[ 160 ] 



XV. Students' Simple Apparatus for Determining the Me- 

 chanical Equivalent of Heat. By Prof. W. E. Ayrton 

 and H. C. Haycraft*. 



I. The Object to be Attained, 



SOME time ago the authors considered the possibility of 

 constructing an apparatus for the determination of the 

 mechanical equivalent of heat which could be placed in the 

 hands of junior students, and which would enable a sufficiently 

 accurate result to be obtained without the introduction of 

 troublesome corrections. For such a purpose the electrical 

 method was naturally adopted ; for now that the commercial 

 values of the electrical units are known with considerable 

 accuracy in the C.G.S. system, it is possible to measure 

 energy in foot-pounds by means of a good commercial 

 ammeter voltmeter and watch with greater ease and certainty 

 than by any mechanical dynamometer. 



Farther, it has become easy to obtain as much electric 

 power as is wanted for the experiment at a small cost, for the 

 price of half a horse-power for ten minutes is only one-third 

 of a penny, at 6d. per Board of Trade Unit. Hence there is 

 not the practical objection to the electrical method that was 

 so formidable when currents of 20 or 30 amperes could only 

 be obtained by setting up a large battery of Grove or Bunsen 

 cells. Indeed every properly organized physical laboratory 

 is now provided with accumulators, from which a quarter or 

 half a horse-power may be readily obtained for use in such 

 experiments as those to be described ; or, when accumulators 

 are not available, power may be taken from the mains of one 

 of the numerous electric-supply companies. 



II. Design of the Apparatus. 



The authors therefore set themselves to design an apparatus 

 which, when used with a good commercial ammeter voltmeter, 

 thermometer, and watch, would give the value of the me- 

 chanical equivalent of heat correct to one per cent, without any 

 corrections having to be made even for the heat lost by radia- 

 tion, convection, and conduction, and without any special 

 manipulative skill being required on the part of the observer. 



Broadly, the experiment consists in passing a known cur- 

 rent through a resistance immersed in a known mass of water, 

 and measuring the rise of temperature in a given time, and 



* Communicated by the Physical Society : read November 23, 1894. 



