A simple earthquake detector

Some of my physics students decided to construct an earthquake device. They suspended a cubic block of metal so that it hung between two plates of equal rectangular surface area. The two plates were then conected to a 12V battery so that effectively we had two capacitors in series with the gap between plates and the sides of the block equal(0.82mm). The thread which the block hung from was connected firmly to the earth so that any tremor would cause a movement of the block and therefore change the distance between the plates and the block. A separate voltmeter was connected from the centre of the block to one side of the circuit.
Initially since there is the same distance between plates and block, with equal capacitance, the voltage across each is 6.0 V. As soon as the earth tremors, the block moves and the distance between block and plates are not equal since capacitance and distance are inversely proportional, capacitance will increase or decrease and the side connected to the voltmeter will show this ( V = Q/C ) A decrease of about 0.1 mm will lead to a voltage drop of about 1 volt, so the slightest tremor will be detected.
With movement of people across a wooden floor affecting the apparatus, they linked the apparatus to the concrete foundations.
Next step is to link voltage change to the computer and finally to make comparisons with national seismographic data so they can convert voltage change to Richter scale.
They are great kids.


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