Variation of the Rigidity of Phosphor-Bronze. 423 



an effect on ttie period especially when the period is small, 

 as the viscosity causes the zero to move periodically. In the 

 case of phosphor-bronze the viscosity could not have been 

 very great as the logarithmic decrement was never very 

 large. To test what effect altering the period (and hence the 

 damping) had on the rigidity the following experiment was 

 made. Two brass circular nuts o£ total weight 19*5 grams 

 moved on a brass screAv 9 cm. long and 6*1 grams in mass. 

 Ine moment of inertia of the system could then be varied 

 very readily without varying the weight. By this means 

 the time of swing was made to vary in the ratio of 2:1. 

 This alteration in period was not accompanied by any appre- 

 ciable alteration in the rigidity. For a light load a slightly 

 different procedure was adopted. A rod was soldered to the 

 strip, and then to vary the moment of inertia it was bent into 

 the the shape of an inverted V. Again the rigidity did not 

 vary for the same weight. It was concluded that the rigidity 

 does not depend on the damping. In further proof of this 

 it may be mentioned that the strips which had had the 

 discrepancy removed from them by annealing showed no 

 change in the damping of their oscillations. 



Influence of 2 emperature on tlie Discrepancy. 



To test whether raising the temperature of the strip for a 

 short time and then allowing it to cool influenced the variation 

 of rigidity the following experiment was tried. The strip 

 was placed in a glass tube which was then closed and put in 

 a steam-jacket and steam was allowed to flow through briskly 

 for about an hour. When taken out the behaviour of the strip 

 was found to be exactly the same as before. It was therefore 

 concluded that temperature had no influence on the result. 

 In later experiments this view was found to be erroneous, as 

 when the strips were heated strongly enough and sufficiently 

 long to anneal them the effect was largely removed. This 

 will be considered in more detail when an explanation of the 

 discrepancy is given. 



The experiments described above were made to discover the 

 properties of this discrepancy between fact and theory; those 

 described below were made to test possible explanations of it. 



1. It is well known that in abifilar suspension the restoring 

 couple per unit angle of twist is proportional to the weight 

 supported. It is possible that a strip of metal will show a 

 bifilar property.' To test this a steel strip was set up 67 cm. 

 long, -64 cm. wide, and *04 cm. thick. It was gripped 

 firmly at the top by a pin-vice, and supported in a horizontal 

 position a light rectangular wooden platform provided with 



