Verification of Thermometers at Freezing-point of Mercury. 27 



another, in a motor they act in such a manner as to mutually 

 assist each other. Thus, with the strength of field, the current 

 in the conductor, and the speed the same in the two cases, it 

 will be seen that in a motor the eddy-currents in the iron core 

 of the armatures will be greater than in a generator, and 

 therefore the heat-loss in the former will be more than in the 

 latter. There is little doubt that this is the cause of the lower 

 efficiency of motors than of generators; and it points to the 

 advisability of giving even greater attention in the former to 

 those principles which are well understood for the reduction 

 or elimination of eddy-currents. 



V. Note on the Versification of Thermometers at the Freezing- 

 Point of Mercury. By GL W. Whipple, B.Sc, F.R.A.S., 

 Superintendent of the Kew Observatory *. 



IT is not many years since the operation of freezing mer- 

 cury was considered a scientific feat, and a party of 

 gentlemen were invited to the Kew Observatory to assist at 

 the performance of the operation by the late Mr. R. Addams. 

 That gentleman produced the solidified mercury by the 

 action of ether upon carbonic acid gas in the solid form, and 

 with his assistance considerable quantities of mercury were 

 frozen, and a number of observations were made with care- 

 fully constructed thermometers, which gave the value for the 

 melting-point of mercury which has since been accepted as 

 the fixed point. Prof. Balfour Stewart communicated an 

 account of these experiments to the Royal Society, in whose 

 publications they will be found (Phil. Trans. 1863, pp. 

 425-435). 



Since that date the determination of the low-temperature 

 fixed point of thermometers has become a regular operation 

 in the verification department of the Kew Observatory, 

 and some eight or nine dozen instruments are compared in 

 melting mercury annually, most of them being destined for 

 use in Canada and other cold countries. 



As the mode of making this comparison is not described in 

 any readily accessible publication, we have frequently been 

 requested to give an account of it, and have accordingly 

 drafted this paper with a view of putting it on record. 



The carbonic acid in its liquid form is purchased generally 

 of Orchard, of High Street, Kensington, in the well-known 



* Communicated by the Physical Society : read November 14, 1885. 



