14 
On some of the Microscopic Errecrs of the ELEcrRic 
Spark. By R. T. Lewis. 
(Read at the Quekett Microscopical Club, September 28th, 1866.) 
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, say- 
ing 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 
by 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 dis- 
rupted 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? 
A number of curious facts, which were detailed some years 
ago in ‘ Recreative Science,’ and which seemed to bear upon 
the subject, led me at first to suppose that the shape of the 
holes might possibly be due to the sparks having taken a 
definite form from the microscopic shape of the points of the 
terminals from which they had been discharged. I therefore 
perforated some pieces of paper and card by sparks: passed 
between the points of two sewing-needles, also between the 
ends of pieces of copper-wire simply cut from a length and 
without preparation; but in each instance all those holes 
which were clear, and through which the sparks had passed 
in a direction at right-angles with the surface of the paper, 
were, as before, five-sided ; and I afterwards found that the 
effect was the same when wires of different metals were used, 
* From an inspection of Mr. Lewis’s drawings we feel bound to say that 
the perforations appear to us more frequently Aevagonal than pentagonal.— 
D. 
+ Vol. i, p. 188. 
