264 HIGH AND LOW IKMPERATURE TESTS. 



(c) The provision of means for reading this tempera- 

 ture. 



(d) The provision of means whereby the extensions of 

 the bar can be ascertained during the test. 



The same are true for low temperature tests. 



The application and measurement of the load is effected 

 in the usual way, in some cases the holding shackles being 

 attached to the test piece outside the heating apparatus, 

 while in others the test bar is attached to the ends of two 

 special bars which themselves pass into the heating bath. 



In order to maintain the oar at the required uniform 

 temperature the usual plan adopted is to have it immersed 

 in a bath of some liquid which can be kept at any desired 

 temperature. The liquid used may be oil of high flashing 

 point, if the required temperature is not to be above about 

 570 deg. Fah. ; while, for higher temperatures, an alloy 

 must be used, such as one consisting of tin and lead. 



The temperature of the fluid in which the test bar is 

 immersed, and consequently of the bar itself, may be 

 measured by means of a high temperature mercury thermo- 

 meter. Up to a temperature of about 600 deg. Fah. the 

 ordinary type of thermometer may be used, but for tem- 

 peratures beyond this a thermometer having compressed 

 nitrogen above the mercury column is necessary. 



Where extensions of the test piece are to be taken 

 during the progress of the test, two measuring pieces must 

 be attached to the bar, one at each end of the measured 

 portion, these pieces protuding from the liquid and the 

 measurements taken direct from them, it being assumed 

 that two points on these are separated to precisely the same 

 extent as the two points on the bar itself. As the specimen 

 will contract laterally as well as extend under the load, the 

 measuring pieces must be attached to the bar through 

 spring clips so as to compensate for the contraction. 



One form of vessel for maintaining a test bar at a high 

 temperature during a tensile test is shown in section on 

 Fig. 130.* This is the apparatus used for testing firebox 

 stays at the Midland Locomotive Works, Derby. The 

 testing machine is here a horizontal one, made by Messrs. 

 Whitworth. In the figure the bar which is being tested 

 is marked D, and it is connected to the shackles of the 

 machine through two bars C C. These pass into the heating 

 tank A, through two stuffing boxes B B, so that there can 

 be no escape of the fluid from the box, and at the same 



* Proc. lost. Mech. Eog., 1898. 



