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Jonahan Hare's Diary

Day 15: Seismograph

Cold start with the challenges
It's a terrible thing when the Earth rocks and rolls,
Earthquakes spread out and chaos unfolds
All things are impermanent that much can be seen
From atoms to mountains just part of the shifting scene
A bridge swing motion is not difficult to detect
But shifting granite and bedrock even explosives neglect.

Photo: Kathy and Jonathan make a pendulumKate gives us our challenges for the third program of the series. Kathy and I have a seismograph challenge and they tell us that on day 3 we can test it in a quarry with explosives!!

Kathy and I make up a basic "pendulum" machine/instrument. This is basically a heavy weight suspended and able to move from side to side. In actual fact, if the Earth moves the idea is that the weight doesn't move and so there is a relative motion between them that can be measured. We are not sure, however, if it will be sensitive enough to pick-up an explosion in the quarry. After all it can only measure the amount by which the Earth moves, and this is a lot for an earthquake proper but won't be much for a few sticks of dynamite however loud the explosion.

To make a record of the results we take a clock apart and attach a paper disc to the minute hand. In principle then this should turn the paper 360 degrees in 1 hour. This records nicely, but there is a problem sometimes getting the paper to be level and this leads to the chart recording dropping off the paper/not being reliable.

Tomorrow we are going to put the prototype onto the Franz Josef Bridge and record the movement due to the traffic going across. Also think about an electronic circuit to boost the sensitivity.

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Photo: Jonathan Hare
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