Mechduicaf EqulvaUnt of Heat. 15:$ 



when set in toi-sional oscillation (iL-cieJises ]>y lO per cent, per 

 vibration, from which it can be sliown that the friction in very 

 small. A mirror is attached to the shaft of the stator, and its 

 movement observed by lamp and scale. With the apparatus 

 arranged for a determination of the mechanical! equivalent, the 

 stator oscillations are not critically damped; frictional resistance 

 arising from the viscosity of water is added, till the damping? is 

 critical. 



Couple. — Two thin wires (see Fig. 1) pass from the circumference 

 of the wheel over two other wheels (see Fig. 2), and then to the 

 weights. With the axis of the stator vertical (which it is to within 

 12' of angle) these wires should Ix? parallel and horizontal, con- 

 ditions which are readily fulfilled. 



The design, construction and testing of the wheels shown in Fig. 

 8 has requii-ed a great deal of attention. Ball, roller, and cone 

 bearings Avere tested, and found to possess far too much friction to 

 be suitable for these Avheels, and so a knife edge was used. While 

 this bearing is quite free from friction, and practical in use, it in 

 necessary to locate the position of the knife edge relative to the 

 centre of the wheel, a test which is not so easily made as might be 

 expected. 



Measurements. 



The relation 



J = 7rl)7W/(m, -7/0/ -J (M,-M.,)(^.,-^,) I- 

 indicates what degree of accuracy is necessary in the varioun 

 quantities in the right hand mend)er, if J is to Ix? correct to 1 in 

 10,000. 



D, the diameter of the wheel, is 20 cm., and can Ik? measurcil 

 to the necessary accuracy. The revolutions, n, are counttnl. The 

 acceleration of gravity, y, is accurately known for Melbounic. The 

 ma.sses, m^ and m, gm., aie readily found to tiu- rccpiired preii- 

 sion. 



With e.^-e^ = lifC it is necessary to dctennim' tiiis difference 

 to I/IOOO^^C, which is alout the limit of accuracy obtainable with 

 a platinum, or, in fact, any thermonicter. The water Hows past 

 the platinum thermometers contained in Dewar Hasks, as shown in 



Fig. 2. 



The water, after passing thrmigh the calorimeter is o.lleclvd in 

 a copper can. A two-wav tap turns it into this can at the U-gm- 

 ning of the experiment, and at the same time starts the chronc 

 graph record; at the end of the experiment turning the tap cauBt^ 

 the water to flow to a different vessel, and also st<,ps i\x. chronograph 

 record. The water collected is weighed. 



