60 Mr. R. A. Lelifeldt 



on a 



tional to the square of the vibrating period of the fork ; or, 

 if the length of the tube be adjustable, i. e. if the tube be 

 telescopic, the temperature of the air in the tube (therefore 

 of the heated receptacle or what not, in which the tube is 

 placed) is evidently proportional to the square of the length 

 of the adjustable tube. 



Zero Centigrade being 273 above the absolute zero, the 

 Centigrade zero is then, of course, 273 when expressed as 

 absolute temperature. At 273 above zero C. (which is 546 

 of absolute temperature) the molecules of air have, of course, 

 twice the energy they have at zero C. At 4 x 273 absolute 

 temperature (819 above zero C.) the molecules of air will 

 possess, in the same way, four times the energy possessed by 

 them at zero C, and therefore twice the velocity which they 

 had at zero C. (velocity being as the square root of energy). 

 Sound would then be propagated in air (in such a tube) 

 whose temperature is 819° C. at twice the velocity of its 

 transmission in air at 0° C. So that a tube of air at a tem- 

 perature of 819° C. would be in resonance with the same 

 tuning-fork as a tube of half the length resounds to at 0° C. 

 This is sufficiently clear; and so it is evident that varying the 

 length of a telescopic tube and employing a constant fork 

 serves the same object as (conversely) varying the vibrating 

 period of an (adjustable) fork and keeping the length of the 

 tube constant. Some may see a way to propose some modi- 

 fication of the above, in a mechanical point of view, which 

 might be more sensitive. It is the principle, or rather 

 the theoretical aspect of the case, with which we are chiefly 

 concerned. 



Hamburg, May 1, 1891. 



IX. On a Modified Form of Wheatstone's Bridge. By E. 

 A. Lehfeldt, B.A.i B.Sc, Demonstrator at Firth College, 

 Sheffield*. 



IN comparing the resistances of two nearly equal coils, a 

 fine adjustment may be obtained either by a divided wire 

 of low resistance in series with the coils, or by a resistance- 

 box in parallel with them. The latter method, involving only 

 a piece of apparatus that exists in every laboratory, appears 

 to be somewhat simpler than the more common one, for 

 which a specially mounted wire is required ; and it avoids, 



* Communicated by the Author. 



