Final answer:
An older type of seismometer that utilized a weighted pen and a rotating drum to record ground movements is known as a seismograph. Modern devices like geophones serve a similar purpose but with greater accuracy. These instruments help geologists calculate the distance to an earthquake's epicenter by analyzing the arrival times of P-waves and S-waves.
Step-by-step explanation:
The older type of seismometer referred to in your question is known as a seismograph. It consisted of a weighted pen suspended above a rotating drum. This drum moved in tandem with the Earth's ground movements. When an earthquake occurred, the ground movement would cause the drum to rotate while the pen stayed stationary, thereby creating a record of the seismic activity on the paper wrapped around the drum.
Geophones are devices which essentially perform a similar function as a seismograph. They convert ground movement into voltage, and this voltage is recorded and analyzed to understand the Earth's structure. Modern seismometers are more sensitive and can measure ground movements with greater accuracy than older seismographs.
Using the precise measurements of seismic waves, such as the arrival times of P-waves and S-waves which travel at different speeds, geologists can calculate the distance to an earthquake's epicenter with great precision. The difference in arrival times of these waves is critical to this process, and can be affected by any uncertainty in wave propagation speeds.