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321
LIGHT AND ELECTRICITY
adequate means at our disposal for indicating with sufficient
sharpness the beginning and end of such a short interval.
If we wish to measure a length correctly to the tenth part
of a millimetre it would be absurd to indicate the beginning
of it with a broad chalk line. If we wish to measure time
correctly to the thousandth part of a second it would be absurd
to denote its beginning by the stroke of a big clock. Now
the time of discharge of a Leyden jar is, according to our
ordinary ideas, inconceivably short. It would certainly be
that if it took about the thirty-thousandth part of a second.
And yet for our present purpose even that would be a thousand
times too long. Fortunately nature here provides us with a
more delicate method. It has long been known that the dis-
charge of a Leyden jar is not a continuous process, but that,
like the striking of a clock, it consists of a large number of
oscillations, of discharges in opposite senses which follow
each other at exactly equal intervals. Electricity is able to
simulate the phenomena of elasticity. The period of a single
oscillation is much shorter than the total duration of the dis-
charge, and this suggests that we might use a single oscillation
as an indicator. But, unfortunately, the shortest oscillation
yet observed takes fully a millionth of a second. While such
an oscillation is actually in progress its effects spread out
over a distance of three hundred metres; within the modest
dimensions of a room they would be perceived almost at the
instant the oscillation commenced. Thus no progress could
be made with the known methods; some fresh knowledge was
required. This came in the form of the discovery that not
only the discharge of Leyden jars, but, under suitable con-
ditions, the discharge of every kind of conductor, gives rise to
oscillations. These oscillations may be much shorter than
those of the jars. When you discharge the conductor of an
electrical machine you excite oscillations whose period lies
between a hundred-millionth and a thousand-millionth of a
second. It is true that these oscillations do not follow each
other in a long continuous series; they are few in number
and rapidly die out. It would suit our experiments much
better if this were not the case. But there is still the possi-
bility of success if we can only get two or three such sharply-
defined indications. So in the realm of acoustics, if we were
M. P.
Y