In practice, a rather elaborate series of observations is gone through for the purpose of elimination of various errors in both D and H. (See MAGNETOMETER.) The classical method of de termining the inclination is by the use of the dip circle, in which a magnetic needle is mounted so that it can turn in a vertical plane which is adjusted to lie in the magnetic meridian. In order to obtain an accurate result from the instrument many sources of error must be recognized and eliminated by a systematic series of observations. For the standard procedure see INCLINOMETER.
Electrical Methods.—Within recent years, the principle of com paring the earth's horizontal intensity with that produced by a measured current in a coil of known dimensions has been occa sionally employed in observatories in one or other of various possible forms. In an instrument designed by F. E. Smith, follow ing the suggestion of Sir Arthur Schuster, the principle is ") : A coil of the Helmholtz-Gaugain type, designed so as to secure a field constant to one part in Ioo,000 within a sphere of 3o mm. diameter, is used to produce the magnetic field to be compared with that of the earth. At the centre of this coil a small magnet is suspended, and the principle of the method involves balancing the earth's horizontal intensity with the field produced by the coil, which field is given by Fi, where i is the current and F a constant calculable from the geometrical constants of the coil. One could secure the balance between Fi and H by adjusting i to secure an infinitely long period of vibration of the suspended magnet, but, a more practicable method, and the method fol lowed, is one which invokes the fact that if the axis of the coil be set at an angle a to the magnetic meridian, it is possible to adjust i so that the magnet sets itself perpendicular to H in which case The procedure, except for details is as follows : The axis of the coil is first set so that on starting or stopping the current, no deflection is obtained in the suspended magnet. The axis is then in the magnetic meridian, and the direction of the field of the coil is the same as that of H, the horizontal component of the earth's field. The axis is now turned through a slight angle a. The current in the coil is reversed and adjusted so that the axis of the magnet becomes perpendicular to the magnetic meridian. For convenience, the final adjustment of this condition is made by adjusting a rather than i. Next, keeping i constant, the axis of the coil is turned to approximately an equal angle 0 on the other side of the magnetic meridian, so that the magnet swings through 18o°. The angle 0 is adjusted so that the angle through which the magnet swings is exactly 18o° and the magnitude of the horizontal intensity is then given by The precision of the method is such as to permit of an accu racy of a few parts in ioo,000 by measurements lasting only a few minutes, although in seeking this accuracy, attention must be given to many details not mentioned here").
A method to some extent analogous to the foregoing was also devised by W. M. Hicks") and submitted to experimental test by
W. A. Jenkins. Yet another absolute instrument for the purpose takes the form of a sine galvanometer designed by S. J. Barnett").
For the determination of the dip, the dip circle has been super seded in many cases by the earth-inductor"), in which a coil is rotated about an axis in its own plane, the axis being adjusted until no current is obtained in the coil. In this case the axis coincides with the direction of the earth's lines of force. The earth inductor must, of course, be provided with a commutator, and the galvanometer used must be situated sufficiently far from the apparatus to insure that its stray field shall not affect the measurements.
In sea observations''), it is customary to use for declination a type of pivoted magnet system which is immersed in a liquid so as to remove much of the weight from the pivot and so reduce the friction. The geographical meridian is determined by observa tions on celestial bodies.
For inclination both the earth inductor and a specially mounted dipping needle have been employed.
Total intensity observations have been made at sea by a method devised by E. Lloyd. The method involves two operations, during both of which the dip circle is placed so that the suspended needle swings in the plane of the magnetic meridian. In north magnetic latitudes, for example, the inclination is measured with a needle having a weight added to its south end. Then the needle is used to deflect a second dip needle when placed at right angles to its undeflected direction and at a definite distance. The first measurement serves to give a relation between R, the weight, and the moment M of the loaded magnet. The second measurement gives a relation between M and R, so that from the two relations M may be eliminated, leaving an expression for R in terms of the loading weight.
Measurements of the horizontal intensity are made by measuring the angle of deflection produced in the liquid compass by a magnet placed above it with its center on the vertical axis through the pivot, and with its length horizontal and perpendicular to the deflected direction of the axis of the pivoted needle. Under this condition where u is the angle of deflection, m is the moment of the deflect ing magnet, and c is a constant depending upon the position of the deflecting magnet, etc. The constant quantity me is evaluated by making measurements with the instrument at a fixed observa tory where H is simultaneously measured by the usual methods.