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Magnetic observation in Tokyo

Site

35°41′N, 139°45′E, 21 m above sea level
(Central Meteorological Observatory, by Kitahanebashi Gate in Main Citadel of Edo Castle)

System

The site’s Absolute House had a roof that opened in the middle to allow azimuth measurement with Polaris sighting. A small window on the north side provided a view of the golden cross of Nikolai Cathedral in Surugadai as an azimuth mark. The Variation House, 18 m east of the Absolute House, was a wooden double-roofed structure with a 3 meter-deep basement. Its interior was divided by a staircase into an eastern room with an autograph variometer and a western room with an identical one for direct visual reading.

Instruments

　 The instruments for absolute measurement were Tanakadate magnetometers (theodolite) for D and H observation and a Casella dip circle for I observation.
　Mascart variometers were used for the monitoring of D, H and Z, all of which relied on magnets. The magnet for D was suspended by a single string and that for H by two. The instrument for Z was a balance type, with a magnet held horizontally.

Observation

The absolute measurement for H was based on the Gauss-Lamont method, with values calculated from oscillation (M H) and deflection (M/H) of magnets with moment H. D and I were based on the same principle with declinometer/inclinometer and dip circle data, respectively, with declination and inclination determined from direct reading of a freely rotating magnetic needle. Observation was carried out monthly from 1897 to 1901 and twice a month from 1902 onward.
Recording was largely as per that for the suspended magnet variometer used until relatively recently, with light from a paraffin lamp shone onto a mirror attached to the suspended/balanced magnet system and the reflection photographically recorded using bromide.

The emitted light is reflected by a mirror that moves together with a magnet suspended on a string, thus projecting a light spot onto the drum, which makes one rotation per day. Magnet displacement is thus magnified to represent geomagnetic field variations.