant results than instruments of the same size made in the ordinary way. In the course of a number of experiments with this coil, my friend held a card in the path of the sparks between the terminals ; and although these were several inches apart at the time, every spark passed through the card, making the well-known raised burr round each perforation. This done, he tossed the card to me, saying in joke, “There, I’ll make you a present of that as a memento.” On reaching home, my microscope being at hand, I placed the card upon the stage to see what might be the microscopic peculiarities, if any, of the burrs surrounding the perforations. My attention was, however, at once arrested Ly observing that the shape of the holes themselves was not circular, as might have been expected, but clearly and sharply pentagonal.* Many holes were filled up by portions of disrupted fibre which had fallen into them; others had been made in so oblique a direction that their actual shape could not be very well made out ; but the remainder—some thirty in number—were, as I have stated, five-sided ; and the question at once arose, to what cause is this peculiarity of shape due ?
Supposing the perforations to be six-sided, as they appear to us, and not five-sided, as Mr. Lewis thinks they are, an explanation is not so difficult. The shape of the spark itself is, in all probability, that of a more or less regular cylinder, -whose section is a circle. The resistance offered to the passage of the spark by the perforated paper or card, acts at the point of passage on its cylindrical form in a manner analogous to that in which equal pressure from all sides acts on a solid cylinder, rendering it hexagonal; though the spark must not be regarded as anything but a condition of the atmosphere. The cases of the basaltic pillars of the Giant’s Causeway and of agglomerated soap-bubbles are well-known instances of this law of pressure.—ED.
