426 



Dr. S. Bidwell. On the Changes of 



[Apr. 11,. 



right-hand margin of the diagram. Curve (d) indicates the contrac- 

 tion in ten-millionths of length due to the mechanical compression, 

 and is derived from (c) as follows : — If P = the weight supported per 



Fig. 4. — (D) shows change of thermoelectric power ; (b) change of length 

 (e) lifting power ; (d) mechanical compression, deduced from (c). (E) = 

 (b) + (d), and is the " corrected " curve of change of length. 



square centimetre for a given value of H, as shown in (c), and M = 

 Young's modulus in grammes weight per square centimetre, the 

 mechanical contraction in .ten-millionths = Px 10 7 /M. The value of M 

 is taken as 2 x 10 9 . (It might without sensibly affecting the form of 

 the curve have been 1*9 x 10 9 or 2*1 x 10 9 , and its actual value no 

 doubt comes within these limits.) Hence, Px 10 7 /M = .P/200, or the 

 mechanical contraction in ten-millionths is numerically equal to the 

 grammes weight supported divided by 200 ; ordinates of curve (d) are 

 therefore simply P/200. In the following table values of P are 

 obtained by measuring ordinates of curve (c) ■ elongations and retrac- 

 tions (E) are similarly derived from the published original of 

 curve (&).* 



Ordinates of the three curves (b), (d), and (E) are referred to the 

 scale of ten-millionths given in the middle of the diagram. For the 

 sake of easy comparison this scale was so chosen that the heights of 

 the thermoelectric and the corrected elongation curves (D) and (E) are 

 about equal. As before remarked, the close agreement between the 



* Loc. cit. 



