A seismometer is a scientific instrument used to detect and measure seismic waves caused by earthquakes, volcanic eruptions, and other sources of ground motion. Seismic waves are vibrations that travel through the Earth's crust and are detected by sensitive sensors called seismometers.

A typical seismometer consists of three main components: a base, a mass, and a sensor. The base is usually anchored to the ground to prevent it from moving during an earthquake. The mass is suspended from the base by springs or other means, and it is free to move in response to ground motion. The sensor measures the movement of the mass relative to the base, and it generates an electrical signal that is recorded by a computer or other data logger.

There are different types of seismometers, but the most common type is the "pendulum seismometer." This type of seismometer uses a pendulum to measure ground motion. The pendulum is typically a long, thin rod with a heavy weight at one end. The weight is suspended above a coil of wire, and when the ground moves, the weight moves relative to the coil, generating an electrical signal that is proportional to the ground motion.

Modern seismometers are highly sensitive and can detect very small ground motions, allowing scientists to study earthquakes and other phenomena in great detail. They are used in earthquake monitoring networks around the world to provide real-time data on seismic activity and to help protect people and infrastructure from the effects of earthquakes.