74 Hoy at Society: — 



axis in terms of that at the centre of the great dynamometer of 

 the British Association, whose power is known in absolute measure. 



The intensities Mere compared by varying currents sent opposite 

 ways through each, till the action on a small magnet at their 

 common centre was nil. 



The intensity at each of a series of points being known for a given 

 current, the difference of magnetic potential at the two ends for 

 that current was obtained by integrating with respect to the length 

 between limits corresponding to the end of the helix. 



For this Weddle's rule was used, viz. 



h 3 



n x <lvz= — h K + ^ + ^ + ^ + s^ + iO + gx}, 



where G* is the length of the helix and u x the magnetic intensity 

 at any point. 



The difference of magnetic potential at the ends for a certain 

 current being known, the strength, N, of the helix (which is the 

 ratio of this difference to the current, or the difference of mag- 

 netic potential which would be due to a unit current) is known, and 

 is a number, because current and magnetic potential are of the same 

 dimensions. 



In the helix used, which was about 26*34 centhns. in length 

 and 13 centims. in diameter, we had 



N= 10752. 



The absolute value of the degrees of a tangent galvanometer 

 was also determined by placing it under the dynamometer. 



To determine the rotation of the plane of polarization, a Mcol's 

 prism, set in a circle, was used, and the light was polarized by 

 means of a prism invented by Professor Jellett, and described by 

 him in vol. xxv. of the Transactions of the Eoyal Irish Academy. 



It was constructed of Iceland spar, and its field of vision con- 

 sisted of a circle divided by a line, the light of one half of which 

 was polarized in a certain plane, and the light of the other half in a 

 plane inclined at about 2° to that of the first. The intermediate 

 position of the Nicol, when the whole field was equally dark, 

 could be determined with some accuracy. 



The water was contained in a tube with glass ends, of the same 

 length as the helix, and placed with it. The polarized ray was 

 sent through it, and a current, whose intensity, C, was measured 

 by the tangent galvanometer included in the circuit, was sent 

 through the helix first in one direction and then in the other, and 

 the plane of polarization observed. Half the difference of the 

 readings was the rotation produced by the current. 



If we call d this rotation expressed in circular measure, and 

 define Verdet's constant as the rotation which a unit current in 

 a unit coil could produce in unit of length of distilled water, we 

 have 



d 



