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Geophysical instruments have developed and progressed throughout the past century. The seismometer demonstrates a device that has changed with the times, becoming more and more useful in measuring the movements of the Earth. Seismographs have developed greatly since the 1930s and even earlier. Between 1880 and 1895, the team of John Milne, James Alfred Ewing, and Thomas Gray worked in Japan to create an early version that used damped horizontal pendulums. In the post-World War II era, these were adapted into the Press-Ewing seismometer. Used in the late 19th and early 20th centuries, a simple early instrument had a pendulum that swung freely in any direction along with a scribe at the bottom that touched a smoked glass plate. This early device didn't provide information such as time of the shock or details of distant earthquakes, but they did give initial shock directions accurately. Additionally, many of these early geophysical instruments helped with detailed analysis of the 1906 earthquake in San Francisco many years later. Modern day seismometers have improved upon older methods of measuring the Earth's movements. These devices use electronic sensors, recording devices, and amplifiers. When it comes to recording, these days seismometers record using a computer with an analog-to-digital recorder, a disk drive, and an Internet connection. Most record continuously, but some only record when they detect a signal. Today's instruments come in three broad ranges. These include geophones, which range from 50 to 750 V/m, local geologic seismographs, at about 1,500 V/m, and teleseismographs, at about 20,000 V/m and utilized for world survey. Additionally, there are three main varieties: short period, long period, and broadband. When it comes to the short and long varieties, they are very sensitive. They measure velocity, but can go off-scale with ground motion that people can feel. One example of a modern instrument is the strong-motion seismometer, otherwise known as an accelerograph. As the name hints, this device measures acceleration. The strong-motion seismometer is an important modern advancement as it help aide the study of how seismic activity affects man-made structures. Advanced Geosciences. Inc. (http://www.agiusa.com) is a manufacturer of geophysical geophysical instruments and IP imaging systems
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