The
instrument applied to the determination of the velocity of shot is
described thus by Wheatstone:--"A wooden ring embraced the mouth of
the gun, and a wire connected the opposite sides of the ring. At a
proper distance the target was erected, and so arranged that the least
motion given to it would establish a permanent contact between two
metal points. One of the extremities of the wire of the electromagnet
(before mentioned) was attached to one pole of a small battery; to the
other extremity of the electromagnet were attached two wires, one of
which communicated with the contact piece of the target, and the other
with one of the ends of the wire stretched across the mouth of the
gun; from the other extremity of the voltaic battery two wires were
taken, one of which came to the contact piece of the target, and the
other to the opposite extremity of the wire across the mouth of the
gun. Before the firing of the gun a continuous circuit existed,
including the gun wire; when the target was struck the second circuit
was completed; but during the passage of the projectile both circuits
were interrupted, and the duration of this interruption was indicated
by the chronoscope."

Henry.

Professor Joseph Henry (_Journal Franklin Inst._, 1886) employed a
cylinder driven by clockwork, making ten revolutions per second. The
surface was divided into 100 equal parts, each equal to 1/1000 second.
The time marks were made by two galvanometer needles, when successive
screens were broken by a shot. Henry also used an induction-coil spark
to make the cylinder, the primary of the coil being in circuit with a
battery and screen. This form of chronograph is in many respects
similar to the instrument of Konstantinoff, which was constructed by
L.F.C. Breguet and has been sometimes attributed to him (_Comptes
rendus_, 1845). This chronograph consisted of a cylinder 1 metre in
circumference and 0.36 metre long, driven by clockwork, the rotation
being regulated by a governor provided with wings. A small carriage
geared to the wheelwork traversed its length, carrying electromagnetic
signals. The electric chronograph signal usually consists of a small
armature (furnished with a style which marks a moving surface) moving
in front of an electromagnet, the armature being suddenly pulled off
the poles of the electromagnet by a spring when the circuit is broken
(_Journal