ON THE MECHANICAL EQUIVALEXT OF HEAT 423 



In devising a new method a great rise of temperature in a short time 

 was considered to be the great point, combined, of course, with an accu- 

 rate measurement of the work done. For a great rise of temperature 

 great work must be done, which necessitates the use of a steam-engine 

 or other motive power. For the measurement of the work done, there 

 is only one principle in use at present, which is, that the work trans- 

 mitted by any shaft in a given time is equal to 2/r times the product of 

 the moment of the force by the number of revolutions of the shaft in 

 that time. 



In mechanics it is common to measure the amount of the force 

 twisting the shaft by breaking it at the given point, and attaching the 

 two ends together by some arrangement of springs whose stretching 

 gives the moment. Morin's dynamometer is an example. Him 41 gives 

 a method which he seems to consider new, but which is immediately 

 recognized as Huyghens's arrangement for winding clocks without stop- 

 ping them. As cords and pulleys are used which may slip on each other, 

 it cannot possess much accuracy. I have devised a method by cog- 

 wheels which is more accurate, but which is better adapted for use in 

 the machine-shop than for scientific experimentation. 



But the most accurate method known to engineers for measuring the 

 work of an engine is that of White's friction brake, and on this I have 

 based my apparatus. Him was the first to use this principle in deter- 

 mining the mechanical equivalent of heat. In his experiment a hori- 

 zontal axis was turned by a steam-engine. On the axis was a pulley 

 with a flat surface, on which rested a piece of bronze which was to be 

 heated by the friction. The moment of the force with which the fric- 

 tion tended to turn the piece of bronze was measured, together with 

 the velocity of revolution. This experiment, which Him calls a balance 

 de frottement, was first constructed by him to test the quality of oils used 

 in the industrial arts. He experimented by passing a current of water 

 through the apparatus and observing the temperature of the water be- 

 fore and after passing through. He thus obtained a rough approxima- 

 tion to Joule's equivalent. 



He afterward constructed an apparatus consisting of two cylinders 

 about 30 cm. in diameter and 100 cm. long, turning one within the 

 other, the annular space between which could be filled with water, or 

 through which a stream of water could be made to flow whose tempera- 

 ture could be measured before and after. The work was measured by 

 the same method as before. 



41 Exposition de la Theorie Mecanique de la Chaleur, 3 m 6d., p. 18. 



