A seismogram is a record of the seismic waves from an earthquake.
A seismograph or seismometer is the measuring instrument that creates the seismogram.
Almost all seismometers are based on the principle of inertia: a suspended mass tends to remain still when the ground moves.
Here we present a short (and very truncated) history of seismometer design.
The earliest records of earthquake measurements are from the Chinese philosopher and polymath Zhang Heng who describes a mechanical device, which can indicate the direction that earthquake waves have travelled from through a mechanism which drops balls into the mouths of frogs.
No images or artefacts of this device exist, however several attempts have been made to reconstruct what it might have looked like.
In the 18th century several Italian scientists experimented with using simple pendulums to record the ground motion of earthquakes, including Father Andreas Bina, a teacher of philosophy in the Benedictine monasteries of Italy.
Bina's simple pendulum was suspended from the upper floor of a building and registered a seismoscope trace on a bed of sand.
These simple pendulums could produce a record of the movement during an earthquake, however they lacked the ability to record a true seismogram, i.e. they did not record the time history of the motion, just its totality.
British scientists in the Victorian era were fascinated by earthquakes also, so when a small earthquake swarm struck the Scottish border town of Comrie in 1870 the local gentlemen scientists built a dedicated observatory called earthquake house.
The Comrie earthquake house contained a series of small wooden posts, balanced on their ends and designed to fall over when an earthquake struck.
Unfortunately, the sensitivity of such a system is very low and no earthquakes were recorded by this system.
From 1876 to 1895 John Milne worked as lecturer in Tokyo, during which time he became interested in earthquakes and how to measure them.
Milne returned to the UK in 1895 and set up a seismic observatory on the Isle of Wight from where he managed a global network of seismometers and published the Shide Circulars of seismic activity. This work continued after his death in 1913, eventually becoming the International Seismological Centre whose work continues to this day.
Seismometers at this time still used pendulums to register relative ground motion but were able to record the time history of the ground motion on a clockwork drum. Initially using simple pen and paper arrangements but later making use of (frictionless) light beams on photographic paper. Such instruments could detect the faint signals from distant large earthquakes anywhere in the world.
At the beginning of the twentieth century American scientists were busy studying the local earthquakes of California. They used a small but sensitive instrument that used the torsion of a taut wire as the suspension for a small mass and mirror arrangement which could photographically record the faint motions of small local earthquakes.
In 1935 Charles Richter used the signals from this instrument to define his magnitude scale for earthquakes; Richter (1935) An instrumental earthquake magnitude scale