232 R1EHL15-GRAY AUTOGRAPHIC ' APPARATUS. 



of the specimen the beam is again raised, and the contact 

 effected, and so in this way an automatic balance is main- 

 tained. There is a provision whereby the speed of running 

 out the weight can be varied to suit the particular kind of 

 test which is being made. 



The drum which carries the autographic diagram will 

 be seen in the illustration. The diagram is drawn by the 

 drum being rotated by the stretch of the specimen, com- 

 bined with the vertical movement of the pencil due to the 

 load variation. The stretch is taken directly from two 

 clips attached to the specimen, and transmitted to the 

 drum by means of a small shaft and mitre gearing, the 

 linear movement being converted into rotary motion and, 

 between the specimen and the drum, being magnified five 

 times. The vertical movement of the pencil is effected by 

 means of a screw which derives its motion from the poise- 

 weight screw, but on a reduced scale. 



This machine can be used with or without the automatic 

 attachment, and the machine can be controlled entirely by 



hand. When the automatic gear is used it is only 



i i 



necessary to put the straining mechanism in action, and, 



by means of the apparatus, the load on the specimen is 

 automatically adjusted so that balance is always maintained. 

 The speed at which the poise-weight travels along the 

 weigh-beam can be varied at the will of the operator. 



As in the case of Unwin's apparatus, and others of 

 the same type, there is the objection, which has already 

 been pointed out, that the load movement of the 

 pencil is only an accurate measure of the load on 

 the bar so long as the beam is in equilibrium. This is, 

 however, very nearly approximated to by the use of the 

 electrical apparatus. The movement of the weight is, 

 however, only an outward one, and the portion of the 

 diagram near the end of the test when the bar is in its 



flastic stage cannot be drawn correctly by this apparatus, 

 f it is wished to obtain this portion of the diagram for a 

 ductile material, the electrical apparatus will have to be 

 put out of gear and the machine controlled by hand. 



129. Riehle-Gray Autographic Apparatus. One pecu- 

 liarity of this instrument is that, instead of drawing only one 

 diagram, it produces two, one for the complete test up 

 to the breaking load, and the other, a diagram showing the 

 relation between load and strain, up to a point just beyond 

 the elastic limit. On Fig. 110 is shown the instrument in 



