The Lehman seismograph can be built for about $120 worth of common materials. Modifications are made in the upper and lower frame cross-pieces, the boom and the damper. Building tips are given for the coil and levelers.
The construction of the Lehman seismograph was described several years ago by Walker (1979), and its utility was discussed by Barker (1983). The seis- mograph can be rather easily built for $120 worth of ordinary lumber, hard- ware, plumbing and electronic parts, plus a necessary strip-chart recorder. The seismograph seems to have received little attention from geology and earth-science teachers and it appears that perhaps less than a dozen are in operation in the United States (G. Barker, personal communication). In spite of its simplicity, the Lehman seismograph is capable of recording earthquakes of Richter magnitude 5 which occur in the continental United States, and mag- nitude 6 elsewhere. It is sensitive enough to detect the tilt of a concrete basement floor as a person approaches or withdraws from it.
Our department had an old but operable strip-chart recorder on hand, and I recently built the seismograph. It has become quite popular with our students, and we intend to more fully utilize it in our introductory geology course in the future.
The seismograph consists of a horizontal boom, weighted at one end, that is free to pivot against a supporting frame of plumbing pipes (Figures 1 and 2). A horseshoe magnet attached toward the end of the boom interacts with a pick- up coil. Relative motion between the boom and the coil is translated into a current that is amplified and sent to the recorder.
Walker covers the construction of the seismograph in sufficient detail, and I will not repeat the details of its overall construction or operation. I found, however, that there are several steps in the construction that can be simplified. I incorporated these changes in my seismograph and they, along with other tips on the construction and operation, are described in the following paragraphs.
The upper cross-piece of the Lehman frame consists of an oil burner nozzle glued into a large hole cut into the cross-piece pipe. Instead of using a nozzle, I drilled and tapped a hole in the center of the pipe to accept a 1/4-inch hex-head machine bolt. (Note: Bolts, nuts and plumbing pipes are sold almost exclusively with English system dimensions in the United States. None of the sizes given are critical and readers who use metric sizes can make approximate conversions.)
A V-shaped notch filed into one of the edges of the bolt head acts as a pivot point for the cable (Figure 1). I drilled only one hole, but holes through both sides of the pipe could be use, though this would require a drill press and a single-step tapping procedure to assure that one gets a congruent thread pattern in both holes. My single hole provides a solid enough mount. The bolt can be screwed in and out to provide lateral adjustment of the cable pivot point. A 2-inch bolt is sufficiently long.
Here Lehman used a metal bar bolted to the vertical pipes. I substituted tees, 2-inch pipes (nipples, in plumber's jargon), and a pipe union (Figure 1). One of the flats of the union acts as the bearing plate for the boom and replaces the metal plate and bolt assembly of the Lehman model (Figure 2). The flat on the union may have to filed smooth.
The pipe frame is assembled by first attaching the upper cross-piece to the elbows and vertical pipes. The bolt should tilt about 30 degrees up from the horizonal. Next, firmly attach the tees, the 2-inch pipes and finally the union. Some adjustment of the lower cross-piece will be necessary to obtain the same width as the upper cross-piece.
The cable pivot point should lead the boom bearing point by about 1cm to start. This distance can be changed to allow for adjustment of the oscil- lation period of the boom.
Rather than a plain steel rod, I used a 3/8-inch threaded rod for the boom. The use of a threaded rod allows the use of nuts and washers for attaching the magnet, weight, and damping assembly to the boom.
Instead of the magnetic damper used by Lehman, I used an oil damper as suggested by Barker (1983, and personal communication). Damper style is not critical. My damper is a sheet of brass soldered to brass stock (Figure 3). The damping medium is motor oil in a coffee can, so the damper paddle should be sized to fit into the can without touching the sides. Actual size is not important, as damping is adjustable by raising or lower- ing the paddle and/or the oil level.
Our seismograph rewarded us shortly after its installation with a seismogram of the May 7, 1986 Aleutian earthquake. The seismograph can be attractive low-budget addition to an earth-science/geology program. Seismograms obtained from a seismograph that students can see in operatin are much more exciting than text book examples. Its construction makes an interesting class or club project.
I thank Gary barker, who teaches at the Center for Arts and Sciences in Saginaw, Michigan, for his encouragement and answers to my questions while I was building the seismograph.
Larry Cochrane - www.seismicnet.com/contact.html