222 



AUTOGRAPHIC APPARATUS. 



Of these two movements, the one representing the 

 strain of the specimen is taken direct from the specimen, 

 and exactly corresponds to the relative movement of two 

 fixed points on the bar, either as this movement actually 

 exists or in a magnified form ; where the elastic extensions 

 of a bar are to be recorded, a system of levers is inter- 

 posed between the specimen and the pencil carriage. This 

 strain movement is not difficult to arrange for, but, un- 

 fortunately, the same cannot be said of the stress move- 

 ment. In the great majority of tension machines the 

 observer obtains his information as to the load on the 

 specimen at any moment from the position of a known 

 weight on the weighing lever. A pointer attached to this 

 weight travels along a scale attached to the beam, and the 

 load on the specimen is in this way indicated. The weak 

 point about tnis is that the reading is only true so long as 

 the beam is floating freely between its stops ; if the beam 

 rests on one of the stops the indication is rendered 



FIG. 106. Kennedy's Apparatus for Drawing Autographic Diagrams. 



useless. But a man who is in the habit of manipulating a 

 testing machine is fully aware of this and is accordingly 

 careful to keep his lever always floating when taking read- 

 ings. In some forms of autographic apparatus the load 

 movement of the pencil is controlled by the movement of 

 the jockey weight, and is, in fact, a facsimile, on a reduced 

 scale, of its motion, the reduction being effected by a 

 system of tooth gearing or pulleys. An arrangement of this 

 kind only produces a semi-autographic diagram because 

 the accuracy of the load movement of the pencil depends 

 on the care and skill with which the machine is manipu- 

 lated. A truly autographic apparatus must draw a diagram 

 quite independently of the observer who is controlling the 

 machine. 



124. Dr. Kennedy's Apparatus. This appliance may be 

 taken as representing one of the most successful attempts to 



