Marc Wynne, age 14, of Great Falls, Mont., for his question:
HOW DOES A SEISMOGRAPH WORK?
A seismograph is an instrument that amplifies and records small movements of the ground. From these records, scientists called seismologists can determine the location and size of earthquakes.
A seismograph draws a wavy line to show the motion of the earth during an earthquake. The main parts of the instrument are a recording device and a weight. Any movement of the earth causes the seismograph frame to move up and down, but the weight does not move. The writing device attached to the weight marks a record of the earth's motion on the recording device.
The most sensitive seismographs magnify ground motion by as much as 10 million times. They consist of a weight suspended from a frame by a delicate spring. The frame moves with the ground but the weight, due to its inertia, tends to remain stationary.
The relative motion between the weight and the frame is magnified by using an electromagnetic transducer and an electronic amplifier. The transducer, a coil attached to the weight, moves in the magnetic field created by a magnet attached to the frame. This movement indicates an electric voltage in the coil which passes to the amplifier. The amplifier voltage controls the recording device that marks the ground motion on a moving sheet of paper.
The seismograph can detect ground motion as small as 39 ten billionths of an inch. This is the same as one hundred millionth of a centimeter.
Different types of seismographs are used to measure short and long seismic waves. The Press Ewing seismograph records long waves at least 500 miles in length. The Benioff linear strain seismograph measures changes in distance between two piers attached to the ground.
Seismographs are used in groups of three to measure separately three types of ground motion: up down, north south and east west. There are more than 500 seismograph stations throughout the world.
Scientist also use seimsograph8 to hunt for oil, to study the earth's interior and to find the thickness of glaciers.
The equipment used for scientific studies and prospecting are tiny and rugged. Scientists place hundreds of these seismographs around the site being studied. Then, the scientists set off explosives to create seismic waves, which travel to underground rock layers and are reflected.
The seismographs measure the reflected waves and indicate what lies beneath the surface.
The Apollo astronauts placed five special seismographs called seismometers on the moon. These instruments detected seismic waves caused by tiny moonquakes and sent information about the waves back to earth by radio. The seismometers indicated that the moon has a thick, strong crust.