The SciAm Magnetic Gravimeter

First created: Mon Jan 10 2000
Last update: Wed Jan 12 2000
The Amateur Scientist column in the January 2000 issue of Scientific American describes a magnetic gravimeter/seismometer. The instrument was designed by Roger Baker and the article was writen up by Shawn Carlson, regular Amateur Scientist column writer and the founder of the Society of Amateur Scientists (SAS). SAS maintains a web site that includes a forum for discussing Amateur Scientist projects.

Scientific American article (a local copy).
Roger Baker's gravimeter web page (local copy).
Society for Amateur Scientists (SAS),
SAS Forum,
SAS Gravimeter Forum.

Developing The Model

A good mathematical model that captures the physics of the device is essential to understanding the instrument's sensitivity to gravity and temperature. The expected sensitivity to gravity of this device is very poor and much smaller than its sensitivity to temperature or air pressure.

Some physical data about magnets.

Building The Probe

The probe, as described in the original Sci. Am. article, is a magnetic floater held in suspension between two magnetic stacks. The rigid framework of the device is made of cut glass held together by silicone cement. A very pleasant combination of materials to work with.

The front end detector uses a paper flag attached to the floater and oscillating between a bright diode and a phototransistor.

The front end signal is amplified and controlled by a differential amplifier and feedback through a magnetic coil.

The device is very temperature sensitive so temperature monitoring and temperature control are essential.

Experimental Results

My best results to date make use of the feedback circuit and the temperature control circuit. Still no tidal signal ...