330 MAGNETIC METHOD [Chap. 8 



the magnet is So , the reading with the north pole of the magnet down is 

 S2 , and the reading with the north pole up is Si (repeated by turning the 

 magnet over two or three times and using two or three distances), the 

 scale value'^ 



4M..k . s, + & ,,„^(,heck), (8-26a) 



(s2 — Si)r^ ' 



where k is given by eq. (8-24d) and Ma is the moment of the auxiliary 

 magnet. One of the auxiliary magnets should be retained for use as a 

 standard and not be taken into the field. For the determination of the 

 moments of other magnets from the standard, the following formula ap- 

 plies, provided the unknown magnet and the standard are used in the 

 same distance, 



M, = M.t . ')'' ~ ''l^ . ^ ; (8-266) 



(Sl — S2)at kz 



kg</ki = 1 if the magnets used for comparison are of the same length. 



Since magnets lose their strength with time and must be recalibrated 

 at intervals, Helmholtz coils (Fig. 8-21) are often preferred. Current is 

 supplied from dry cells, controlled by rheostats, and read on a milliam- 

 meter. If / is the current in milliamperes, and C the "coil constant" 

 (deflection per mil.), the scale value 



^^^ (8-26c) 



(S2 — Sl) 



The Helmholtz coil may also be used for calibrating magnets by producuig 

 a number of readings (si — Sz) with the magnet first and then with the 

 coil, and by adjusting the current until magnet and coil deflections are 

 equal. Then 



e(si - 52) = 4 ^^ = 2IC , (8-26d) 



where r is the distance of the magnet, C the coil constant, / the current. 

 Hence 



M^ = IC/ /2k. 



Observations in different azimuths are used when deflection magnets or 

 coils are not available. Readings are taken in the E-W and N-S positions 

 of the instrument; then the scale value 



— ^-^ , (8-26e) 



2f(SE - sn) 



^^ For the vertical magnetometer. 



