Meclumical Equivalent of Heat. 151 



anents, the api^aratus now beinp; described was desi<:ned so that 

 the inlet and outlet water temperatures would determine the surface 

 temperatures of the calorimeter, and so for a given temperature 

 of the surroundings of the calorimeter, determine the loss of heat, 

 L, independently of the rate of development of heat in the calori- 

 meter. 



The equation we have given assumes that three axes are parallel, 

 namely, the axis of rotation of the magnetic field, the axis aljout 

 which the copper cylinder is free to rotate, and the axis of the 

 couple produced b}^ the two masses m. This condition is fulfilled to 

 tha required accuracy in the apparatus as we are using it. 



In another paper,'' the theory of the electrical device, which we 

 have used in these experiments, is given. It is there shown that 

 the couple i// dyne. cm. acting on the stator is given by the ex- 

 pression 



where N revolution per sec. is the speed of the rotoi-, ()> maxwell 

 is the flux crossing the copper cylinder, p is proportional to the 

 resistance from end to end of the cylinder, and X cm. is a certain 

 inductance. 



Design and Operation of the Apparatus. 



It will be convenient to call tlie rotating field magnets, the rotor, 

 and the copper cylinder and the iron cylinder which it encloses, the 

 isliator. 



The rotor (see Figs. 1, 2, 3 and 4) is mounted on ball bearings, 

 with its axis vertical. The field magnet windings are connected 

 through slip rings to a lead .storage battery. The rotor is belt- 

 driven by a shunt motor, and the speed of the former is deter- 

 mined by means of a worm gear, which, at the completion of every 

 100 revolutions (that is, about every four seconds), moves a pen 

 Tvriting on a chronograph ; the pen also indicates seconds, asi given 

 by a standard clock. In thi.si' way, the rate of rotation and number 

 of revolutions is recorded. 



The rotor, is pierced with eight sighting holes (see Figs. 3 and 4) 

 for adjusting the axis of the stator parallel to that of the rotor. 

 These holes, if fully open, thoroughly ventilate and cool the field 

 magnets. 



The lower bearing of the rotor is rigidly held by an iron l>ed 

 plate bolted to a brick foundation. In order to prevent vibration 

 in the plate, which carries the upper bearing of the rotor and the 



4. J. K. Roberts. The Design of a Motor with Large Air Gap and Rotat- 

 g Field Magnets. Proc. Roy. Soc. Vict.. XXXII., h>io, \<. i.-)6. 



