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From: fowler@uw-beaver (Rob Fowler)
Newsgroups: net.physics
Subject: Re: Low frequency RF
Message-ID: <1427@uw-beaver>
Date: Wed, 24-Jul-85 15:05:26 EDT
Article-I.D.: uw-beave.1427
Posted: Wed Jul 24 15:05:26 1985
Date-Received: Sat, 27-Jul-85 04:13:45 EDT
References: <3202@decwrl.UUCP> <226@fear.UUCP>
Reply-To: fowler@uw-beaver.UUCP (Rob fowler)
Organization: U of Washington Computer Science
Lines: 29

How far do you have to go to get away from 60Hz hum?  I used to do
computerized cartography applied to doing near shore analytic tidal
predictions.  As such I had contact with people who actually go out
measuring currents on the continental shelf in up to 300- 400 feet of water
south of Long Island, N.Y.  (there are others). They had to redesign their
electro magnetic current sensors because of 60 Hz interference on the ocean
floor.  I won't get into the details of how they work, but they rely on
using currents through moving salt water to generate detectable magnetic
fields.  It's necessary to use low freq. AC to do this in order to cancel
out the earth's magnetic field, to cancel out chemical effects at the
electrodes, and otherwise make the field detectable .  If you use some
random frequency and phase then your measurements contain an interference
component generated by your signal beating against the background hum.  The
solution they used was to phase lock the oscillator that generates the AC
with the background hum.  Presto, all the problems go away.  This apparently
works anywhere on the continental shelf off the East Coast.  Modulating the
entire US power grid for communication purposes would be another story
though.  I have heard rumors that the Russians do have the potential to use
part of their power grid as antennae for emergency submarine communication.

An interesting experience I had related to background hum occured about 10
years ago.  I went to a screening of a documentary that was done on a
European tour of Julian Beck's Living Theater.  At one point the performers
started chanting and "OM"ing.  It was really awful, unaesthetic, bad.  They
seemed to be out of tune with the universe.  The reason?  They were doing a
European OM in resonance with the 50Hz grid and it sounded flat and
unharmonious to an audience sitting in 60Hz Berkeley.

-- Rob Fowler (uw-beaver!fowler or fowler@washington.arpa)