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A seismoscope is a scientific device that signals the occurrence of an earthquake, possibly providing information about the timing and size of the quake as well. It should not be confused with a seismometer, a measuring instrument, or the closely related seismograph, which generates a record of the shaking. Geologists do not widely use seismoscopes, because the range of data they can record is limited when compared with more sophisticated equipment.
The earliest known seismoscope appears to date to 132 BCE, when a Chinese inventor developed a device capable of dropping balls to alert people to the occurrence of an earthquake. This device did not offer information on when the earthquake occurred, unless someone happened to be present when a ball dropped. It also did not measure the size of the seismic event; a ball would drop for a relatively mild quake or a very large one. Records indicate it was capable of measuring distant quakes, as on at least one occasion people thought the indicator was false and later learned of an earthquake in a neighboring region.
Over time, seismoscope technology became more sophisticated. Inventors worked on devices capable of measuring the intensity of earthquakes so they could collect better data, and they also developed seismoscopes with timers to tell them when quakes occurred. The development of the seismograph and seismometer contributed even more useful information; early devices used a simple pendulum attached to a stylus to record an earthquake in full, allowing people to track the intensity of the shaking and look at the pattern of movement inside the Earth's crust.
The seismoscope is useful primarily as an alert system to let people know an earthquake happened. More modern scientific equipment provides information about the details and does not just signal to alert people to the occurrence of a quake. By using a network of equipment to measure seismic activity, researchers can pinpoint the epicenters of earthquakes, track earthquake activity, and learn more about the origins of quakes. These devices are also useful for activities like issuing tsunami warnings.
It is possible to build a simple seismoscope at home, and directions can be found in scientific activity books as well as online. One problem with this and other devices to measure earthquake activity is the degree of sensitivity. The machine may issue a false alarm for a passing heavy truck, or fail to detect a very distant quake because the Earth's motion is so subtle. Researchers use tools like probes located underneath the Earth to confirm that shaking is caused by an earthquake and to collect data about the directionality of the movement.