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.
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