Magnetic Tractive Force. 159 



o-' = (T{l + 0-5(m-l)-0-277(m-l) 2 + 0-13(m-l) 3 }*. 



<r' is proportional to the ballistic sensitiveness of the galvano- 

 meter corrected for the damping of the oscillations. 



Table I.— Temp. 21°-3 0. 



R. 



<T. 



m. 



<s\ 



7-82 



2112 



1-231 



2333 



10-82 



2142 



1-187 



2325 



12-82 



2149 



1-184 



2327 



1482 



2164 



1-180 



2340 



17-82 



2165 



1161 



2326 



22-82 



2191 



1143 



2335 



The value of a' is therefore within \ per cent, constant ; 

 in other words, the influence of the resistance of the secondary 

 circuit on the ballistic sensitiveness of the galvanometer is 

 completely, though indirectly, explained by its well-known 

 influence on the damping of the oscillations, and can be 

 eliminated, was as done in all the galvanometer observations. 

 Moreover, the damping was too small to have any effect on 

 the proportionality of throw to quantity of electricity flowing 

 through the galvanometer, as I convinced myself by special 

 observations. 



The areas of the two isthmus coils were compared electro- 

 magnetically in the uniform field between flat pole-pieces with 

 a standard coil made of a single strip of phosphor bronze 

 wound on a circular glass plate which had been carefully 

 turned, ground, and measured. The connecting strips of the 

 standard coil were exactly laid one above the other so as to 

 enclose no area parallel to the plane of the coil. 



In using this coil the influence of its resistance on the 

 sensitiveness of the galvanometer was of course, as explained 

 above, taken into account. 



The total areas of the two isthmus coils were thus found to 

 be 1*235 and 2'553 sq. centim. respectively, that of the 

 standard coil being 1*530 sq. centim. These areas were also 

 calcnlated from the previously measured diameters of the 

 brass bobbin and of the wire, and the calculated values 



* Ayrton, Mather, and Sumpner, Phil. Mag. [5] xxx. p. 69 (1890). 



