Circular brass base in the form of an annulus mounted on a horizontal tripod, adjusted screws under each arm, mounted on a square wooden plate 20 cm ´ 20 cm. Annulus has angular gradations down to 0.5° resolution, and is inset with a matching brass disk rotating about a central axis, also mounted on the tripod. The inner disk has attached a vernier scale by a screw clamp to the annulus, with a fine rotation screw, for measurement of angular position.
A horizontal, roughly-rectangular platform mounted on the inner disk with a spirit level below to allow accurate levelling via the tripod screws. A vertical box at the back of the platform (15cm ´ 15cm ´ 4cm deep) with glass windows front and back (the rear smoked). Box contains a magnetised needle mounted on an axle suspended freely on two agate edges on metal posts. A knob on the right-hand side of the box raises one support to center the axle. The back window of the box hinges open horizontally. A vertical annulus mounted on the platform via two cylindrical posts, in a plane parallel to the box face, with 0.5° angular markings, and a rotating centrepiece attached via a vernier scale, containing at opposing ends a small microscope used to view the ends of the needle. A swivelling piece on the same axis, with magnifying glasses at opposing ends, used to view the vernier scale. Separate from the apparatus (all contained in wooden box) are two long permanent magnets used to remagnetise the needle. Used to measure the three-dimensional orientation of the prevailing geomagnetic field, to obtain the heading of the vertical plane containing it, and the inclination in that plane. Returns a horizontal heading relative to an arbitrary reference, and vertical inclination, as angular measures in degrees of arc.Does not measure the actual field strength.
The apparatus in the museum, has the inscribed annotation "Supplied by the Cambridge Scientific Instrument Company". However, there is no serial number (so the time of manufacture can not be ascertained), and it is likely that it was built under licence. Identical designs are present in the Casella catalogue, for example, and refer to the apparatus as a Kew Pattern Dip Circle, indicating the source of the design. It is expected that the needle is frequently remagnetised by hand, to ensure that its field axis lies along the line between the needle points. This is necessary to ensure accurate operation, although the effect of small errors across the plane of the needle is accounted for by the averaging of needle-reversed measurements. Special permanent magnets are provided with the apparatus, for this purpose.
SG
