Made in 1895.
|The first version of this instrumemt, made by Ernst von Rebeur-Paschwitz, had only one pendulum. This same first instrument, installed in 1889 in Potsdam and in 1892 in Strasbourg took the first recordings of signals from distant earthquakes.|
|On the 17th of April 1889, the ground's vibrations caused by a distant earthquake were recorded for the very first time. The recording was made in Potsdam, while the earthquake was in Japan! Three years later, on the 19th of December 1892, a similar recording was made in Strasbourg of an earthquake in Beloutchistan. For this record, an identical instrument as the one in Postdam was used in the basement of the Astronomy Observatory of Strasbourg. This was a small instrument of about 40cm (16 inches) in diameter, made of a single pendulum capable of swinging horizontally. All of these small instruments have been lost. |
These two experiments marked the beginnings of modern seismology.
Reinhold Ehlert continued von Reuber-Paschwitz's work and in 1895 produced an instrument which was in use in Strasbourg from 1895 to 1906, firstly in the basement of the Astronomy Observatory too, and, from 1900 on, in the new seismological station.
The original model was improved on three points. The first version had only one pendulum, Ehlert used three horizontal pendulums, in an effort to have a more precise signal. The mass was increased to allow modification of the eigenperiod. Finally, the rotation of the recording cylinder was accelerated from 11cm (4.5 inches) per hour for the Reuber-Paschwitz to 36cm (14.5 inches) per hour for the Ehlert version.
The instrument was used in a dark room. Each pendulum is equipped with a mirror connected to the rotating axe. Windows at the front of the apparatus allow a beam of light to fall on the mirrors. The beam thus reflected is recorded on a photographic plate; its movement is an image of the ground's movement. The amplification is directly dependent on the distance between the mirror and the photographic plate of the recording roll.