In the summer of 1930 the Civil Engineering Department of the Massachusetts Institute of Technology undertook to design and construct an accelerometer capable of recording an earthquake‐shock at a point comparatively near the epicenter. Mills W. Braunlich was appointed Research Associate in the Department to carry out this project.
The instrument is essentially a torsion‐pendulum, the “inertia‐mass” being mounted eccentrically on a taut wire similar to that used in the Wood‐Anderson seismometer. The particular form of the inertia‐mass is that suggested by Dr. Frank Wenner of the Bureau of Standards. It consists of a rectangular metal loop about 1 inch by 3/8 inch, the longer side being attached to the supporting wire, which is 4 inches in length. The free‐period is one twenty‐fifth of a second. That side of the loop which is farthest from the torsion‐wire is made of sheet copper one‐hundredth of an inch thick, bent into a curve so that it can vibrate between the poles of a magnet. The air‐gap is one thirty‐second of an inch. In the experimental instrument an electromagnet was used in order to permit variations in the damping‐action; this will be replaced later by a permanent magnet. In addition to the magnetic damping, it was found necessary to use special damping to reduce lateral (“violin‐string”) vibrations of the wire. This was accomplished by placing bits of felt, soaked in oil, just above and just below the inertia‐mass.
After long delays in securing the instrumental equipment, the Technology Seismological Station was put in operation July 5, 1932. This seismograph is on the grounds of the Surveying Camp operated by the Massachusetts Institute of Technology, and is just north of United States Route 1, on Gardner Lake, about six miles east of Machias, Washington County, Maine. The equipment consists of two horizontal components of the Wenner seismometer, mounted on a low concrete‐pier about four feet by six feet, resting on an outcropping ledge. The building is of concrete with enclosed airspace on the walls and ceiling, and also on the roof. The room is entered through two outer rooms. The walls are banked with earth up to the eaves. The rather serious moisture‐difficulties encountered at first now appear to have been overcome.
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