374 
VELOCITY AND PRESSURE INSTRUMENTS. 
shall sever the primary current, and thereby produce a spark from the secon¬ 
dary, is shown in Plate VI. Pigs. 1, 2, and 3, representing a longitudinal 
and a transverse section of the bore, B } of the gun, along which the shot, A , 
is moving. 
A hollow plug, (7, is screwed into the gun, carrying at the end next the 
bore a cutter, B, which projects slightly into the bore. The cutter is held 
in this position by the primary wire, e, which passes in at one side of the 
plug, C, then through a hole in the cutter, B, and out again at the other 
side of the plug. The two ends of this wire are connected with the main 
wires leading to the instrument when the plugs have been fixed in this gun. 
When the shot. A, is fired, it presses the cutter into the second position 
as shown in the figure, thereby severing the primary wire, and causing the 
induced spark instantaneously to pass from the discharger to the disc, its 
passage being marked by the spot left upon the edge, as already described. 
To prevent a possibility of the cutter being forced down by any gas that 
may escape past the projectile, a safety-pin, f } secures the cutter firmly in 
its place. This safety-pin is cut simultaneously with the primary wire. 
The mode of reading the observations is as follows :—After an experiment 
has been made, and the series of sparks has been found on the discs, the 
mark on No. 1 disc is, by means of a micrometer-screw, brought precisely 
opposite to the point of the discharger; the vernier, V y is then attached to 
the extremity of the shaft, S, is firmly fixed there, and is set at zero. 
The spark of No. 2 disc is then brought precisely opposite to its dis¬ 
charger, the vernier-reading is recorded, and so with the other discs in 
succession. 
It is obvious that the results furnished by an instrument intended to 
measure the very small intervals of time necessary for the investigations in 
which the Committee have been engaged, would be liable to be received 
with great suspicion, were there no means of testing the accuracy of the 
instrument; accordingly, one of the chief objects in its design was to arrange 
that the accuracy of the indications could at any time be tested with facility. 
Each disc, discharger, and induction coil, form, so to speak, an inde¬ 
pendent instrument for recording the spark when the primary wire is 
broken; and it is obvious that, if the whole of the primaries are cut simul¬ 
taneously, the sparks on all the discs should be in a straight line, and the 
deviations from a straight line—that is, from an absolutely simultaneous 
record—give the instrumental errors. 
Great difficulties have, however, been experienced in securing a simul¬ 
taneous rupture of the primary wires, and the method found most satisfactory 
has been to connect each circuit with a fine insulated wire, and to wind each 
of these fine wires tightly round the body of a detonator, containing about 
100 grs. of fulminate of mercury. On firing the detonator by electricity, 
tbe whole of the wires may be cut nearly simultaneously, and the action of 
the instrument examined. 
The following table shows the mean errors of several consecutive readings 
of the instrument, the primary wires having been severed in the manner 
described; and it will be observed how exceedingly slight they are. It is 
also more than probable that, small as these errors are, they must be in 
great part ascribed to the impossibility of securing an instantaneous rupture 
of the currents in all the wires. 
