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FORCE USED IN FILLING TEETH. 239

be done. They must not be too strong nor too light or they will
fail to fulfill the purpose intended. In each case the area of the
plugger point must be fitted to the pounds pressure of the blow.
The instrument we use, and which receives and transmits the
blow of the mallet, communicates the accumulated force of the
mallet, less the retarding power of its own weight, to the con-
densation of the gold. It is the stress between the instrument
point and the gold that we employ.
The impact or force with which the instrument strikes the
gold may be measured in pounds or kilograms stress in several
different ways. For the purpose of illustration, we may use:
(1) a thrust dynamometer to measure the force of thrusts or
pressure in pounds or kilograms (2) a falling weight apparatus
;
for obtaining a variety of uniform blows by mallets of definite
weights falling from definite heights; (3) instriunents with
points of definite area; (4) a specified cardboard and boxwood
blocks cut from one piece of the wood. This prepared wood may
be placed on an anvil to furnish a definite resistance. The instru-
ments used are turned to very nearly the size required and then
tempered. After tempering, they are ground to exact size and
the ends perfectly squared. The edges are made sharp. Figures
311, 312. The sizes of the points used are 0.5, 0.75, 1.0, 1.25,
1.5, 2.0, 2.5 millimeters in diameter. All are roimd.
The instrument measuring 2.5 millimeters in diameter is
placed in the instrument sleeve of the falling weight apparatus.
Figures 313, 314, and a mallet that weighs approximately three
ounces (93.270 grams), let fall 85 centimeters (33.5 inches) and
strike the instrument. Under the instrument point is a polished
block of boxwood, the fiber of which is on end, and over this a
piece of hard cardboard 1-100 inch thick. The boxwood block
rests on an anvil. The effect of the blow is to cut a clean hole
in the cardboard and implant the piece cut out in the wood
exactly level with its surface. This blow reckoned in foot pounds,
with its result spread over one second of time, would generate
a force of a little more than five-tenths of a foot pound. Now
transfer the same boxwood block and the same cardboard to
the thrust dynamometer. Figure 315, and with a precisely simi-
lar instrument point measure the pressure necessary to produce
a similar result. This is found to require 250 pounds pressure.
This is the real force of the blow at the instant of contact.
Place the instrument 2 millimeters in diameter in the falling
weight apparatus and let the same mallet fall from a height
of 44.8 centimeters (17.5 inches) under otherwise the same con-