An Improved Instrument for Measuring the Extension of Test Bars or Extensometer.
Year: 1887

page_text: Date of Application, 12th Jan., 1887
Complete Left, 11th Oct., 1887
Complete Accepted, 11th Nov., 1887
PROVISIONAL SPECIFICATION.
An Improved Instrument for Measuring the Extension of Test Bars
or Extensometer.
WILLIAM CAWTHORNE UNWIN of 7 Palace Gate Mansions Kensington W. Civil
Engineer do hereby declare the nature of this invention to be as follows:-
Two clips are fixed on the Test Bar which grip it each between a pair of points in
the middle of the thickness. The clips are spaced apart by a distance piece resting
on knife edges on the clips. The upper clip carries a steel roller & the lower clip a
spring finger which presses on the roller. Hence as the specimen extends the roller
rotates by friction against the spring finger. A mirror is carried by the roller
on the same axis & the angular movement of the roller is measured by a telescope
and scale.
Dated this Tenth day of January 1887

page_text: I, WILLIAM CAWTHORNE UNWIN of 7 Palace Gate Mansions, Kensington W. in
the County of Middlesex 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:-
My invention relates to an improved construction of instrument for measuring the
extension of test bars wherein the parts are so arranged that the measurements taken
are the mean of the extensions of the two sides of the bar. According to the present
construction, two clips are fixed on the test bar gripping it each between a pair of
points in the middle of the thickness. The clips are spaced apart by a distance piece
resting on knife edges on the clips. The upper clip carries a steel roller and the
lower clip a spring finger which presses on the roller. Hence as the specimen
extends, the roller rotates by friction against the spring finger. A mirror is carried
by the roller on the same axis and the angular movement of the roller is measured by
a telescope and scale.
Fig. 1 shews a front elevation Fig. 2 a side view and Fig. 3 a plan of one
arrangement of the above described construction ab are the clips which grip the
bar between pointed set screws shown at c d. The lower. clip is supported by the
set screw e The upper clip rests on the lower clip by the distance piece, the clips
having knife edges resting on the latter. A roller and mirror m are carried by the
upper clip and a finger g attached to the lower clip presses against the roller. If the
bar extends, the roller approaches the lower clip and turns against the finger g. The
angular rotation of the mirror determined by a reading telescope and scale gives a
measure of the extension.
Fig. 4 shews a side view another modification in which, in place of the finger g of
the first arrangement a loose bar g' is employed, having knife-edge projections
bearing respectively upon the lower clip b and on the roller h of the upper clip, the
bar being held firmly against these parts by caoutchouc springs i i.
The construction and action of the apparatus is otherwise the same as before
described.
Having now particularly described and ascertained the nature of this invention and
in what manner the same is to be performed, I declare that what I claim is :.
An instrument for measuring the extension of a test bar wherein two clips, attached
to the test bar a certain distance apart, have each an attachment on opposite sides of
the bar and are held in position by a distance piece, the one clip having an adjusting
screw for supporting it, and a finger situated mid-way between the points of attach-
ment while the other clip has a roller on which the said finger bears and a mirror
connected to the axis of the said roller, the several parts being arranged and operating
substantially as herein described.