154 Lahy (tvd Roheris: 



Distilled water contained in two thermally insulated tanks, so 

 arranged as to give a constant head of about 240 cm. (8 feet) is used 

 in the experiments. 



The precision of the experiments will be limited (1) by the steadi- 

 ness in the rate of generation of heat, and (2) by constancy of the 

 loss of the heat, L, in the heavy and liglit experiments. Ihe first 

 of these has been satisfactory in the preliminary experiments, and 

 if necessary, could be improved. The principal loss of heat, no 

 doubt, occurs through the walls of the vacuum jacket of the calori- 

 meter, and is proportional to the excess of the temperature of the 

 inner wall (that is, the temperature of the inlet water B^ above 

 the outer wall (that is, the air temperature 6^. This difference 

 {0-^ — 0^) degree can be determined, it is expected, with sufficient 

 accuracy to attain the desired precision. 



Only preliminary determinations of J have so far been made, 

 further experiments are now in progress. 



We have to thank Mr. R. Berryman for the care he has taken 

 and the success he has achieved in constructing the apparatus shown 

 in tlie figures. 



DESCRIPTION OF PLATES. 



Plate X. 

 Fig. 2. — General view of apparatus. 

 ,, 3. — Rotor with top remoTed, sliowing pole pieces and wind- 

 ings. 

 ,, i. — Rotor mounted. 



Plate XI. 



,, 5. — The two parts of the stator, on the left the iron cylinder 

 in the channels for water, on the rijjht the copper 

 cylinder fitting over it. 

 Fig. 6. — Stator attached to torsion wheel with iiask removed. 

 ,, 7.- — Stator with flask attaclied, sliowing plate witli levelling 



screws resting on the top plate of Fig. 5. 

 ,, 8. — Knife edge bearings. 



[Note added 10th March, 1 920 : — Iniprovement in the brush con- 

 tacts on tl>e rotor lias led to greater steadiness in the rotor field 

 magnet current and therefore in the couple ; this has made the water 

 damping device mentioned in the text unnecessary. 



