50 MKTHODS OF AIM'LYINC THE L.OAM. 



pull musi come on the fixed end in order to prevent its 

 being pulled away. So that the same load is really being 

 applied to both ends simultaneously. It is, however, 

 customary to speak of the straining apparatus being 

 applied to the movable end, and the measuring appliance 

 at the stationary end, although in reality both are exerting 

 the same pull. In some few cases the measurement of the 

 load and the straining are effected at the same end of the 

 specimen. 



The two methods most in vogue for applying the load 

 are those of hydraulic power and screw gearing. In all 

 the larger testing machines used in this country the former 

 is almost universally employed on account of its great 

 capability of being easily controlled and manipulated, and 

 by reason of its greater efficiency as compared with screw 

 gearing. 



In many of the smaller machines, and also in one or 

 two of the larger machines used in America, screw gearing 

 is employed. By means of a direct-acting screw, or a worm 

 wheel rotated by a worm and acting as the nut on a fixed 

 screw, bars of small dimensions can be readily strained and 

 broken by the application of manual power, although the 

 mechanical efficiency of such arrangements is extremely 

 low, in some cases as much as 90 per cent or more of the 

 work done by the operator being lost in overcoming 

 frictional resistances. With a conveniently quick move- 

 ment of the straining shackle the limit of load which can 

 be applied by manual power is probably from two to three 

 tons. Beyond loads in this neighbourhood manual power 

 must be abandoned, and steam or other mechanical power 

 made use of. 



23. Measurement of the Load. The magnitude of the 

 load is, in almost all cases, measured by some kind of lever 

 Aveighing machine. In this way small movable weights, 

 applied at great leverage, which may be variable or constant, 

 may be used to measure loads very many times their own 

 magnitude. It is according to the manner in which 

 these weighing appliances are arranged and used that the 

 various testing machines are divided into several distinct 

 classes. They may be placed as follow : 



I. SINGLE -LEVER MACHINES. (a) High power, with 

 constant leverage, and movable small weights. (6) Low 

 power, with variable leverage and constant large weight. 



II. MULTIPLE - LEVER MACHINES. (a) High power, 

 vith three or more levers, with variable moving weights on 



