322
XX
LIGHT AND ELECTRICITY
denied the continuous tones of pipes and strings, we could get
a poor kind of music by striking strips of wood.
It almost
We now have indicators for which the thirty-thousandth
part of a second is not too short. But these would be of little
use to us if we were not in a position to actually perceive
their action up to the distance under consideration, viz. about
ten metres. This can be done by very simple means. Just
at the spot where we wish to detect the force we place a con-
ductor, say a straight wire, which is interrupted in the middle
by a small spark-gap. The rapidly alternating force sets the
electricity of the conductor in motion, and gives rise to a
spark at the gap.
The method had to be found by experience,
for no amount of thought could well have enabled one to
predict that it would work satisfactorily. For the sparks are
microscopically short, scarcely a hundredth of a millimetre
long; they only last about a millionth of a second.
seems absurd and impossible that they should be visible; but
in a perfectly dark room they are visible to an eye which has
been well rested in the dark. Upon this thin thread hangs the
success of our undertaking. In beginning it we are met by a
number of questions. Under what conditions can we get the most
powerful oscillations? These conditions we must carefully
investigate and make the best use of. What is the best form
we can give to the receiver? We may choose straight wires
or circular wires, or conductors of other forms; in each case
the choice will have some effect upon the phenomena. When
we have settled the form, what size shall we select?
We soon
find that this is a matter of some importance, that a given
conductor is not suitable for the investigation of all kinds of
oscillations, that there are relations between the two
which remind us of the phenomena of resonance in acoustics.
And lastly, are there not an endless number of positions in
which we can expose a given conductor to the oscillations?
In some of these the sparks are strong, in others weaker, and
in others they entirely disappear. I might perhaps interest
you in the peculiar phenomena which here arise, but I dare
not take up your time with these, for they are details-
details when we are surveying the general results of
an investigation, but by no means unimportant details to
the investigator when he is engaged upon work of this kind.