As well known, the method by which glass barometer tubes are made gives
them variable calibers. Not only do the different tubes vary in size,
but even the same tube is apt to have different diameters throughout
its length, and its sections are not always circular. Manufacturers
of barometers often have need to know exactly the dimensions of the
sections of these tubes, and to ascertain whether they are equal
throughout a certain length of tube, and this is especially necessary in
those instruments in which the surfaces of the sections of the reservoir
and tube must bear a definite ratio to one another. Having ascertained
that no apparatus existed for measuring the caliber of these and
anolagous tubes, and that manufacturers had been accustomed to make the
measurements by roundabout methods, Colonel Goulier has been led to
devise a small apparatus for the purpose, and which is shown in the
accompanying cuts.
[Illustration: GOULIER'S TUBE GAUGE. (Plan and longitudinal and
tranverse sections.)]
The extremity of a brass tube, T, 0.5 to 0.6 of a meter in length and
smaller in diameter than the tube to be gauged, is cut into four narrow
strips a few centimeters in length. The extremity of each of these
strips is bent toward the axis of the tube. Two of them, m and m',
opposite each other are made very flexible, and carry, riveted to their
extremities, two steel buttons, the heads of which, placed in the
interior, have the form of an obtuse quoin with rounded edge directed
perpendicular to the tube's axis. The other extremities of these buttons
are spherical and polished and serve as caliper points in the operation
of measuring. These buttons are given a thickness such that when the
edges of their heads are in contact, the external diameter of the tube
exceeds the distance apart of the two calibrating points by more than
one millimeter. But such distance apart is increased within certain
limits by inserting between the buttons a German silver wedge, L,
carried by a rod, t, which traverses the entire tube, and which is
maneuvered by a head, B, fixed to its extremity. This rod carries a
small screw, v, whose head slides in a groove, r, in the tube, so as
to limit the travel of the wedge and prevent its rotation. Beneath the
head, B, the rod is filed so as to give it a plane surface for the
reception of a divided scale. A corresponding slit in the top of the
tube carries the index, I, of the scale. The principal divisions of the
scale have been obtained experimentally, and traced opposite the index
when the calibrating points were exactly 7, 8, 9 etc., millimeters
apart. As the angle of the wedge is about one tenth, the intervals
between these divisions are about one centimeter. These intervals are
divided into ten parts, each of which corresponds to a variation in
distance of one tenth of a millimeter.
To calibrate a glass tube with this instrument, the tube is laid upon
the table, the gauge is inserted, and the buttons are introduced into
the section desired. The flat side of the head, B, being laid on the
table, arranges, as shown in the figure, the buttons perpendicular to
it. Then the measuring wedge is introduced until a stoppage occurs
through the contact of the buttons with the sides of the tube. Finally,
their distance apart is read on the scale. Such distance apart will be
the measure of a diameter or a chord of the tube's section, according as
the buttons have been kept in the diametral plane or moved out of it. In
order that the operator shall not be obliged to watch the position of
the line of calibrating buttons in obtaining the diameter, the following
arrangement has been devised: The sides of the measuring wedge are filed
off to a certain angle, and the ends of the corresponding strips, d and
d', are bent inward in the form of hooks, whose extremities always rest
on the faces of the directing wedges. The length of these hooks and the
angle of the wedge are such that the distance apart of the rounded backs
of the directing strips is everywhere less, by about one-thirtieth, than
that of the calibrating buttons. From this it will be seen that if the
wedge be drawn back, and inserted again after the tube has been turned,
we shall measure the diameter that is actually vertical. It becomes
possible, then, to determine the greatest and smallest diameters in a
few minutes; and, supposing the section elliptical, its area will be
obtained by multiplying the product of these two diameters by pi/4.
From the description here given it will be seen that Colonel Goulier's
apparatus is not only convenient to use, but also permits of obtaining
as accurate results as are necessary. Two sizes of the instrument are
made, one for diameters of from 7 to 10.5 mm., and the other for those
of from 10 to 15.5 mm. It is the former of these that is shown, of
actual size, in the cuts.
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