L, that forms a passage way between the fluid chamber and the dial gauge,
is provided on one side with a small screw hydraulic pump, with a
reservoir supply of fluid. This part is shown in longitudinal section;
the steel plunger, I, is firmly secured to wheel, F, the long hub, H, of
which is provided with a screw thread on its inner side, which thread
screws upon the exterior of pump barrel, K. After first filling the
interior of the pump barrel with fluid, the said hub is screwed upon the
pump barrel, causing the plunger, I, to force the fluid into the fluid
chamber and passage way leading to the dial gauge, causing the hand or
pointer to move to any predetermined pressure on dial, in advance of
pressure applied in the high pressure chamber at D. The purpose
accomplished in this act is to give the least possible movement of the
pointer to record any maximum pressure, as, for example, assuming that
20,000 lb. was the expected pressure from any one explosive, then the
pointer, by the means above described, can be set at, say, 18,000 lb., in
which event the pointer is reduced to the minimum movement of only 2,000
lb. to register 20,000 lb.

It will be evident that much greater accuracy of measurement of maximum
pressures can be obtained by the minimum movement of the pointer, as both
the inertia and the momentum are reduced to the minimum quantity. The
subsidence of pressure resulting from explosives being about as sudden as
the creation of pressure, causes the pointer to move too rapidly for
correct ocular observation, on which account a static electric current is
employed, causing a stream of electric sparks to shoot off from the end
of the pointer, B, to the brass outer ring, M. The gauge is insulated for
that purpose by glass plate, S, which is secured concentrically to the
gauge proper and the ring, M. Binding posts for the electric wires are
provided at O and P, which wires are shown in Fig. 2. A spring clamp, N,
Fig. 2, enables the insertion of chemically prepared or other paper,