XX
323
LIGHT AND ELECTRICITY
They are the peculiarities of the instruments with which he
has to work; and the success of a workman depends upon
whether he properly understands his tools. The thorough
study of the implements, of the questions above referred to,
formed a very important part of the task to be accomplished
After this was done, the method of attacking the main prob-
lem became obvious. If you give a physicist a number of
tuning-forks and resonators and ask him to demonstrate to
you the propagation in time of sound-waves, he will find no
difficulty in doing so even within the narrow limits of a room.
He places a tuning-fork anywhere in the room, listens with
the resonator at various points around and observes the in-
tensity of the sound. He shows how at certain points this is
very small, and how this arises from the fact that at these
points every oscillation is annulled by another one which
started subsequently but travelled to the point along a shorter
path. When a shorter path requires less time than a longer
one, the propagation is a propagation in time. Thus the prob-
lem is solved. But the physicist now further shows us that
the positions of silence follow each other at regular and equal
distances from this he determines the wave-length, and, if he
knows the time of vibration of the fork, he can deduce the
velocity of the wave. In exactly the same way we proceed
with our electric waves. In place of the tuning-fork we use
an oscillating conductor. In place of the resonator we use
our interrupted wire, which may also be called an electric
We observe that in certain places there are sparks
at the gap, in others none; we see that the dead points follow
each other periodically in ordered succession.
Thus the pro-
pagation in time is proved and the wave-length can be
measured. Next comes the question whether the waves thus
demonstrated are longitudinal or transverse. At a given
place we hold our wire in two different positions with refer-
ence to the wave: in one position it answers, in the other not.
This is enough-the question is settled: our waves are trans-
versal. Their velocity has now to be found. We multiply
the measured wave-length by the calculated period of oscilla-
tion and find a velocity which is about that of light. If
doubts are raised as to whether the calculation is trustworthy,
there is still another method open to us. In wires, as well as
resonator.