Final answer:
A seismograph uses a device called a geophone to record the drum's vibrations. A geophone converts ground movement into an electrical voltage signal, which is then recorded and analyzed for the structure of the earth.
Step-by-step explanation:
A seismograph uses a device called a geophone to record the drum's vibrations. A geophone is an instrument that converts ground movement into an electrical voltage signal, which is then recorded and analyzed for the structure of the earth. The vibrations are detected by the geophone and converted into voltage, creating a record or graph of the seismic waves produced by an earthquake or other geological activity.