Riding the wave

Bamberg's Theodolite

The main feature of this instrument, built by Carl Bamberg in Berlin-Friedenau after 1871, is that it associates two distinct parts: the magnetic theodolite, for measuring the declination, and the chamber to measure the oscillation period. The apparatus comes with the chronograph by Matthaeus Hipp (1813-1893) for the accurate measurement of time.

The theodolite, in brass, is placed on a wooden tripod. The suspension wire is made of brass, is 40 cm long, has a diameter of 0.3 mm and is contained in a brass tube top ending with a turning head graduated in steps of 5°; at bottom there's a hook that can be locked with an external clamp.

The horizon ring has a diameter of 27.2 cm, is graduated every 5' and by means of micrometer microscopes, artificially lit, provides 5/100 of minute. It is possible to estimate down to the hundredth of minute. The autocollimating terrestrial telescope with artificial lighting is equipped with a small vertical ring with a vernier in minutes, the objective has 32 mm of aperture and the eyepiece has variable magnification. On the counterweight supporting arm is engraved Friedenau Carl Bamberg - Berlin No 7904.

The middle case, where the magnet is hung, is equipped with two horizontal rods that support the magnet during the deviation measurement. They are made as hollow cylinders to allow adjustment of the mirror. The graduation of the rods, 260-290 mm divided in mm, had been carefully determined, so that the distance of the magnet during the experience of deviation was known to one hundredth of a millimeter. In the case was placed a small oil container (now missing) to damp the oscillations of the magnet.

The theodolite has 5 magnets, one for declination (a tungsten steel cylinder with an interior mirror at one end), two deviatoring magnets drilled for deviations, a magnet of weak magnetic moment for the measurement of the deviation induced by the deviatoric magnets, a magnet for measuring the torsion of the wire, with size and weight identical to the one for the declination (with a mirror at one end) and a non-magnetic cylinder to keep the wire in tension, with the same weight of previous magnets, but smaller dimensions.

(Taken from Two centuries of geomagnetic instruments in Italy (1740-1791) , 1997)
Bamberg's Theodolite

Magnetic theodolite property of the Hydrographic Institute of the Navy.
By Paola Presciuttini
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