Determination of Poisson's Ratio. 



889 



Second Method. 



In order to increase the transverse deflexion I made use of 

 a device due to Lord Kelvin — the bifilar mirror. 



A brass rod, CAEF, bent twice at right angles as shown in 

 fig. 4, was soldered to the face AC at the middle point of the 



Fig. 4. 



;» 



P Q 



u 



M 



K 



bar. The end G of a second rod, GHK, was soldered to the 

 face BD, exactly opposite the point of attachment of the first 

 rod. When a couple is applied to the bar the points K and 

 F move in opposite directions, approximately at right angles 

 to the plane of the paper, and the mirror M, which is 

 suspended by silk fibres from the needles P and Q, rotates. 

 A damping vane, immersed in oil, was attached to the mirror, 

 and the latter was completely protected from draughts by 

 means of a cardboard case fitted with a glass window. 



Let #i = the total angle turned through by AC 

 and BD ; 

 a = mean heights of the needle points P and Q 

 above the mean line of the bar ; 



b = the mean distance between the fibres. 

 The mirror M turns through an angle 2 such that 



0, 



■e x . 



The formula used in the first method now becomes 



R _S_ L . b 

 2Si ' B a 





Ri 



D 



As a was about 20 cm. and h about 1 cm., the deflexion on 

 the transverse scale was 20 times as great as in the first 



