426 Anomalous Variation of Rigidity of Phosphor-Bronze. 



about 4 mm. in diameter. It was then straightened out as 

 far as possible by hand, and its behaviour tested. The dis- 

 crepancy was found to have almost disappeared, and when 

 the strip was left standing for four weeks it did not alter 

 perceptibly, so that the disappearance of the effect seemed to 

 be permanent. This is shown in fig. 2, graphs I. and IV '. 

 It is seen that the rigidity for the heavy loads has scarcely 

 altered, but that for the light loads has been greatly increased. 

 Other specimens gave similar results. It was not found 

 possible to treat the very thin strips in this manner as they 

 invariably broke in the process. The conclusion drawn was 

 that the bending overstrain removed the discrepancy. A 

 later experiment threw a little doubt on this ; for when an 

 attempt was made to straighten the strip a little more it went 

 back to the form of a spiral such as it had been into before ; 

 thus proving that the bending overstrain had not been 

 removed by the straightening, but had superposed on it 

 another overstrain largely of the nature of a twist which 

 could be more easily removed. We have, therefore, the 

 alternative explanation that the original effect remains 

 unaltered, but that we have now in addition an opposite 

 effect nearly neutralizing the other owing to the overstrain 

 we have added. This, however, appears to be very unlikely, 

 as the twist we have given to the strip in overstraining it 

 will not produce a contrary effect, but would increase the 

 amount of the original effect. The explanation that the 

 discrepancy is due to overstrains given to the strips during 

 the process of manufacture thus receives ample proof from 

 experiments on bending overstrain, and more particularly on 

 the annealing of the strips. 



Sir William Roberts-Austen found that the addition of 

 2 per cent, of potassium to gold diminished the greatest 

 pull it would sustain without breaking to ^ of its former 

 value. If the presence of a small amount of foreign substance 

 has such a large influence on the strength of a material, it 

 would seem to be very likely that when the relationship of the 

 foreign ingredient, which in the case of phosphor bronze is 

 phosphorus, is altered with regard to the metal, such as 

 would occur with an overstrain, then a large alteration in the 

 elasticity will take place. 



Summary* 



1. When phosphor-bronze strip is very hard, in which 

 condition it usually is when sent out from the manufacturer, 

 it shows an anomalous variation of its coefficient of rigidity, 

 the rigidity for small loads being much less than for heavy 

 loads. 



