146 



Mr. J. E. H. Gordon on the 



[June 15, 



with those of coils of kno^Ti areas. Two coils were used, a small one 

 and the great electro-djnamometer of the British Association. With the 

 small coil the same currents were sent through a coil and helix, and the 

 distances from the suspended magnet varied ; while with the large one 

 the distances were the same and the currents varied. These latter expe- 

 riments were made by Prof. Maxwell. 



The foUo^-ing values were then obtained for the area of the helix : — 



By the author with small dynamometer, 



S(A)= 77417'2 sq. centims. ; 

 by Prof. Maxwell with large dynamometer, 



2(A) = 77488-8 sq. centims.; 

 the mean, 77453-0, of these was adopted. 



Calculation of the strength of the current in terms of the deflection I of the 

 magnet suspended outside the helix and in the hisecting plane perpendi- 

 cular to its axis. 

 The author shows that this is 



where r is the distance from the suspended magnet to either end of the 

 axis of the helix, and H the horizontal component of the earth's mag- 

 netism at the time and place of observation. 



Formula for w. 



w is the rotation of the polarized ray expressed in "circular measure 

 between two points in its path, whose magnetic potential differs by 

 unity; thus 







where L and M are the ends of the tube, and Q is half the difference of 

 the cii'cle readmgs expressed in circular measure. 



An approximation is given for the difference of potentials at the ends 

 of a tube (A B) of finite length projecting at each end of the hehx (L M). 

 The letters being in the order A, L, M, B, the formula for w becomes 



^-'^-(^^^^))(iA^ + MATMB) 



Hr^ tan a 

 2(A) 



Tan }). 



The author explains at length the method of adjusting the telescope 

 and scale. 



A formula for deducing the angular deflection from the scale-reading 

 is obtained. 



