Page 629 - My FlipBook

Fig. 400.
Fic. 400. The author's micrometer for tlie Tiieasurement of the shrinkage or expansion of ainal-
eain, in its final form. The circle of the scale plate is twelve inches in diameter. The metals entering
into its construction are so arranged as to eliminate errors from temperature changes, a. A wheel
attached to a threaded shaft passing through a metallic block to the lever b. The lever b
is pushed to the left by turning the wheel a. This moves the touch point f, which passes through
the lever B, and the point of which impinges upon the Lube c. away from the lube. .\t the same
tinir, the necrllc on tht- di.il i)late is caused to swing away from the center toward the right hand. By
tiu'ning the hncr i» around to the left, the grasp of a bolt held by a spiral spring against the bottom
uf tube c, is loosened and the tube may be removed, another placed or the same one returned. When
in place, the lower side of this tube c rests on two points fi.ved in the bottom of the slot which
receives tube c. There are screws arranged to take up wear of the points. The face of tube c is
caught by three lugs when the bolt is allowed to come against it. The tube c must be turned so that
the mark just under the letter c shall coincide with the point on the lever e when it is turned over it.
Then by a backward turn of the wheel a, the touch point f comes to rest on the contents of tube c,
and the needle registers the measurement. When a first measurement of an amalgam filling in a tube
has been registered, the tube may be removed, and returned for measurement at any future time. If
at another time the needle swings to the left of the former measurement, the amalgam has shnmk. If
it stops to the right, the amalgam has expanded. The amount of variation is read in gradations of
one ten-thousandth of an inch. The filling is supposed to be finished so that the tube will be level
full, but a variation of one or two, or even three, hundredths of an inch is unimjiortant. Any number
of fillings may be measured and laid away, and remeasured at any time. The total movement of the
lever B, which carries the toucli point, is six-lumdredths of an inch, while the needle swings from -to
(iiK) of the scale. .\ setting bluek, in the form of one of the tubes, is provided as a test of accuracy of
measurements, and for the further purpose of correcting the instrument itself for any errors of adjust-
ment. This is made of hardened steel and the face ground.
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