Mechanical Equivalent of Heat. ]:,\ 



ments, the apparatus now beiiip: descriUil wax .Ic^i^r.HMl ro that 

 the inlet and outlet water temperatures would detenuine \hv surface 

 temperatures of the calorimeter, and so for a jriven U-mperature 

 of the surroundings of the calorimeter, determine tlie Iohh of heat, 

 L, independently of the rate of development ef hent in the calori' 

 meter. 



The equation we have given assumes that three axen are parnlU-I, 

 namely, the axis of rotation of tlie magnetic field, the &x\h alK>ut 

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

 couple produced by the two masses ??i. This condition is fulfilled to 

 the. required accuracy in the apparatus a« we are using it. 



In another paper, 4 the theory of the electrical device, which we 

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

 the couple xj/ dyne.cm. acting on the slator is given hv the ex- 

 pression 



M'here N revolution per sec. is the speed of the rotor, <f> maxwell 

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

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

 inductance. 



Design and Operation of the Apparatus. 



It will be convenient to call the rotating tield magnets, the rotor. 

 and the copper cylinder and the iron cylinder wliich it encloses, tlie 

 •istjator. 



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

 with its axis vertical. The field magnet windings are connected 

 through slip rings to a lead storage battery. The rotor is 1-^lt- 

 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 yvw 

 writing on a chronograph; the pen also indicates seconds, ast given 

 by a standard clock. In thi.«3*way, the rate of rotation and numi»er 

 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 tliat of the rot-or. 

 These holes, if fully open, thoroughly ventilate and c(.ol the field 

 magnets. 



The lower l>earing of the rotor is ligidly held bv an iron l)ed 

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

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



4. J. K. Roberts. The Design (.f ;. Mof.r with I^.rRe Air Gap aiul H»t.ii. 

 ing Fiehl Magnets. Prof. Roy. .S,,,-. Vlrt.. X.XXII . 1-A'. )• !••<«. 



