Riding the wave

D, H, Z Variograph

The original device consists of a single wire declinometer, a two wire magnetometer, a scale and three recording chronographs. It allows to obtain records of time variations of declination, horizontal intensity H and vertical Z. A single light source, switched off every hour, illuminates the three slits facing the three devices. The mirrors on the moving elements reflect the image of the slit on three drums, rotating at a speed of 2 cm/h, wrapped with photographic paper; the images are focused into points by a lens system. The 8 sec hourly switch off is produced by electrical contacts on the pendulum, and allows to synchronize the drums and time stamp the records. The daily magnetogram of each element is composed of a base line, drawn by the light reflected from the fixed mirror and the curve showing the movement of the magnet.

The D variograph consists of a base ring with three leveling screws, on which is placed the box housing the magnet and the torque tube. The turning head 0°-360° is divided in degrees, and marked every 10°. The base ring has just one small index 0-10. The box housing the magnet has two doors and is engraved in Gothic with the manufacturer's name Otto Toepfer & Sohn, Potsdam.

Each door carries a copper jaw with an adjustment mechanism, so it can go closer or farther from magnet end. The diamond shaped magnet is 14.5 cm long, 0.8 cm wide and 4 mm thick, and weighs about 40 g.

The original suspension wire, missing in the sample described here, was in platinum, 50 cm long with a diameter of 0.03 mm.

Suspended with the magnet there's a mirror and a metallic part of it carries a scale divided into 10 parts, while the fixed mirror that traces the baseline is on the bottom. The mirrors are rectangular, 4.0 cm long and 2.5 cm wide; damping was achieved with two jaws having a very small torsional constant.

(Taken from two centuries of geomagnetic instruments in Italy (1740-1791), 1997)
D Variograph

D Variograph, owned by the Hydrographic Institute of the Navy.
By Paola Presciuttini