15S 



Presents, 



[Dec. 5, 



the potential difference to be measured. The general working of the 

 intrnment is also considered, and numerical results and curves are 

 given for the electrometer used by the authors. 



In Appendix II is considered the question of the reduction of the 

 readings to absolute measure, or, in other words, the calculation of 

 the effective area of the attracted disk. Starting from an expression 

 given by Professor J. J. Thomson, a formula is deduced for the effec- 

 tive area of the disk when in the plane of the guard-ring, which is 

 probably more accurate than that given by Maxwell. It is then 

 shown that, in addition to the well-known advantages of the hetero- 

 static over the idiostatic method of measurement, there is the addi- 

 tional advantage that the error in the ordinary assumption as to the 

 value of the effective area is of far less importance in the former 

 than in the latter method. Finally an investigation is given in which 

 Schwarz's method is applied to determine the error in the ordinary 

 assumption as to the value of the effective area for the case when the 

 disk and guard-ring are not quite in the same plane. 



V. " Variations in the Electromotive Force of Clark Cells with 

 Temperature." By W. E. Ayrton, F.R.S., and VV. R. 

 Cooper, M.A., B.Sc. Received October 29, 1895. 



[Publication deferred.] 



Presents, December 5, 1895. 



Transactions. 



Baltimore : — Johns Hopkins University. Circulars. Vol. XV. 



No. 121. 4to. Baltimore 1895. The University. 



Batavia: — Koninkl. Natuurkundige Vereeniging in Nederlandsch- 



Indie. Natuurkundig Tijdsckrift voor Nederlandsch-Indie. 



Deel LIV. 8vo. Batavia 1895. The Society. 



Boston : — American Academy of Arts and Sciences. Proceedings. 



Vol. XXX. 8vo. Boston 1895. The Academy. 



Kew: — Royal Gardens. Bulletin of Miscellaneous Information. 



1895. Appendices 2, 3. 8vo. London. The Director. 



London : — British Astronomical Association. Journal. Vol. VI. 



No. 1. 8vo. London 1895. The Association. 



Clinical Society. Transactions. Vol. XXVIII. 8vo. London 



1895. The Society. 



Geologists' Association. Proceedings. Part XIV. Part 5. 8vo. 



London 1895. The Association. 



