Patent GB-534795-A (GB)
Title: An improved extensometer
IPC: G01B11/16
Filed: 1939-12-23
Abstract: 534,795. Extensometers. CALLENDER'S CABLE & CONSTRUCTION CO., Ltd., HOWSON, A. V., and REED, N. S. Dec. 23, 1939, Nos. 32849/39 and 13626/40. [Class 106 (ii)] Comprises a system of reflecting surfaces arranged to bring the images of two reference marks on the test piece into view in close and defined proximity, one or more of the reflecting surfaces being adjustable under control of an operator, so that the relationship of the images is maintained during separation of the marks, the final position of the control serving as an indication of the maximum extension. A vertical tube 1 is supported by a frame 2 carried by a bar 3 so as to be parallel with the specimen. At the centre of the tube 1 is an eye piece 4 in front of which within the tube are prisms 5, 6 so arranged that they reflect images into the eyepiece from the top and bottom of the tube respectively. Within the tube near the ends are mirrors 7, 8 each adapted to be turned about on horizontal axis by a handle 12 through a 2:1 reduction gear 20, 21. The mirrors are turned so that two marks in the specimen appear side by side in the eye-piece. The specimen is then tensioned and the mirrors simultaneously turned so that the marks are kept side by side. Each lever 12 has a slot 26 in which engages a pin 25 on a vernier guided on a rod 24 and moving over a scale 22. In a modification, the top mark is viewed directly and only one tilting mirror provided for the lower mark, the whole tube being moved parallel to the specimen by rack and pinion gear to keep the top image in view. A further modification, Fig. 8, is used with a known construction of testing machine in which a pull applied at 66 to a sliding crosshead 64 is transmitted through the specimen 61, guide rod 58, chain 57 and drum 53 to a spindle 52 on which is an arm 55 carrying balance weights 56. A balance weight 62 is attached to a second drum 54 and the rotation of the spindle 52 is transmitted through quadrant and pinion 67, 68 to a pointer 69 moving over a scale 70. The movement of the arm 53 is transmitted by link 77 to a slide block 75 carrying an electrode from which a high tension current at intervals passes to a backing plate through the chart. The extensometer comprises a pillar 30 on which two sleeves 33 may be adjusted by pinions 34 engaging a rack 32 on the pillar. Each sleeve carries a tubular elbow 37, one outlet of which is directed upwards and the other horizontally towards the test piece, each elbow being off-set from the other. Telescoped into the horizontal portion of each elbow is a tube 38 carrying a lens and at the rear is an inclined mirror 40 or prism projecting a parallel beam upwards to a collecting monocular 41 on top of the pillar. Each beam is projected through a lens 45 in a tube 44 to an inclined mirror 47 and hence through a prism to an eye-piece 49. The marks are kept side by side in the eye-piece by turning the handwheels 35 and each time a contact carried by a vernier 72 on the lower sleeve passes over a stud 79 on a scale 71 carried by the upper sleeve a spark passes through the chart marking the load at the corresponding extension.