Marine Earth-Inductou 201 



Scheme of Observation. 



The galvanometer is shunted so that its turning-points can be read satisfactorily. 

 The horizontal circle of the inductor is set with the rotation axis in the magnetic meridian. 

 The vertical circle is then set approximately 1 degree less than the actual magnetic inclin- 

 ation and the coil is spun by turning the crank in a right-hand direction at the fixed speed 

 adopted for the particular observations, and rapid, successive readings of the galvanometer 

 are made during the period of spinning. The coil is next spun in the opposite direction 

 at the same speed and similar galvanometer readings are made. Similar readings follow for 

 the vertical circle set about 1 degree greater than that of the magnetic inclination. From 

 these readings the sensibility of the instrument is determined, as also, by linear interpola- 

 tion, the setting for which the mid-point will be independent of the direction of spin. Thus, 

 if <S„ is the nth vertical-circle setting, the reading of the vertical circle for the position of 

 balance, assmning no appreciable variation in themio-electromotive force due to possible 

 changes in temperature at the commutator, would be 



dn 



\d„ — d„+iJ 



where d„ is the difTerence between the galvanometer readings for right-hand and left-hand 

 rotation of the crank for the vertical-circle setting S„, d„+i the corresponding quantity for 

 the (n -f 1) setting of the vertical circle, and A is the difference between the two settings 

 of the vertical circle, i. e.,A = S„+i — *S„. Usually the observation is begun with the vertical 

 circle east of the magnetic meridian, so that a second detennination, similar to the one 

 outlined above, is then made with the vertical circle west. From these two interpolated 

 positions of balance the angle of dip may be determined readily by combination of the 

 results. It is to be noted that the scheme of observation makes n always an odd number. 

 The speed used in rotating the coil is 120 turns of the crank per minute; by reason of the 

 multiphcation of the gear system this is equivalent to 500 rotations of the coU per minute. 



As the interpolation term may be subject to error because of any change in the sensi- 

 bihty of the galvanometer, or in the quaUty of insulation, or in the resistance of the various 

 contacts and portions of the circuit, a sharp lookout is kept for any leakage, this being 

 indicated by a decrease in the apparent sensibility of the galvanometer. To prevent leak- 

 age, all insulated surfaces are greased with vaseline or good kerosene and wiped off and 

 greased again frequently. The term may also be subject to error if there is variation in 

 the thenno-electromotive force, due to changes of the temperature at the commutator 

 during the observation. The commutator is, therefore, kept well lubricated with vaseline, 

 in order to reduce the possibiUty of such variation. 



For the above scheme of observation four persons are required : two at the galvanom- 

 eter, one to read and the other to record ; two at the earth inductor, one to make the circle 

 settings and to rotate the coil, the other to record the settings and to keep the meridional 

 plane of the instrument in the magnetic meridional plane. The last operation is of the 

 same importance as in the use of the dip circle, and is maintained by reference to a standard 

 Ritchie compass, where simultaneous observations are being made by another observer in 

 connection with the horizontal-intensity observations. 



In the first work on board ship it was attempted to form mental means of the galvanom- 

 eter deflections. This was found to be fairlj' satisfactory when the sea was smooth, but 

 impracticable under ordinary conditions, on account of the swinging of the gunbal. The 

 method finally adopted and used is to make 20 readings of the galvanometer scale for each 

 operation of the earth inductor. Four determinations of the dip, namely, circle east and 

 circle west, with the gunbal rings both direct and reversed, are necessary in order to obtain 

 one result. The whole operation as outlined requires from 13 to 16 minutes. 



