The electrical target usually employed in determining velocities of
projectiles consists of a wooden frame on which is strung a copper
wire so as to make a continuous circuit arranged in parallel vertical
lines about one inch or one and one half inches apart.
It frequently happens that a projectile will pass through this target
without breaking the circuit, either by squeezing between the wires or
because, when last repaired, the target was short-circuited unnoticed,
so that the cutting of the wires did not break the circuit. The repair
of this target takes considerable time.
_______________________________________________________
| {
| +-------------__ --------------__---------------- }
| | _ // \\ // \\ } {
| | |_| || C_{0} || A || || || A { }
| | P \\ // \\ // } {
| +------------- --------------- ---------------- }
| F {
|_______________________________________________________}
Plan.
P C
=|= _________
|===| ========= A A
========| | S |========\_______/=================
|spring | | |
| | | | |
|S_ | __| |__ __|
||| | |
___________|||______________| |_____________________
|
| Section.
H /
\+/
|
|
|
|
|
|
_____+_____
| |
| W |
| |
|___________|
Besides these objections to this target, another and more serious one
is the irregularity in the manner of breaking the circuit. It has been
proved that times required for a flat headed and an ogival headed
projectile to rupture the current are very different.
To remedy these defects a new and very ingenious target has been
devised and used with great success at the United States Military
Academy at West Point. The top of the target is a wooden strip, F, on
the upper side of which are screwed strips of copper, A A, about 1/2
in. wide, and 1/8 in. thick. The connection between two adjoining
strips is made by a copper cartridge, C, which is dropped in a hole in
the frame bored to receive it. This cartridge is the one used in the
Springfield rifle. Inside the cartridge is a spiral spring, S, which,
acting on the bottom of the hole and the head of the cartridge, tends
to make the latter spring up, and so break the circuit.
To the hook, H, which is attached to the cartridge, is suspended, by
means of a string, the lead weight, W, thus drawing down the cartridge
and making the circuit between A and A'. All the weights being
suspended the current comes in through the post, P, passes along the
copper strips and out of the corresponding post on the other end.
On firing the projectile cuts a string, and the spring at once causes
the cartridge to spring up, thus breaking the circuit.
It is not possible for the projectile to squeeze between the strings
and not break the current, for in so doing the cartridge is tipped
slightly, which is sufficient, as it breaks the current on one side.
This target is used in connection with the Boulenge chronograph. Two
targets are established at a known distance apart, say 50 ft., and the
time required for the projectile to pass over this distance is
determined by finding the difference in the time of cutting of the two
targets, by finding the difference in the time of falling of the two
rods, caused by the demagnetization of two electromagnets in the same
circuit with the targets.
By means of a disjunctor both rods are dropped at the same time, the
shorter one releasing a knife blade which makes a cut on the longer
one. Now both rods are hung from the magnets again and the gun is
fired.
The projectile passes through the first target, breaks the circuit,
demagnetizes the magnet of the longer rod, and it begins to fall. On
passing through the second target, the projectile causes the shorter
rod to fall. This releases the knife blade, and a second cut is made.
The time corresponding to the distance between these cuts is the time
the longer rod was falling before the second rod began to fall or the
time between the cutting of the two targets by the projectile.
The distance between the cuts is measured, and the time corresponding
to it can easily be found. Then the velocity of the projectile is
equal to 50/t.
To repair this target, strings are prepared in advance of suitable
length and looped at both ends, so that by placing the hook of the
cartridge in one loop and that of the weight in the other the repair
is quickly made.
This target has been used on the West Point proving ground to
determine velocities over distances of 100 ft. interval to distances
of only 9 ft. interval, and has given most satisfactory results.
* * * * *
[Continued from SUPPLEMENT, No. 786, page 12566.]