[From ms. op Science anu Arts, Voi,. I Jan. 1871.1 
ON THE DURATION OF FLASHES OF LIGHTNING. 
By OGDEN N. ROOD. 
After the completion of mv first set of experiments on the 
duration of the discharge of a Leyden jar, I became anxious to 
make some measurements of the duration of a flash of ordinary 
lightning, which may be considered as equivalent to the dis- 
charge of an immense jar with an enormous striking-distance. 
The results of Feddersen have shown that the duration of the 
discharge is increased by an addition to the size of the jar, as 
well as by augmentation of the striking-distance, and as both 
these quantities are so large with a flash of lightning, it was 
reasonable to expect that the duration of its discharge would 
be prolonged in some corresponding ratio. During the violent 
thunder-storm of last August, which occurred in the evening, 
I happened to be at a house commanding an unobstructed view 
of the horizon, and this circumstance taken in connection with 
the frequency and proximity of the electrical discharges, in- 
duced me, although entirely unprovided with apparatus, to at- 
tempt a measurement of their duration. A circular disc, five 
inches in diameter, was hastily cut from white cardboard, while 
a steel shawl-pin served as an axis, on which it was made to 
revolve by constantly striking its edge tangentially with the 
right hand, the pin being held in the left. The maximum veloc- 
ity attainable in this way was always employed. The general 
indications at the time were that the rate thus obtained was 
considerably more uniform than might have been expected, 
and subsequent quantitative experiments have confirmed this 
idea. The first experiments were made by observing black 
figures traced near the circumference of the disc, which was 
illuminated solely by the rapidly recurring flashes, and it often 
happened that the figures, with their details, were seen quite as 
clearly and sharply as though the disc had been stationary; on 
the other hand, sometimes the edges seemed blurred, as though 0. N. Rood on the duration of Flashes of Lightning. 
2 
the disc had moved through a few degrees during the act of 
discharge. The result being doubtful, the mode of experi- 
menting was quickly changed; about fifteen narrow radial 
apertures were made near the circumference of the disc, and 
the flashes and illuminated clouds were observed through these 
openings, the disc being made to revolve as before. The dis- 
tance of the eye from the apparatus was about eight inches, 
and it was of course adjusted so as to obtain distinct vision of 
the disc. The result was that sometimes the openings were 
seen quite unchanged in appearance, but more frequently they 
were most distinctly elongated into well defined streaks some 
degrees in length. They were observed often and without dif- 
ficulty, but as farther confirmation I may add that I requested 
Prof. Joy, who was ignorant of the actual form of the aper- 
ture, to state his opinion of their apparent shape while the disc 
was in rotation. The reply was that they resembled Prince 
Rupert’s drops—a not unfair description of the phenomena in 
question. Repeated estimates of their size were then made 
with paper and pencil. Some time afterward I measured the 
velocity which I could communicate to this disc in the manner 
above described, by attaching to it a small hollow axis through 
which the steel pin passed, the disc being then caused to wind 
up a thread stretched by a small weight. The rate of rotation 
thus attainable was found to be about twelve revolutions per 
second, which is a little more than I had anticipated. The 
average size of the streaks was 9°, corresponding to a duration 
°f vItt of a second. It hence results that the duration of the 
flashes of lightning on the occasion referred to was in round 
numbers about of a second, some of them, however, seem- 
ing to be confined to smaller limits. 
I know of only a single circumstance which might militate 
against the correctness of the above conclusion, and it is but 
fair to give it such weight as it may carry. Becquerel has 
succeeded with some difficulty in observing a faint phospho- 
rescence when an electric discharge is passed through rarefied 
air, and it is not absolutely impossible that the effects observed 
by me were due to a cause of this kind. 
This point can hereafter readily be decided by observing with 
a revolving disc, not the distant clouds, but a sheet of white 
paper, placed so as to receive the light from the electrical flashes. 
Columbia College, Nov. 10th, 1870. ,