The supports for the needle, and a box to protect the needle from draughts, as well as the vertical circle, can be rotated about a vertical axis, and their azimuth read off on a horizontal divided circle. There are also two adjustable stops which can be set in any position, and allow the upper part of the instrument to be rotated through exactly 180° without the necessity of reading the horizontal circle.
When making a determination of the dip with the dip circle, a number of separate readings have to be made in order to eliminate various instrumental defects. Thus, that side of the needle on which the number is engraved being called the face of the needle, and that side of the protecting box next the vertical circle the face of the instrument, both ends of the needle are observed in the following relative positions, the instrument being in every case so adjusted in azimuth that the axle of the needle points magnetic east and west:-
i.' Face of instrument east and face of needle next to face of instrument;
ii. Face of instrument west and face of needle next to face of instrument;
iii. Face of instrument west and face of needle away from face of instrument;
Face of instrument east and face of needle away from face of instrument.
Next the direction of magnetization of the needle is reversed by stroking it a number of times with two strong permanent magnets, when the other end of the needle dips and the above four sets of readings are repeated. The object in reading both ends of the needle is to avoid error if the prolongation of the axle of the needle does not pass through the centre of the Vertical circle, as also to avoid error due to the eccentricity of the arm which carries the reading microscopes and verniers. The reversal of the instrument between (i.) and (ii.) and between (iii.) and (iv.) is to eliminate errors due to (a) the line joining the zeros of the vertical circle not being exactly horizontal, and (b) the agate knife-edges which support the needle not being exactly horizontal. The reversal of the needle between (ii.) and (iii.) is to eliminate errors due to (a) the magnetic axis of the needle not coinciding with the line joining the two points of the needle, and (lr) to the centre of gravity of the needle being displaced from the centre of the axle in a direction at right angles to the length of the needle. The reversal of the poles of the needle is to counteract any error produced by the centre of gravity of the needle being displaced from the centre of the axle in a direction parallel to the length of the needle. For use at sea the dip circle was modified by Robert Were Fox (Annals of Electricity, 1839, 3, p. 288), who used a needle having pointed axles, the points resting in jewelled holes carried by two uprights, so that the movement of the ship does not cause the axle of the needle to change its position with reference to the vertical divided circle. To counteract the tendency of the axle to stick in the bearings, the instrument is fitted with a knob on the top of the box protecting the needle, and when a reading is being taken this knob is rubbed with an ivory or horn disk, the surface of which is corrugated. In this way a tremor is caused which is found to assist the needle in overcoming the effects of friction, so that it takes up its true position. In the Creak modification of the Fox dip circle, the upper halves of the jewels which form the bearings are cut away so that the needle can be easily removed, and thus the reversals necessary when making a complete observation can be performed (see also l'l¢GNETO-METER).
Induction Inclinorneters.~The principle on which induction inclinometers depend is that if a coil of insulated wire is spun about a diameter there will be an alternating current induced in the coil, unless the axis about which it turns is parallel to the lines of force of the earth's field. Hence if the axis about which such a coil spins is adjusted till a sensitive galvanometer connected to the coil through a commutator, by which the alternating current is converted into a direct current, is unreflected, then the axis must be parallel to the lines of force of the earth's field, and hence the inclination of the axis to the horizontal is the dip. The introduction and perfection of t-his type of inclinometer is almost entirely due to H. Wild. His form of instrument for field observations* consists of a coil 10 cm. in diameter, containing about 1000 turns of silk-covered copper wire, the resistance being about '40 ohms, which is pivoted inside a metal ring. This ring can itself rotate about a horizontal axle in its own plane, this 'axle being at right angles to that about which the coil can rotate. Attached to the axle of the ring is a divided circle, by means of which and two reading microscopes the inclination of the axis of rotation of the coil to the horizontal can be read. The bearings which support the horizontal axle of the ring are mounted on a horizontal annulus which can be rotated in a groove attached to the base of the instrument, as so to allow the azimuth of the axle of the ring, and hence also that of the plane in which the axis of the coil can move, to be adjusted. The coil is rotated by means of a flexible shaft worked by a small cranked handle and a train of gear wheels. The terminals of the coil are taken to a two-part commutator of the ordinary pattern on which rest two copper brushes which are connected by flexible leads to a sensitive galvanometer. The inclination of the axis of the coil can be roughly adjusted by hand by rotating the supporting ring. The final adjustment is made by means of a micrometer screw attached to an arm which is clamped on the axle of the ring.
When making a measurement the azimuth circle is first set horizontal, a striding level placed on the trunnions which carry the ring being used to indicate when the adjustment is complete. The striding level is then placed on the axle which carries the coil, and when the bubble is at the centre of the scale the microscopes are adjusted to the zeros of the vertical circle. A box containing a long compass needle and having two feet with inverted V's is placed to rest on the axle of the coil, and the instrument is turned in azimuth till the compass needle points to a lubber line on the box. By this means the axis of the coil is brought into the magnetic meridian. The commutator being connected to a sensitive galvanometer, the coil is rotated, and the ring adjusted till the galvanometer is unreflected. The reading on the vertical. circle then gives the dip. By a system of reversals slight faults in the adjustment of the instrument can be eliminated as in the case of the dip circle. With such an instrument it is claimed that readings of dip can be made accurate to ==o-r minutes of arc.
The form of Wild inductor for use in a fixed observatory differs from the above in that the coil consists of a drum-wound armature, but without iron, of which the length is about three times the diameter. This armature has its axle mounted in a frame attached to the sloping side of a stone pillar, so that the axis or rotation is approximately parallel to the lines of force of the earth's field. By means of two micrometer screws the inclination of the axis to the magnetic meridian and to the horizontal can be adjusted. The armature is fitted with a commutator and a system of gear wheels by means of which it can be rapidly rotated. The upper end of the axle carries a plane mirror, the normal to which is adjusted parallel to the axis of rotation of the armature, A theodolite is placed on the top of the pillar and the telescope is turned so that the image of the cross-wires, seen by refection in the mirror, coincides with the wires themselves. In this way the axis of the theodolite telescope is placed parallel to the axis of the armature, and hence the dip can be read off on the altitude circle of the theodolite. Authorities.—In addition to the references already given the following papers may be consulted: (1) Admiralty Manual of Scienttfic Inquiry, which contains directions for making observations with a dip circle; (2) Stewart and Gee, Elementary Practical Physics, which contains a full description of the dip circle' and instructions for making a set of observations; (3) L. A. Bauer, Terrestrial Magnetism (Igor), 6, p. 31, a memoir which contains the results of a comparison of the values for the dip obtained with a number of different circles; (4) E. Leyst, Repertoriurn f12r Meteorologie der kaiserl. Akad. der Wiss. (St Petersburg, 1887), ro, No. 5, containing a discussion of the errors of dip circles; (5) H. Wild, Bull. de l'Acad. Imp(des Sci. de St Pétersbourg (March 1895), a paper which considers the accuracy obtainable with the earth inductor. (W. WN.)
1 Repertorium far Meteorologle der kaiserl. -Akad. der 'Wissensch.
(St Petersburg, 1892), 16, No. 2, or Meteorolog. Zeits. (1895), rz, p. 4I.