254 Mendenhall and Waidner — 



Changes of zero, and changes in sensibility. — In general 

 both the zero and the sensibility of a galvanometer will be 

 affected by the changes constantly going on in the direction 

 and magnitude of the earth's horizontal force, and it becomes 

 of interest to know whether there is any position of the gal- 

 vanometer with respect to the magnetic meridian which will 

 give minimum values for these disturbances. An examination 

 of the magnetic records of the Naval Observatory shows that 

 rather quick changes of H to the amount of nearly 1 per cent 

 may be expected rarely, and to the amount of \ per cent fre- 

 quently, while the declination sometimes changes rapidly by 

 19' and frequently by 3 7 . A consideration of the vector dia- 

 gram previously given, involving the earth's horizontal force 

 H, the force due to the control magnet M, and the resultant 

 force R, supposing the above changes to take place in the 

 direction and magnitude of H, and assuming various directions 

 for R, shows that there is no position of the galvanometer for 

 which the resulting changes of zero or sensibility are markedly 

 small. For null methods, however, in which changes of sensi- 

 bility are of less moment than changes of zero, it would be 

 somewhat advantageous to have the plane of the coils parallel 

 to the magnetic meridian. It is interesting to note the magni- 

 tude of the changes produced by the above mentioned changes 

 in H and the declination in the case of two identical systems, 

 both arranged to have a complete period of 10 s in the galva- 

 nometer, but one having a period of I s in the earth's field, 



while the other is astaticised to a 4 3 period. In the first case 



Ti- 

 the resultant field in the galvanometer would equal — -, in the 



TT 100 



second case — — ; and we would have, if the plane of coils is 

 perpendicular to magnetic meridian : — 



(a) non-astatic, 



change of zero due to 19' change in declination, = 0° 



a it a « a a jj of -^ _ ao o 



change of sensibility due to 19' change in declination = 55$ 

 " " " " " " H of \<f> = 5Qf 



(b) astatic 



change of zero due to 19' change in declination = 0° 



" " « " " " H of Ifo = 3°40' 



change of sensibility due to 19' change in declination = S.5f 



it a a a ct n a H of 1^ = \<f> 



With the astatic system, but for the smaller changes in II and 

 declination of \ per cent and 3', the change in zero would be 

 less than 30' (or 18 mm at l m distance), and the change of sensi- 

 bility about 0*6 per cent. This emphasizes the great importance 

 of careful astaticising unless magnetic shielding is available. 



