Improvements in Apparatus for Measuring Small Strains.
Year: 1894

page_text: Date of Application, 24th July, 1894
Complete Specification Left, 23rd Mar., 1895-Accepted, 27th Apr., 1895
PROVISIONAL SPECIFICATION.
Improvements in Apparatus for Measuring Small Strains.
I, JAMES ALFRED EWING, of Langdale Lodge, Cambridge, Professor of
Mechanism in the University of Cambridge do hereby declare the nature of this
invention to be as follows -
The object of the invention is to provide a convenient and accurate means of
measuring small strains of extension or compression, such as the elastic strains of
specimens of metals subjected to stress in testing machines, or those of the strained
members of loaded structures.
The apparatus is self-contained and is supported by the rod or other body the
strain of which is to be examined. It comprises two rigid pieces which are
separately pivotted to the rod at separate places along it, each by two pivot points
on opposite sides of the rod. Between the two pairs of pivot points is the length on
which the strain is to be measured.
No part of the apparatus touches the rod elsewhere than at the two pairs of pivot
points. Each of the two rigid pieces separately would be free to oscillate about a
line through its two pivot points as axis, but the two pieces are caused to engage
with each other by means of a screw on one the point of which presses into a hole
or V slot cut in the other, with the effect that the system is steady so long as the
distance between the two pairs of pivots does not change. One of the two pivotted
pieces is a bar or lever which stands at right angles or nearly at right angles to the
rod under test, and the screw point which forms the connection between the two
pivotted pieces is so placed as to tilt this piece when the rod is strained or when
the screw is turned. This tilting is observed by means of a microscope which hangs
from the other pivotted piece and sights a line or mark on the tilted bar.
The
microscope has a micrometer eye-piece to measure the strain by measuring the
tilting of the pivotted bar, which is proportional to the strain; but the screw may
be used to bring the tilted bar back until the mark on it is in optical coincidence
with a cross-hair or other mark in the microscope, and the micrometer scale in the
microscope may in that case be dispensed with, the strain being then measured by a
divided head on the screw. When a micrometer eye-piece used the screw serves
to standardise the scale in it, by tilting the pivotted bar through a known distance.
It serves in all cases to adjust the bar to a convenient position for sighting, and also
to measure strains which are so comparatively large as to tilt the sighted mark out
of the field of the microscope.
To enable the apparatus to be readily adapted to specimens of various lengths
one of the two pivotted pieces may be made in two portions united by a telescopic
joint or equivalent device.
DL: 0011
T 1
1000

page_text: Improvements in Apparatus for Measuring Small Strains.
I, JAMES ALFRED EWING, of Langdale Lodge Cambridge, Professor of
Mechanism in the University of Cambridge do hereby declare the nature of this.
invention and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
The object of the invention is to provide a convenient and accurate means of
measuring small strains of extension or compression, such as the elastic strains of
specimens of metals subjected to stress in testing machines, or those of the strained
members in loaded structures.
The apparatus is self-contained and is supported by the rod or other body the
strain of which is to be examined. It comprises two rigid pieces which are
separately pivotted to the rod at separate places along it, each by two pivot points
on opposite sides of the rod. Between the two pairs of pivot points is the length
on which the strain is to be measured. No part of the apparatus touches the rod
elsewhere than at the two pairs of pivot points. Each of the two rigid pieces
separately would be free to oscillate about a line through its two pivot points as
axis, but the two pieces are caused to engage with each other by means of a screw
on one the point of which presses into a hole or V slot cut in the other, with the
effect that the system is steady so long as the distance between the two pairs of
pivot points does not change. One of the two pivotted pieces is a bar or lever
which stands at right angles or nearly at right angles to the rod under test, and the
screw point which forms the connection between the two pivotted pieces is so placed
as to tilt this piece when the rod is strained or when the screw is turned.
This
tilting is observed by means of a microscope which hangs from the other pivotted
piece and sights a line or mark on the tilted bar. The microscope has a micrometer
eye-piece to measure the strain by measuring the tilting of the pivotted bar, which
is proportional to the strain, but the screw may be used to bring the tilted bar back
until the mark on it is in optical coincidence with a cross-hair or other mark in the
microscope, and the micrometer scale in the microscope may in that case be
dispensed with, the strain being then measured by means of a divided head on the
screw. When a micrometer eye-piece is used the screw serves to standardise the
scale in it, by tilting the pivotted bar through a known distance. It serves in all
cases to adjust the bar to a convenient position for sighting, and also to measure
strains which are so comparatively large as to tilt the sighted mark out of the field
of the microscope. To enable the apparatus to be readily adapted to specimens of
various lengths, one of the two pivotted pieces may be made in two portions united
by a telescope joint or equivalent device.
The drawing hereto annexed shows one form of the apparatus. A A is the rod
or specimen the extension of which is to be measured, and to which the apparatus
is applied.
The piece B, which is here made in two parts B, and B,, is attached to the rod
by the points of a pair of screws bb one of which is seen in the drawing, and is,
separately, free to oscillate about the line joining these points.
The piece C is attached to the rod by the points of a pair of screws cc, set so as
to be parallel to the screws bb, and is separately free to oscillate about the line
joining the points of the screws c c.
But these freedoms to oscillate which B and C have separately are removed by
making the point P on the piece B gear into a hole in the piece C.
The point P
is so attached to the piece B that any oscillation of B about its pivot points bb
would necessarily carry P towards or away from the rod under test, and such
oscillation is prevented by constraining P to remain in the hole in C.

page_text: The microscope M is suspended from the piece B and is arranged with a joint
at E and screw to allow it to be focussed. The object sighted by the microscope
is a stretched wire W on the end of C, and one side of this wire is used in taking
the observations. The wire W, the point P, and the axis c c are in one plane.
There is a micrometer scale in the eye-piece by which readings are taken of the
displacement of the wire on C when the rod under test is strained. The point P is
the point of a screw of known pitch, with a divided head, and the scale in the
eye-piece is calibrated by observing the displacement of the wire when this screw
is turned.
In the example here illustrated the screw P engages in a hole in the piece C.
A slot cut parallel to the axes b b and c c may be used in place of a hole, and this
construction is to be preferred when the piece B is rigid as regards motion in all
directions. But it is sufficient to make B rigid as regards motion about the axis of
its pivot points b b. As regards motion at right angles to that axis B may be made
flexible or jointed, and this will allow P to engage in a hole instead of a slot. In
the example shown in the drawing this is done by making the piece B consist of
two parts B, and B, which move as one rigid whole so far as oscillation about bb
is concerned, but allow the point P freedom to adapt itself to the hole in C by
movement perpendicular to the plane of the drawing. The connection between
the two parts of B is formed by making two pins on B₁ gear into a hole and slot
in Bg.
In the example shown in the drawing the arms on either side of the rod under
test, namely cP and c W are of equal length, and the wire therefore moves through
twice the distance representing the extension of the rod, but these lengths may have
any other ratio.
In cases where it is desirable to have all the apparatus situated on one side of the
rod under examination (as may happen in measuring the strains in the members of
structures) the construction of the apparatus is varied by placing the point P and
the wire or other mark to be sighted both on the same side of the rod, at different
distances from the axis of the pivot points c c.
To facilitate the application of the apparatus to the rod under test I occasionally
use a clamp by means of which the pieces B and C are temporarily fixed in relation
to one another, with the lines of the pivot points parallel and at the desired distance
apart. This clamp is used in fixing the apparatus to the rod and is removed before
the observations of extension are taken.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
(1) Self-contained apparatus for measuring strains of extension or compression
comprising two rigid pieces which are simply pivotted to the rod or other strained
body and are caused to engage with each other in such a manner that the system
is steady so long as the distance between the pivots does not change, in combination
with a microscope for observing the relative displacement of the pieces which
results from the strain.
(2) The combination, for the purpose of measuring small strains, of two pieces
pivotted to the strained body with a screw point on one engaging in a hole or slot
in the other in such a manner that the strain causes one of the pieces to be tilted
relatively to the other, with a microscope for observing the tilting attached to one
of the pieces, substantially as described.
(3) The apparatus for measuring small strains substantially as described.
Doted this 22nd davr of March 1895