Comment: 08:31 - 09:35 (01:04)
Source: Annenberg/CPB Resources - Earth Revealed - 9. Earthquakes
Keywords: "James Sadd", earthquake, seismograph, China, "initial ground motion", vibration, "duplex pendulum", "San Francisco Earthquake", strength, duration, epicenter
Our transcription: In the second century the Chinese built a device that could detect the initial ground motion during an earthquake.
It consisted of eight metal balls ranged around the circumference of a large sphere.
If the Earth shook hard enough, a small pendulum inside the sphere swung back and forth knocking one of the balls off its stand.
This indicated the direction that the earthquake vibrations came from.
At the turn of the century a more advanced device was designed, one to make a permanent record of ground motion during an earthquake.
This instrument, called a duplex pendulum seismograph recorded the shaking of the ground as a continuous squiggle on a piece of paper.
It was this device that recorded the ground motion of the 1906 San Francisco Earthquake.
The invention of a device that could accurately measure the strength of an earthquake was a significant scientific achievement.
These modern seismographs operate on the same general principles as the early pendulum instruments, but they are much more sensitive.
Not only do they record vibrations, but they can be used to measure earthquake strength and duration, and also to determine the location of the earthquake almost immediately.
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