Home made Magnetometer

Many years ago I subscribed to Popular Electronics Magazine.
since then I have moved twice and lost most of my old issues.
Anyway, one issue dealt with making a home made magnetometer to sense the change in the local earths magnetic fields as it bent around metalic objects as they moved. It used a pair of neon transformer secondary coils on a soft iron bar for the reciever.
It then took the microscopic induced voltage generated in that coil assy and amplified it to a level that could be heard with a regular set of headphones.
there was a very high gain ultra sensitive op amp used in the first stage.

Does anyone have that circuit? or know of how that circuit can be built?

I thought it would be a neat toy to let the local elementary kids play with and get a better understanding of how the earths magnetic field moves around in their Earth Science class. Or just for me to play with too!

I do have a couple of those circuits, they use instrumentation amplifiers on the front end, but do not have a scanner to post it here. Give me a few days perhaps I can ask someone to scan.

It supposedly can detect an automobile passing a block away; and it is not more than a zillion turns of hair-fine enameled wire on an iron bar. The instrumentation amplifier is fed by the coil ends and the output is a millivoltmeter with its pointer centered at midscale as zero.

Miguel

Super! I figured there would be someone here that would know what I was looking for. I am patient. post it when you have time. thanks