The Seismometer

            A seismometer, or seismograph,  is a precise instrument used to measure and record the force of an earthquake. This is usually referred to seismic waves. The first seismograph that could accurately measure earthquakes was invented by John Milne in 1893. However, Thousands of years ago, the Chinese used a vase with steel balls and a heavy pendulum. Whenever an earthquake struck, the pendulum lost its balance and knocked one of the steel balls down. Even though many improvements were made to John's seismometer, his principle remained the same.

            To get an accurate measurement of the seismic waves, seismometers are often rooted to the earth's bedrock. This eliminates the "seismic garbage" often created by underground subways or other artificial forces created by human. Seismometers are placed in many locations. Though the means of wireless transmission , the data is transferred to a central building, where the information is printed on paper or stored in computer files. Often, one building could monitor up to 100 areas across a state or region.

            But what exactly do these instruments measure? They measure the earth's sudden movements, or more often referred to as earthquakes. If the earth does not make sudden movements (i.g. Continental Drift. Slow movement of the earth's crust.), than the seismograph does not pick it up. However, seismographs are used in a much wider array of applications. For instance, just by looking at the recordings of a seismometer, you can tell the density of the recorded area. This useless information can show unstable areas on the surface of the earth, and therefore predict future earthquakes. There are many other clues that the recordings of a seismograph can tell us if we know how to decipher it.

            The recordings of a seismometer during a moderate earthquake would be many large, squiggly lines. If there is no earthquake, than the seismograph records a fairly staight line. If the seismometer records some small, squiggly lines and you don't feel an earthquake, than there is a microquake. A microquake is a very small earthquake that humans cannot feel. From this information, you could probably guess that seismometers are very sensitive to motion. Even though the recordings are easy to read like this, and it gives you a good idea of the force of the earthquake, it still needs to be put on a scale so that it can be read. The scale is called a Richter scale. It is numbered from 1 to 10. The greater the mumber, the greater the force of the earthquake. Thus, if the lines on the recording are large, the number on the Richter scale would also be large, probably around a 6 or a 7.

            My seismometer works on the same principle as a standard mechanical seismometer. However, my instrument measures 2 axis instead of three. There is one fault in it which makes it unable to measure diagonally in one direction. Otherwise, everything is the same. The most important part of the seismograph is the lead pendulum which hangs on an aluminum rod supported by stainless steel hooks. All this hangs on an aluminum support attached to the base with machine screws. When the base is secured firmly to a surface, the lead pendulum should stay nearly still while the surface is moving.

The next most important part is the lever, or amplifier. The amplifier amplifies the small movement of the pendulum 4 times. For example, if the pendulum moves 1 mm, than the needle moves 4 mm. This device makes small movement readable. Next is the recording tool, or the .07 mm pencil lead attached to the amplifier. As the lever moves back and forth during an earthquake, the lead records the movement on paper. The 85 ft. by 2 1/4 in. roll of paper records the data. The paper spreads onto the guided aluminum writing board from the original roll to a rubber roller. This rubber roller pulls the paper across the guided writing board since it is attached to a geared motor. The motor is powered by 1 C sized non-rechargeable alkaline battery.To give the rubber roller more friction to the paper, 2 ball bearings press the paper against the roller using a spring. The paper comes out contiuously until the motor is switched off. It can be teared off easily by using the rigid edge located right above the rollers.

Special Notes:

 If the lead is too short to further record, than gently push down the lead rod. Be careful not to break it. If the lead runs out, than replenish with standard .07 mm lead used in mechanical pencils. The paper rolls can be replaced with 2 1/4" by 85' rolls used for thermal calculators.