Bauhaus-Universität Weimar

Cambridge Instrument Company, Inc.: Cambridge Physical Instruments
THE Cambridge Extensometer illustrated in Fig. 17 
has been designed for the measurement of the 
elastic extension and modulus of elasticity of 
specimens of metal under tensile loads. It will be 
found particularly suitable for workshop use, no 
microscopes, mirrors or other delicate parts liable to derange¬ 
ment being used in its construction, while it gives direct 
readings of the extension of the specimen which are accurate 
to within 0-001 millimetre under ordinary conditions of 
The instrument is made in two portions, each of which is 
separately attached to the test piece by hard steel conical 
points, which are driven gently into punch marks made in 
the specimen, and clamped in position, a centring gauge being provided to ensure that the two 
portions of the Extensometer are mounted in the correct position relative to one another and to 
the centre line of the specimen. The lower part of the Extensometer carries a micrometer screw 
fitted with a hardened steel point and a divided head. It also carries a vertical arm, at the top of 
which is fitted a hardened steel knife edge about which the two portions of the instrument are pivoted. 
From the upper portion extends a flexible nickel-plated steel tongue, which acts as a lever magnifying 
the extension of the specimen, so that the movement of the edge of the tongue towards or away from 
the point of the micrometer screw is five times the actual extension. In taking a reading with the 
Extensometer, the steel tongue is caused to vibrate and the micrometer head turned until the point 
of the screw just touches the hard steel knife edge on the tongue as it vibrates. This method of 
setting is extremely delicate, owing to the audible sound produced and the fact that the vibrations 
are quickly damped out immediately contact is made. Direct readings to within 0-001 millimetre 
are given on the micrometer head. Readings may be taken in this way before and after the 
load is applied, the difference in the readings giving directly the extension of the specimen 
under test. 
When the test piece is of small diameter, the flexibility of the specimen may cause vibration of the 
instrument as well as the steel tongue, thus rendering it more difficult to obtain accurate results 
by the method described. In such cases, however, highly accurate readings may be taken by simply 
deflecting the spring with the finger and noting the contact as it passes the micrometer point. No 
damage can be done by advancing the micrometer screw too far forward. 
The Extensometer is suitable for use with specimens up to 20 millimetres or 0-75 inch diameter, 
and is made in two patterns, for centre points 100 millimetres and 50 millimetres apart respectively. 
A complete revolution of the micrometer screw corresponds to an extension of 0-1 millimetre in the 
test piece, one division on the micrometer head being equal to 0-001 millimetre extension. The 
instrument can also be supplied to read in English units, one division on the micrometer head 
corresponding to an extension of 0-0001 inch. A slightly modified form of Extensometer for use 
with a horizontal test piece is also made. For marking off the test specimens, the marking-off tool 
described on page 13 may be used. 
Cat. Xo. Fig. No. 
23112 17 Cambridge Extensometer. 


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