Page 512 - My FlipBook

242 THE TECHNICAL PKOCEDUEES IN FILLING TEETH. :

in their sockets. They give more to the blow, reducing its
pounds force. In using hand pressure, these differences are
of no effect.
In one sense this may be regarded as a crude form of experi-
ment, in which one essential element, the resistance, is not accu-
rately measured. It is a comparison between the work done
by a weighed thrust and by a blow. It is intended to show the
difference between jiounds pressure discharged by a blow at the
instant of contact, and foot pounds, or the motion produced in
the thing struck. Physicists have generally regarded the meas-
urement of this pressure at the instant of contact as practi-
cally impossible because of its very short duration. Instruments
can be built, however, that will measure it very accurately.
This particular plan of measurement is used here because
it may be better illustrated and therefore better understood
than results detailed in figures derived from more accurate
plans of experimental study. Impressions in metals made by
falling weights can be measured in microns, or in ten thou-
sandths of an inch, giving much more accurate results, but even
these are not entirely free from error. So far as the main facts
under which we must work are concerned, this plan of measure-
ment declares them with sufficient accuracy to guide us in the
selection of instrument points and in determining their relation
to the pounds force we may employ.
An area of gold, which, in its condensation, will require
fifteen pounds stress, is not more than .785 sq. mm., or the area
of a plugger that is 1 mm. in diameter. It will be seen by the
table given above that this instrument, 1 mm. in diameter,
requires 40 pounds to bury the cardboard to a level with the
surface of the block. The diameter and the area of the points
are given in the table, and the pounds force developed by four
mallets, each of specified weight, each falling from a specified
height for each instrument.
At the ratio of 15 to 40 pounds, the pounds force required
for the condensation of gold is represented in the following table
Diameter of points
in millimeters. . . 0.5 0.75
Area of points 1963
Pounds stress 3.75
Careful trials at the chair by a number of assistants well
trained in the use of the mallet in condensing gold, have shown
that a blow of twenty-five pounds is seldom exceeded without
an unusual effort to strike hard blows. Forty pounds was