154 Laby and Roberts: 



Distilled Avater contained in two thermally insulated tanks, so 

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

 ill the exper'iments. 



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 light 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 Avails 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 Avater 8^) above 

 the outer Avail (that is, the aii- tenii^erature 0^). This difference 

 (^i"~^^) degree can be determined, it is expected, Avith sufficient 

 accuracy to attain the desired precision. 



Only preliminary determinations of J have so far been made, 

 further experiments are noAv in progress. 



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

 and the success he has achieved in constructing the apparatus shoAvn 

 in the figures. 



DESCRIPTION OF PLATES. 



Pl.\te X. 

 Fig. 2.- General vieAv of apparatus. 

 ,, 3. — Rotor with top removed, shoAving pole j^ieces and wind- 

 ings. 

 ,, 4. — Rotor mountcMl. 



Platk XI. 



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

 in the channels for Avater, on the right the copper 

 cylinder fitting over it. 

 Fig. 6. — Stator attached to torsion Avheel with Hask removed. 

 ,, 7. — Stator witli liask attached, shoAving plate Avitli levelling 



screAv.s resting on the top plate of Fig. 5. 

 ,, 8. — Knife edge bearings. 



[Note added lObh March, 1 920 : — Inipiovenient in the brush con- 

 tacts on tlie rotor has led to greater steadiness in the rotor field 

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

 damping device mentioned in the text unnecessary. 



