DIPPING-NEEDLE, or INCLINATION COMPASS, an instrument for measuring the magnetic dip or inclination; that is, the angle which a magnetized needle, free to move in the plane of the magnetic meridian, and about a horizontal axis, makes with the horizontal plane at the place. 'The dipping-needle is a light mag netized steel bar supported on a horizontal axis which passes, as nearly as possible, through the centre of inertia of the bar. The axis is either a steel knife-edge which rests on agate plates, or a fine wire supported by friction-rollers. A vertical circle surrounds the needle, the axis on which the needle turns being at the centre; the circle is graduated, and the needle moving over the graduations, the inclination to the horizon can be read a by means of it. The vertical circle is mounted on a vertical pillar, which turns round its own axis; and to the pillar there is attached at the foot an arm or pointer, which moves over a horizontal graduated circle. This Iast is supported on three legs furnished with leveling screws.
To observe with the dipping-needle the ver tical circle must be placed in the plane of the magnetic meridian, and then the angle at which the needle is inclined must be read off by means of the graduations on the circle. The plane of the magnetic meridian might, of course, be de termined by means of a separate instrument, such as the declination compass. But this is not necessary, the vertical pillar and hori zontal circle described above furnishing all that is required. A complete observation is made in the following way. The instrument having been properly leveled, the vertical circle is turned round on the pillar till the needle points vertically downward. When this is the case we
know that the needle must be moving ih the vertical plane at right angles to the plane of the magnetic meridian, because it is only in this plane that there is no horizontal component of force acting on the needle. The vertical circle is now turned round on the pillar through 90 degrees, which is done exactly by means of the horizontal circle at the foot. The needle is thus free to move in the plane of the magnetic meri dian, and the inclination is read off.
Corrections for defects in the instrument must be applied in order to make the deter mination exact. In the first place, in order to correct for any error in centring of the axis on which the needle turns, readings are taken at both ends of the needle. Next, because the magnetic axis of the needle may not coincide with the axis of the figure, the needle is turned over on its bearings, and fresh readings are taken. Thirdly, the axis on which the needle turns may not pass exactly through the centre of inertia of the needle. If this were the case the needle would not balance about the axis exactly, one or other end of it being the heavier. To do away with error from this source, after one set of readings have been taken, the needle is lifted from its bearings and remagnetized, the end that was formerly north being now made the south end. A complete new set of readings is taken. The mean all these gives the true inclination. See DECLINATION NEEDLE; MAG NETISM, TERRESTRIAL.