Mea&uniuy the Lateral Contraction of Tie-bar 



421 



possible accuracy. The screws A and B have spherical points 

 in order that they may not cut into the specimen. The in- 

 strument is balanced by the weioht W. The mirrors are of 

 specially worked optically plane olass. The joint at P is 

 made by centring the screws in the inner piece, and is con- 

 structed with the greatest care. 



In order to test the instrument a specially selected bar of mild 

 Bessemer steel was used. It was turned to one inch diameter 

 as given by a Whitworth gauge. After placing in the testing- 

 machine it was loaded several times up to 5 tons. The in- 

 strument was attached and allowed to stand for a short time 

 before readings were taken. 



The load was then applied by successive tons between the 

 limits 1 and 5 tons and the scale-readings observed. The 

 results for three series of increasing and decreasing loads are 

 given in the following table : — 



Table I. 





Scale-Eeadings. 



Differences. 



Load. 



1 

 2 



3 

 4 

 5 

 4 

 3 

 2 



1 



1st 

 Loading. 



195-0 

 197-9 

 200-7 

 203-6 

 206-1 

 203-4 

 200-5 

 197-7 

 194-8 



2nd 

 Loading. 



201-2 

 204-0 

 206-9 

 209-8 

 212-6 

 2100 

 207-4 

 204-6 

 201-8 



3rd 

 Loading. 



201-8 

 204-7 

 207-6 

 210-4 

 213-2 

 210-7 

 207-9 

 205-1 

 202-2 



1st 



Loading. 



2-9 

 2-8 

 2-9 

 2-5 

 2-7 

 2-9 

 2-8 

 2-9 



2nd 

 Loading. 



2-8 

 2-9 

 2-9 

 2-8 

 2-6 

 2-6 

 2-8 

 2-8 



3rd 

 Loading. 



2-9' 

 2-9 



2-8 

 2-8 

 2-5 

 2-8 

 2-8 

 2-9 



! 



Mean diffs. per ton ... 



2-80 



2-78 



2-80 



1 



It was thus seen that the readings could be repeated with 

 considerable accuracy. The above are some of the first 

 readings taken, further experience in the use of the instrument 

 led to still more uniform results. 



Experimental Determination of Poissoii's Ratio. 

 In these experiments the load on the specimen was in some 

 cases varied from 1 to 5 tons, and in others from 1 to 3 tons. 

 In the former the readings were taken at intervals of 1 ton, 

 and in the latter at intervals of one-half ton. In every case 

 the specimens were loaded several times previous to the 

 readings being taken. In order to insure complete immunity 

 from local strains (due to the method of gripping the specimen) 



