60-cycle vibration from heating pad?

[neomsgs]

I'm wondering about what's going on in an effect I've noticed with heating pads (as well as electric mattress pads).

Try this experiment: take an ordinary heating pad (one of those flat rectangular things about 1 foot by 1.3 feet with an electric plug), plug it in, and have a friend sit leaning against it. (Note that the heating pad only needs to be plugged in; it doesn't matter whether it is actually switched on.) Now, with 2 or 3 fingertips, very slowly (moving about 1-2 cm/second) and very lightly brush your fingertips across some of your friend's exposed hairless skin (e.g., forehead, palm, or wrist) - lightly enough that your fingers don't make a depression in their flesh, but not so lightly that you're only brushing their hairs -- you should actually be making very gentle physical contact with their skin. If you do this at the right speed and pressure, you'll feel a mild vibration or buzzing (though it doesn't make a sound that I can hear), which I think is at the 60-cycle-per-second rate of the AC power, as if their skin were covered with tiny ridges and your fingers were quickly (30 or 60 per second?) skipping across their peaks. If you keep doing this and have your friend unplug the heating pad (again, note that merely turning it off will not interrupt the effect), you'll find the buzzing/vibration instantly stops, and if they plug it back in, it will instantly resume.

Can you explain what is happening here? Any idea if it could be dangerous? (A friend of mine who noticed this now refuses to use heating pads, despite living in an apartment with poor heating and not really having the money to run space heaters, which use far more energy than heating pads--over 1000 watts vs. about 20 watts.) I took a year of high school physics and did some basic electronics in college, but it's not obvious to me why this happens. I'd love to get a clear explanation from someone who actually understands what's happening here. Thanks in advance!

 

[Kreemoweet]

You are feeling the 60 Hz induced voltage in your friend's body, brought on by capacitive coupling to the wires in the heating mat. I've felt the same
thing many times on the surfaces of appliances and light fixtures. Those of us who live in the electrified parts of the world have been bathed since
birth in such electromagnetic fields, and I've seen no evidence that there's any harm in it.

 

[Sponsor]

 

[neomsgs]

Fascinating. Thank you for specifying those terms, induced voltage and capacitive coupling!

Now I'm wondering what that actually means is happening in my friend's body? My vague understanding of "voltage" is that it's a difference of charge between two locations. So I guess the induced voltage is a voltage *between* my body and hers? What, physically, creates that voltage? Is it something like free electrons congregating in one part of her body (like, next to the heating pad?) during one phase of the AC and in another part during the opposite phase?

And I guess, as long as the voltage is low, these electrons aren't energetic enough to cause a "shock" or interfere with the operation of the body's nerve impulses, etc? Does that make sense?

And, what happens when I *feel* this voltage? Is it electrons jumping from her body to mine and back? But if so, why don't I feel a steady 60-hz vibration when I just hold my fingers still on her skin? I'm guessing that's because when I do that, it electrically joins our bodies so that there's no voltage between them, and my body then takes on the same 60-hz induced voltage relative to everything else in the room?

I'd appreciate any further explanation you could give.