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60-cycle vibration from heating pad?

  1. Feb 8, 2010 #1
    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 I'm no expert. I imagine this must be some kind of electromagnetic field effect (induction?), but I'd love to get a clear explanation from someone who actually understands what's happening here. Thanks in advance!
  2. jcsd
  3. Feb 12, 2010 #2
    I got this response elsewhere:
    "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."

    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 such a 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 firmly against 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 insights!
  4. Feb 12, 2010 #3
    You say this is happening with the pad plugged in but switched off? Assuming it's a real effect, I'd have to wonder if "Hot" and "Neutral" are incorrectly wired on the power outlet, or if the switch for the pad is switching "Neutral" rather than "Hot" as it should.

    If you are comfortable making electrical measurements, here's a quick test to try:

    Using a DVM or VOM set to AC, measure the voltages between:

    1) "Hot" and "Ground" --> This should be about 110VAC
    2) "Hot" and "Neutral" --> This should also be about 110VAC
    3) "Neutral" and "Ground" --> Should be zero volts

    (You could also just buy a $5 outlet checkers from Home Depot, the kind with lights that indicate the configuration of the outlet it is plugged into.)

    Remember, in the US "Hot" is the *shorter* of the two rectangular slots, "Neutral" is the longer slot. On a three-prong outlet "Ground" is the third, *roundish* hole, but on two-prong (i.e. ungrounded) outlets there are only the two slots so "Ground" may or may not be the metal screw that holds the cover plate to the outlet box (older installations might not have the outlet box itself grounded).

    If you get 110VAC for test 3 and near zero volts in test 1 the power outlet is incorrectly wired and *should be immediately rewired for safety*.

  5. Apr 27, 2010 #4
    Hi. I'm wondering about the same thing neomsgs wrote about.

    I just experienced the exact same phenomenon last night while laying on top of our heated mattress pad.

    My 7 year old son was lying next to me while we watched a movie, and I was gently drawing my fingers over his forehead and immediately felt the same sensation (expertly described by neomsgs below!). To me it felt almost like light stripes of a gluestick had been drawn on his forehead and my fingers were going over them - sticky / not sticky / sticky / not sticky.. but of course, it wasn't actually "sticky".. It was definitely a well-defined pattern that I was feeling. then I realized it was some kind of electrical pulse of some sort. I could only feel it when my fingertips moved across his skin.. if I held them still, I felt nothing.

    I thought it was strange, and then I ran my fingers down the side of his face and felt the same. At that point, I was a little concerned, thinking something was going on with him, or under his skin.. I sent him to get my husband so he could feel it too.

    I called my daugher in, and had her lie down, and I could feel it on her forehead and arm as well. Then, I thought, well maybe there's something wrong with me! I then tried feeling it on myself (we were all on the mattress) and could not feel any sensation.

    I had the kids get off the bed and lie on the floor, and I tried again, but did not feel anything. I said to them that I thought the bed must be charged or something (forgetting completely about the heated pad) and my son looked over and saw that the heating pad was turned on on my husband's side of the bed. I unplugged the master cord from the mattress pad, had the kids get back on the bed and felt their foreheads again, and no odd sensation.

    Now, I have had that heated mattress pad on all winter, and I can safely say that my son has been in my room watching TV with me on more than one occasion, and I am sure that I have run my fingertips across his forehead (this usually puts him right to sleep!) before, but I have never felt that electrical sensation before.

    I plan to have the outlet checked to make sure it is ok, however, I was a little unnerved by this (as was neomsgs' friend)..

    1. Could something have changed? Why wouldn't I have felt this or noticed it before? It was very clear and noticible last night. Could it be some short in the pad itself ?

    2. Is it dangerous? Has it been dangerous to have been sleeping on that all winter long?

    Thanks in advance for any insight here.
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