A dip circle, or dip needle, is an instrument for measuring magnetic dip, consisting of a magnetic needle mounted free to pivot in the vertical plane with a graduated circle. For use, the instrument is carefully leveled and oriented into the magnetic meridian.

 

The magnetic dip or inclination is one of the three elements necessary for complete determination of Earth's total magnetic force at any place. It is defined to be the angle which the magnetic axis of a magnet, swinging freely in the plane of the magnetic meridian, makes with the horizon.

 

The dip circle which is employed to determine its amount at any place, consists essentially, of a magnetic needle suspended in the plane of the magnetic meridian, so that it can indicate, on a graduated circle in its plane, the angle required. One form of the instrument is shown in Figure 1. A is a vertical brass circle, graduated in degrees and fractions of a degree, inclosed between circular glass doors (one of which is seen opened in the figure), and supported on the pillar P. This pillar fits into a socket in the horizontal circular disk B, also graduated at its margin, and from the base of the pillar two flat brass strips, CD, reach on opposite sides to the graduated edge.

 

The horizontal disk is supported on three legs, whose lengths can be adjusted by screwing or unscrewing. On this horizontal circle the vertical circle can be turned in azimuth. The needle, NS, a thin flat piece of steel, pointed at both ends, is suspended in the plane of the vertical circle by means of hard, polished, cylindrical axles passing through its center of gravity, and rolling on the agate edges, EE. When the indications of the needle are not actually being read, the axles may be raised by, so as to rest on, two Y-shaped brass edges, which are lifted or depressed again by turning the handle H. On the outside of the glass door a light cross-bar carries a flat rod, at each end of which a magnifying lens or microscope is placed to read off the dip of the needle as indicated by its ends, on the graduated vertical circle.

 

For the purpose of setting the one circle truly horizontal and the other truly vertical, one level, L, is placed on the horizontal circle, and another, M, on the top of the vertical circle. If the needle is swinging freely in the plane of the magnetic meridian, if its center of gravity lies in the axis on which it is free to rotate, and if its magnetic axis coincides with its axis of figure, it will correctly indicate the dip. But there are numerous errors possible in such an instrument which can only be avoided by taking the mean of two observations which have errors of equal amount but of opposite sign. This duplication of observations has to be carried out for each of the known possible instrumental errors, so that finally the true angle of. inclination or dip is obtained, as the mean of no less than sixteen readings.

 

The errors in question may be due to the following causes : (1) The vertical circle may not be properly set, and in consequence, when the needle is standing vertically, it does not point exactly to 90°, as it should; (2 ) the bearings of the needle may not be exactly in the center of the circle; (3) the center of gravity of the needle may not be in the axis on which it can rotate; (4) its magnetic axis may not coincide with its axis of figure; (5) there may be friction or adhesion at the bearings.

 

In making an observation of the inclination with the instrument, the first requisite is the adjustment to verticality of the vertical circle by means of the levels. The plane of the magnetic meridian must then be found, and the axis turned round until the vertical circle is in that plane. This is done by finding the position in which the needle stands vertically, for in that position, which is at right angles to the plane of the magnetic meridian, the horizontal component of Earth's magnetic force is non-effective, owing to the mode of suspension; the vertical component only is effective, and causes the needle to hang vertically. The plane of the needle is then moved through 90°, and is thus brought into the magnetic meridian. In making this observation of the plane in which the needle stands vertically, it is necessary, in order to eliminate the effect of faults in construction already noted, to take a reading from the lower end, and one from the upper end of the needle; then, turning the circle round through 180°, to make other two in the same way; the mean of these four readings, with 90° subtracted, gives the plane of the meridian. To determine the angle of dip, four readings are taken, two with the face of the instrument towards magnetic east, and two with it facing magnetic west; one of each two being with a marked side of the needle facing the circle, the other being in the reversed position. The needle is then magnetized afresh, so that its polarity is reversed, and another series of four similar readings is obtained. The mean of these eight readings gives the true amount of the inclination. The turning of the instrument through 180° is to avoid any error due to that noted above as (1) ; to counteract (2), readings of both ends of the needle are taken; presenting both faces ( or sides) of the needle, neutralizes the effect of any error such as (3); errors such as (4) are obviated by the reversal of polarity. The dip circle is not of such form as is convenient for continuous registration; for these requirements – i.e. the variations of the vertical component of Earth's magnetic force, the vertical force magnetometer is used.

 

With the development of electronic systems dip circles have become obsolete.