438 



[Jan. 11, 



January 11, 1883. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. "Experiments, by the Method of Lorentz, for the further 

 Determination of the Absolute Value of the British Asso- 

 ciation Unit of Resistance, with an Appendix on the Deter- 

 mination of the Pitch of a Standard Tuning-Fork." By 

 Lord RAYLEIGH, F.R.S., Professor of Experimental Physics 

 in the University of Cambridge, and Mrs. H, SlDGWICK. 

 Received December 8, 1882. 



(Abstract.) 



The experiments described in the present paper were carried out 

 during the spring and summer months of the present year, at the 

 Cavendish Laboratory, and are divided into three distinct series. In 

 the first and second series, the induction coils were situated nearly in 

 the plane of the revolving disk, as in Lorentz's original use of the 

 method ; the difference between the two series relating only to the 

 speed of rotation, which was varied in the proportion of 10 : 16. The 

 third series presents a point of novelty, in that the induction coils 

 were separated from the disk to such a distance as to render the 

 accuracy of the result practically independent of the mean radius of 

 the coils. 



The small resistance, traversed by the battery current, to which the 

 terminals of the galvanometer branch are connected, was obtained 

 indirectly by a method of shunting. Thus in the first series, the prin- 

 cipal part of the battery current passed on one side through two unit 

 coils, placed in multiple arc, and equivalent to J, and only a com- 

 paratively small remainder through a second branch, composed of two 

 coils in series, of values 10 and T \. The terminals of the galva- 

 nometer branch were connected with the extremities of the -fa, and 

 the difference of potentials between them, due to the primary current, 

 was thus reduced to that which would be required to drive the 

 current through a resistance of ■ w ^ u . 



From the first series — 



1 B.A. unit= -98674 x 10 9 C.G.S. 



