A POCKET GALVANOMETEE. 



287 



seen that at each of these points, the lines of force begin to change 

 their curvature, and the field is sensibly constant. By constructing 

 the instrument, so that the base lies just above one of these points, we 

 adapt it for the measurement of very strong currents, such as 100 

 ampères or greater. To facilitate such a measurement a V-groove is 

 cut out along the base in the proper position, and into this the circuit 

 bearing the current is received. To reduce the value thus obtained to 

 what it would have been, had it been placed in the jaw, we have only 

 to multiply the reading by — 3. 



The following table giving the proportional decrement of force 

 for given displacements of the magnets, will show to what amount of 

 displacement we may, without sensible error, assume the uniformity of 

 the galvanometer constant as obtained by calibration for small dis- 

 placements. Computing (F—Fo)/F„ we have 



Along a;-axis. 



Along y-?ixis. 



Along Diagonal. 



Ix/a 



IF/F, 



ly/b 



^F/F, 



V/r 



IFIB\ 



Vi 



1/4161 



1/1 



1/135 



1/4 



1/445 



1/3 



1/1333 



1/3 



1/183 



1/3 



1/132 



1/2 



1/273 



1/2 



1/24 



1/2 



1/21 



1 



1/21 











As long as the deflection is within ka the error will be less than 

 1/2 per cent, and when it is to the extreme limit of a, the error will be 

 about 5 per cent. It is interesting to notice that IF is very nearly 

 proportional to the fourth powers of Ix, ly, Ip, even when these are -ia, 

 \h, ^r. The above table enables us to adjust the range of the index 

 and the jaw of the instrument so as to keep the magnitude of errors 

 within any assigned limit. In the actual instrument constructed 



