112 KENNEDY EXTENSOMETERS. 



the graduated edge. This is usually either a hundredth or 

 fiftieth of an inch ; so that thousandths or five-hundredths 

 can be read. In order that the readings may be accurate, 

 the top of the distance-piece E must be plane and correctly 

 at right angles to the direction of the bar, and also the 

 edges of the gauge must be perfectly straight and free from 

 indentations. In spite of its simplicity the wedge gauge is 

 very little used nowadays, and may almost be described as 

 an obsolete instrument, recent extensometers being capable 

 of more precise readings and, at the same time, more con- 

 venient to use. 



63. Lever Multiplying Extensometers. In all instru- 

 ments of this type the alteration in length of a given portion 

 of the specimen is magnified, so as to be capable of being the 

 more easily measured, by means of a simple lever. This 

 is an arrangement which is at once easy to construct and, 

 at the same time, gives results which are in most' cases 

 sufficiently accurate for ordinary purposes. Of the lever 

 extensometers in use perhaps those of Dr. Kennedy are 

 the best known. These include instruments for the pur- 

 pose of measuring the extensions of both horizontal and 

 vertical tension specimens. Two of these will now be 

 described. 



64. Horizontal Lever Extensometers of Dr. 

 Kennedy.* This instrument is shown in plan and eleva- 

 tion on Fig. 44. In both views, of which the lower one 

 is the plan, A represents the specimen which is under 

 test, held in a horizontal position. At points ten inches 

 apart on the specimen are fixed two clips B', B". These 

 clips are each fixed to the specimen by means of two hard- 

 pointed steel screws C, C, and it is really the distance 

 between these that is measured by the magnifying lever. 

 The clips can either be set on two pairs of centre-punch 

 marks, or be simply screwed up lightly so as to grip the 

 surface of the specimen by slightly penetrating into it. 

 They both serve the purpose of fixed distance pieces, and 

 they also form the only supports which the instrument has, 

 as it has no connection or contact with any outside object. 



To the clip B' is attached the hollow triangular plate E', 

 and similarly, to clip B"is attached another hollow triangular 

 plate, E". The end of each base slides upon and is 

 steadied by the opposite clip. As the bar A stretches, 

 the distance B', B" increases, the clips move relatively to 



* Engineering, 1890, II., p. 305. 



