MEASURING THE VELOCITY OF A SHELL
In at least two of the preceding chapters of this book reference has
been made to the speed at which a shell fired from a gun travels through
the air. Such velocities as 3,000 feet per second have been mentioned in
this connection, and some readers are sure to have wondered how such
measurements could possibly be made. Possibly some sceptics have even
supposed that they were not measured at all but simply estimated in some
way or other. They are actually measured, however, and by very simple
and ingenious means.
Needless to say, electricity plays a very important part in this
wonderful achievement. In fact, without the aid of electricity it is
difficult to see how it could be done at all.
People often ask how quickly electricity travels, as if when we sent a
telegraph signal along a wire a little bullet, so to speak, of
electricity were shot along the wire like the carriers of the pneumatic
tubes in the big drapers' shops. That is quite a misconception, for in
reality the circuit of wire is more like a pipe full of electricity, and
when we set a current flowing what we do is to set the whole of that
electricity moving at once. If we think of a circular tube full of water
with a pump at one spot in the circuit, we see that as soon as the water
begins to move anywhere it moves everywhere. Moreover, if it stops at
one point it stops simultaneously at every other point. While
practically this is the case it is theoretically not quite so, for the
inertia of the water when it is suddenly started or stopped no doubt
causes a slight distortion of the tube itself resulting in a very slight
(quite imperceptible) retardation of the movement of the water.
Electricity also has a property comparable to the inertia which we are
familiar with in the objects around us, and there is also a property in
every conductor which to a certain extent resembles the elasticity of
the water-pipe, whereby it may for a moment be bulged out. In a short
wire, however (up to a mile or so), particularly if the flow and return
parts of the circuit be twisted together, this electrical inertia
practically vanishes and consequently we may say that for all practical
purposes the current starts or stops, as the case may be, at precisely
the same moment in every part of the circuit.
That fact is of great value when, as in the case we are now discussing,
we want to compare very exactly two events occurring very near together
as to time but far apart as to place.
[Illustration: BOMB-THROWERS AT WORK.
Many kinds of bombs are used. One has a metal head and a handle about a
foot long, with a streamer to ensure correct flight; another form
resembles a brush when it is flying through the air; and a third, known
as "the egg," is oval in form.]
We need to compare the time when the shell leaves the gun with the time
when it passes another point, say, one hundred yards away, and then
again another point, say one hundred yards further on still. Supposing,
then, a velocity of 3,000 feet per second, the time interval between the
first point and the second and between the second and third will be
somewhere about a tenth of a second. So we shall need a timepiece of
some sort which will not only measure a tenth of a second, but will
measure for us a very small _difference_ between two periods, each of
which is only about a tenth of a second and which will be very nearly
alike. That represents a degree of accuracy exceeding even what the
astronomers, those princes of measurers, are accustomed to.
This exceedingly delicate timepiece is found in a falling weight. So
long as the thing is so heavy that the air resistance is negligible, we
can calculate with the greatest nicety how long a weight has taken to
fall through a given distance.
Near the muzzle of the gun there is set up a frame upon which are
stretched a number of wires so close together that a shell cannot get
past without breaking at least one of them. These wires are connected
together so as to form one, and through them there flows a current of
electricity the action of which, through an electro-magnet in the
instrument house, holds up a long lead weight.
At some distance away, say one hundred yards, there is a similar frame
also electrically connected to an electro-magnet in the same instrument
house. This second magnet, when energized by current from the frame,
holds back a sharp point which, under the action of a spring, tends to
press forward and scratch the lead weight. The third frame is likewise
connected to a third magnet controlling a point similar to the other.
To commence with, current flows through all three frames so that all
three magnets are energized. The gun is then fired and immediately the
shell breaks a wire in the first frame, cutting off the current from the
first magnet and allowing the weight to fall. Meanwhile, the shell
reaches the second frame, breaking a wire there, with the result that
the second magnet loses its power, lets go the point which it has been
holding back and permits it to make a light scratch upon the falling
weight. This action is followed almost immediately by a similar action
on the part of the third magnet, resulting in a second scratch on the
lead weight.
The position of these two scratches on the weight and their distance
apart gives a very accurate indication of the time taken by the shell to
pass from the first screen to the second and from the second to the
third. From those times it is possible to calculate the initial velocity
of the shell and the speed at which it will move in any part of its
course. Indeed, with those two times as data, it is possible to work out
all that it is necessary to know about the behaviour of the shell.
This is rendered practicable by the fact that the moment the wire is cut
the magnet lets go, no matter what the distance of the screen from the
instrument may be. But for the instantaneous action of the current,
allowance of some sort would have to be made for the fact that one
screen is farther than another and the whole problem would be made much
more complicated.
Even as it is, someone may urge that the magnets themselves possess
inertia and will not let go quite instantaneously, but that can be
overcome by making the magnets all alike so that the inertia will affect
all equally. It is only necessary to have a switch which will break all
the three circuits at the same moment (quite an easy thing to arrange)
and then adjust all three magnets so that when this is operated they act
simultaneously. After that they can be relied upon to do their duty
quite accurately.
Thus by a method which in its details is quite simple is this seemingly
impossible measurement taken.