Made in 1904 and 1909 in Göttingen (Germany). They operated in Strasbourg from 1904 to 1968.
Complete system for recording the ground's movement: one seismometer for recording two horizontal components, and a second seismometer for the vertical component:
These are two good examples of an entirely mechanical design. Due to the rubbing and friction in this model, a large mass is necessary.
The Wiechert horizontal seismometer is an original concept because a single machine can record two horizontal components. It consists of a converted or astatic pendulum. The mass of one ton, in unstable equilibrium on a point, pivots around two axes which are made up of a cardan system.
The horizontal movement of the mass is decomposed into two perpendicular components. It is damped by air pistons and amplified by a lever system. Two fixed points allow the recording of each component on smoked paper. The recording is helicoïdal: the sheet is fixed on a roll which turns and transfers itself. An electrical signal can be sent, for example once every minute, by means of a contact clock. This signal triggers off a rod system which passes under the recording points, interrupting the recording and allowing the user to benefit from "time sign - posts".
The Wiechert vertical seismometer is composed of two interconnected masses weighing 600kg (1300lb) each, suspended from two springs.
The seismometer is entirely streamlined to limit variations in temperature inside the apparatus, since the elasticity of the springs is very sensitive to temperature variations. A thermal compensator based on the principle of rack pendulums allows for correction of the inevitable variations. Thanks to an astatisation system, it is also possible to regulate the machine's eigenperiod.
The vertical movement of the masses is damped by an air piston and amplified by a lever system. A fixed point allows recording of the movement on a sheet of smoked paper, in a system very similar to that of the horizontal Wiechert.