230 UN WIN'S AUTOGRAPHIC APPARATUS. 



diagram is drawn by a pencil C. This pencil is attached 

 bo a small carriage which slides up and down two guide 

 rods, and whose motion is made to depend upon the stretch 

 of the specimen. Two clips, E and F, are fixed to the top 

 and bottom of the specimen respectively, and are such 

 that they are perfectly rigid on the bar. To the lower of 

 these is fixed a small pulley, and to the upper one the fine 

 wire used to transmit the motion is attached. This wire is 

 led downwards over the pulley at the lower end of the 

 specimen, and upwards again, passing over a pulley on the 

 top clip ; from here it is led horizontally to the guide 

 pulley G, and thence direct to the pencil carriage 

 of the instrument. The wire is kept tight by means 

 of the weight H. From this it will be seen that the 

 elongation of the specimen is magnified two-fold before 

 it is transmitted to the pencil. By leading oft' the wire 

 horizontally as shown, and parallel to the knife edge, any 

 error that might arise from the movement of the specimen 

 as a whole is reduced to a negligible quantity. 



The indications of the loads, on the specimen are made 

 in a direction at right angles to that of the extension by 

 the rotation of the drum. The machines to which this 

 apparatus has been applied are of the single-lever type, in 

 which the movement of the poise-weight is controlled by 

 means of a screw, and the motion of the weight, and there- 

 fore of the load on the specimen also, is proportional to the 

 rotation of this screw, and this fact is taken advantage of 

 in the working of the instrument. The motion of the 

 beam screw is transmitted to the small grooved pulley J 

 by means of a catgut band. On the same spindle as the 

 grooved pulley is a worm which gears into the worm wheel 

 of the paper drum. So that, apart from small inaccuracies 

 in the intermediate gearing, the circumferential movement 

 of the paper is proportional to the movement of the poise- 

 weight, and therefore to the load on the specimen. 



In this way a diagram is drawn having for its abscissae 

 the loads on the specimen, as given by the position of the 

 poise-weight, and for its ordinates the extensions taken 

 directly from the specimen itself. 



In the case of a test carried out in the usual way the 

 beam is kept floating, and the position of the weight on 

 the beam is a measure of the load on the specimen, and is 

 always taken as such. Even \vith the most ductile of the 

 metals there is little difficulty in manipulating the beam 

 so as to keep it in equilibrium up to the maximum 



