228 SEMI-AUTOMATIC GEARS. 



introduced. What is most unsatisfactory about the effect 

 of the ram friction on the scale is due to the fact that this 

 friction acts sometimes with the load, and sometimes 

 against it ; and, consequently, the friction may be a plus 

 quantity at one point of test, and a minus one at another. 

 But this hardly affects an ordinary test, where the strain 

 is continuous in one direction. 



As there is a frictional force between the main ram and 

 its packing, so there is also the small amount of friction 

 between the ram of the instrument and its packing, which, 

 if not obviated, would considerably interfere with the 

 movement of the pencil. Mr. Wicksteed has, however, 

 resorted to the very ingenious plan of rotating the ram 

 continuously by means of belt gearing driven from the 

 main shafting, and thereby reducing the frictionalresistance 

 to motion to a minimum. 



126. Semi-Automatic Recording Apparatus. The two 



pieces of apparatus which have so far been dealt with may 

 be describea as truly automatic in their working, that 

 is to say, they are independent of those parts of the testing 

 machine which are used to control the load upon the 

 specimen. In the case of either of these appliances, if the 

 poise-weight were never moved from its initial position 

 on the weighing lever, but the specimen were to be 

 broken by the application of the hydraulic pressure with 

 the beam fixed against its upper stop, precisely the same 

 diagram would be drawn as if the beam had been kept 

 floating during the whole of the test. 



In the examples next to be described, the case is 

 different. In all these the strain movement is, as before, 

 taken direct, in its original or magnified form, from the 

 specimen. The load movement of the pencil is made to 

 depend on the position of the poise-weight, from which it 

 takes its motion. In a test as ordinarily carried out, the 

 load on the specimen at any moment is given by the 

 position of the jockey-weight on its beam, the actual reading 

 being taken on a graduated scale, along which moves a 

 pointer or vernier. This is the only means of ascertaining 

 the load upon a bar, and it must be remembered that it is 

 accurate only so long as the beam is in equilibrium and 

 floating freely between its stops. Assuming therefore that 

 the beam is always floating, then, if the pencil of the 

 autographic apparatus is made to move from its original 

 position a distance always bearing a constant ratio to 





