From: billn@hpcvaac.cv.hp.com (bill nelson)
Subject: Re: Tell me about electric blankets
Organization: Hewlett-Packard Company, Corvallis, Oregon USA
Distribution: na
Date: Sat, 26 Dec 1992 09:20:22 GMT

kolstad@cae.wisc.edu (Joel Kolstad) writes:
: In article <1992Dec14.092120.20097@hpcvaac.cv.hp.com> billn@hpcvaac.cv.hp.com (bill nelson) writes:
: >: 
: >: Your DC blanket will, of course, generate a magnetic field -- just not an
: >: electric one.  Whether or not this is better or worse from the health
: >: standpoint is, as you point out, a good question.
: >
: >Bullshit. It will produce a DC electromagnet field. That field will build
: >and collapse as the blanket thermostat cycles.
: 
: Um, Bill, for someone who works for a very reputable company on the net
: here, I'm surprised you'd use such language.

I am me, not the company. So far, they have not chosen to censor me.
I do not expect them to do so for my using a non-pc term.

: If you're thinking of blankets that just have an on/off cycle, there will
: only be a significant electric field during the moment that the switch does 
: cycle.  There may be small fields due to the change in resistivity of the 
: blanket while it's on, but for the most part _only_ an magnetic field is 
: generated.
: 
: If you have a blanket that switches on and off every, say, 15 minutes, the
: e-field exposure is going to be pretty insignificant compared to a 60Hz AC
: electric blanket.  Consider also that even the electric field generated
: when a switching does occur will have a very small frequency component at
: 60Hz (since the change in the current will be approximately a square wave
: of fundamental frequency 1/(15 min*60 s/min)=1/900 Hz).

An electromagnetic field has electric and magnetic components. As such,
in this case, they cannot be separated.

If the blanket had a simple diode circuit (to produce the dc) - then the
waveform would not be anywhere close to a square wave. It would be a 
rectified sine wave - with a frequency of either 60 Hz or 120 Hz, depending
on whether a half-wave or full wave rectifier was used. You still will have
an electromagnetic field that builds and collapses - just like with an
unrectified supply. To a test meter, you would not be able to tell the
difference - nor would your body.

The only way to avoid this would be to build a DC power supply that has
some capacitors and a voltage regulator to smooth out the DC so there
is no ripple.

: Finally, the concept of a "DC electromagnetic field" is of dubious value
: at best.  When someone says "EM field" the implication is that the two
: fields are linked together from one common source, as is the case in AC.
: That link is, for almost all pruposes, non-existant in this quasi-static
: case.

Either I misunderstand what you are saying, or you are confused. Any time
a current moves in a wire, you have an electromagnetic field. It does not
matter if the current is AC or DC. 

: If you like, I can drudge out Maxwell's equations and show this all
: mathematically, but I don't think there's much point.

What would you try to prove, that a DC current does not generate an
EM field? I don't think you can do so.

: P.S. -- Which division of HP do you work for?  Certainly not one where
: they do anything with fields and waves?

What does that have to do with my training? One of the people in my
group got his degree in religious studies. He still is one of the
brightest engineers in the division.

Bill