Page 44 - My FlipBook

few pounds as we can with this dynamometer, for the ;
reason that we cannot hold the instrument perfectly square
to the work (demonstrating). I used a Httle too much force,
so that the instrument has gone through into the wood
I used twelve pounds ; about six to seven pounds is necessary

to penetrate that paper with that instrument point. Now, 1
will take one seven and a half tenths millimeters. I had that
just about the right thing (demonstrating), square onto the
work. It takes full fifteen pounds to put the instrument
through the paper. I will say that this is a very small handle
and requires a pretty good grip to make fifteen pounds pres-
sure. I will give the young men a chance. (They make some
failures.) This is one millimeter in diameter (taking another
instrument). It cuts through all right, but it takes twenty-
seven pounds. Now I want you to note these differences in
the size of the instrument points. This is fifteen (ly^ milli-
meters) ; I didn't put it through at full forty pounds. We
can just as well lay the block on the table, for the manudyna-
mometer will not register a sufficient number of pounds. I
have intended in these experiments to call your attention
strongly to the difference in condensing power in instrument
points of different sizes. This difference occurs in the con-
densation of gold precisely as it occurs in the penetration
of this paper upon the wood block ; just as you increase the
size of your instrument point you diminish the condensing
power with a given blow or with a given hand pressure.
While with the first instrument, measuring one-half milli-
meter in diameter, we can penetrate this paper with six and
a half pounds, the next size, seven and a half tenths of a
millimeter, requires fifteen pounds by actual trial ; it also re-
quires that if we calculate the area and use the same number
of pounds per square millimeter. If I take an instrument one
millimeter in diameter it requires twenty-seven pounds to
produce the same effect. Now, these are all in the range
that we use in filling teeth. If I take another, one and a
quarter millimeters in diameter, it requires forty-two pounds

to produce the same result. If I take an instrument a milli-
meter and a half in diameter it requires sixty pounds to pro-
duce the same result, and, of course, becomes impossible in
the condensation of gold. While if I take an instrument here
that seems not very large, two and a half millimeters in diani

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