498 Messrs. W. H. Preece and H. R. Kempe. [June 10, 



divisions ; and to avoid parallax the indicator is reflected in a mirror, 

 so that when the image and the indicator are in one line, no error from 

 this cause occurs. 



Mr. Eden, one of the assistant electricians of the Post Office, 

 discovered while experimenting that the galvanometer could be made 

 more sensitive to increments of current for high deflections if it were 

 given a false zero. In fact, if the instrument be " slewed n round so 

 that the plane, of the coil makes an angle of 60° with the meridian, 

 then the instrument becomes twice as sensitive as it was before ; and 

 he suggested a double scale such as is shown in Fig. 1 (p. 497) to 

 utilise this fact. 



This plan has been adopted, and all tangent galvanometers in the 

 Post Office service will be gradually altered to the new scale. 



It is quite clear that it owes its increased sensitiveness to the fact 

 that when the needle reaches its maximum deflection, its angle to the 

 lines of force of the field is in the most favourable direction for 

 deflection by the current. In fact, the plane of the coil becomes 

 parallel to the plane of the needle which is then in the most uniform 

 portion of the field. 



By changing the zero of the instrument the range of movement 

 can be considerably increased. Thus if (3° be the angle which the 

 needle normally makes with the coil, and if a° be the angle to which 

 the needle has been deflected on the other side of the coil, then, / 

 being the deflective force, we have — 



,_, sinO° + 0°) 



/ — Jl o > 



1 COS cc 



where f x is a constant ; but if the needle were parallel to the coils, 

 then — 



f=f l tan « l5 



therefore — 



sin(«° + /3°) 



COS a. 



Now if we have /3° = 60°, and if the current were sufficient to turn 

 the needle through 2/3°, or 120°, that is, if we have a=60°, then — 



sin 120° ., 



— — = tan a-, , 



cos 60° 1 ' 



but sinl20 o =sin(180 o -60 o ) = sin60°, therefore— 



sin 60° 



, o sm a, 

 -= tana x = — -L, 



cos 60° cos a 



or 



^=60°, 



