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Induction cooktop and effect of magnetic field

fishable May 9, 2010 12:28 AM

Does anyone know if an induction cooktop could potentially damage a mechanical watch worn by the cook due to the magnetic field created? I have read online that there has been some question in the past as to whether or not they are safe around pacemakers as well. I am remodeling a kitchen and am trying to decide between gas and induction and would hate to choose induction and then accidentally destroy a watch that is more expensive than the cooktop.

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  1. Politeness RE: fishable May 9, 2010 02:11 PM

    Fishable: "Does anyone know if an induction cooktop could potentially damage a mechanical watch worn by the cook due to the magnetic field created? I have read online that there has been some question in the past as to whether or not they are safe around pacemakers as well."

    The Swiss Federal Office of Public Health has, with usual Swiss thoroughness, investigated the allegation that induction cooktops can interfere with pacemakers or defibrilators. http://www.bag.admin.ch/themen/strahl...

    The report, dated January 21, 2009, was based upon a review of the peer-reviewed literature; in relevant part, its conclusion was:

    "Effect on implanted electronic devices

    "Some studies have looked at the way induction hobs affect implanted electronic devices [7-10]. The possibility cannot be excluded that stray magnetic fields generated by induction hobs may affect implanted electronic devices at short range; this has been demonstrated for unipolar cardiac pacemakers [10]. Also the effect of leakage current on unipolar cardiac pacemakers has to be borne in mind. People with unipolar pacemakers are advised not to touch pans for extended periods and not to use metal spoons for cooking [7]. It is vital for people with implanted electronic devices to read the safety advice provided by the manufacturer and talk to their doctor before using an induction hob. The likelihood of the implanted device being affected adversely is very low if the induction hob is used correctly." I recommend that you read the entire report.

    The part of the Swiss government report that I found the most difficult to assess was the reference to "unipolar" pacemakers as being the only kind that might be affected by induction cooktops. What proportion of pacemakers are unipolar?

    The most important part of the Swiss report's conclusion appeas to be the phrase "short range." In the charts accompanying the summary at the URL above, the magnetic field is negligible (about the same level of radiation exposure as being outside on a day when you can clearly see your own shadow) at distances exceeding 10 cm (about 4 inches) from the induction coil, and approaches zero at about 30 cm (about a foot ).

    Assuming that the Swiss govenment study is correct, unless the person who has a unipolar pacemaker implanted is positioned so that the pacemaker in his or her chest is within 4 inches of the cooktop while BOTH the induction unit is in operation AND when there is no pot or pan covering the operating burner, the risk is nil; just being in the general vicinity in the same room with the cooktop would not expose the person to significant risk, certainly nowhere near the risk that microwave ovens present, or even the risk that being within that same 4 inch distance from a gas or resistive electric coil burner presents when the burner is in operation.

    As to the magnetic watch question, when our first induction cooktop was new in 1999, as an educational exercise, I placed a floppy disk containing data inside a pan and turned on the induction burner, taking the disk out before it got hot enough to melt. The disk was readable after the exercise. Whether that tells anything about the susceptibility of a mechanical watch to the field from an induction burner, I cannot ascertain for certain, but it certainly is suggestive.

    1. Veggo RE: fishable May 9, 2010 05:12 PM

      It's an interesting science question. In my youth, I intentionally magnetized a mechanical movement watch (that's all there was then) to a level where it failed. All magnets degrade, but not at one's chosen pace. Aircraft compasses are often skewed by the magnetic field induced by activating the windshield heating element. Aircraft components and instruments project magnetic fields, but I have never heard of an effect on a pilot's aviator watch, nor do i have an idea of the relative strength of a field surrounding an induction cooktop to conditions I have been able to observe. I will e-mail the question about the stove to two prominent Swiss watchmakers. I am equally curious, I lack the stove. I have almost reflexively kept my mechanical movement watch away from known magnetic fields for exactly that concern. Meanwhile, while cooking, shuck the Rolex and strap on the Timex.

      5 Replies
      1. re: Veggo
        Chemicalkinetics RE: Veggo May 9, 2010 06:53 PM

        Magnetizing a watch to the point of permanent failure is overkilled. I worked in a high field magnetic lab and I have killed my credit cards and I have seen mechnical watches temporary stopped due to the high field. (Technically the magnet I worked with is not so high field by today standard) Basically, the watch was stopped when we moved it close to the superconducting magnet and resume moving when we moved it away. It was not permanently damaged. Situation like this is fine for a watch, but not for a pacemaker. You don't need to permanently destory a cardiac pacemaker to cause adverse effects on the patients, right?

        I have read a couple journal articles on this. Most of the articles point to that the effect from an induction stovetop to a pacemaker is very small. The problem of course is that we are talking about life related issue, so even a very small effect is concerning. Here is an abstract:


        I don't think it is a very serious problem that you should stop inviting anyone with a pacemaker in your house. However, if I have a cardio-pacemaker, then I will think twice about getting an induction cooktop. Keep in mind that most of these scientific studies are done for short term stability, not long term. Just because the magnetic field has no effect on a pacemaker in 2-5 minutes, does not mean it will have no effect in months and years. In addition, the studies only conduct on a few pacemaker (usually brand new) at a time. Having 10-15 pacemakers pass the test is not the same as having 10 000 pacemakers pass the test. I can randomly test drive 10-15 Toyota and none of them will have unintentional acceleration problem. While at it, do you know the chance you can get infected from a HIV needle-stick accident is less than 1%. That is to say, the chance is that you need to stick yourself with a HIV infected needle 1000 times before you catch HIV.

        1. re: Chemicalkinetics
          Veggo RE: Chemicalkinetics May 9, 2010 07:30 PM

          Pacemakers have no moving parts to be effected; extraneous electromagnetic fields can and will induce unwanted electrical flows, which can be attenuated and rectified in the semiconductor design with absorbent capacitors and discharge circuitry, and are designed for this likelihood. In contrast, adjacent tiny, highly kinetic ferrous moving parts in mechanical timepieces are not designed to operate when at first they are magnetized in alignment, and revolve into opposition. The physics are not comparable.

          1. re: Veggo
            Chemicalkinetics RE: Veggo May 9, 2010 07:39 PM


            I am just saying there is a difference between permant damaging something and temparory affecting something. You don't need to destory a pacemaker to get worry. Leakage currents is enough to be concern.

            1. re: Chemicalkinetics
              Veggo RE: Chemicalkinetics May 9, 2010 07:54 PM

              Yes. Please recall that this thread is about the watch thing, although pacemaker reliability should dwarf Swiss watches.
              I enjoy your refreshing knowledge of physics, kinetics, and electronics, and when we can both contribute a litte science here.
              And may minus B plus or minus the square root of B squared minus 4AC over 2A always yield 2 rational numbers in your equations!
              ( But it rarely does..)
              Cheers, Veg the quadratic cactus chewing jackass

              1. re: Veggo
                Chemicalkinetics RE: Veggo May 9, 2010 08:15 PM

                Opps. You are right. Sorry. I mis-read the post. Thanks.

      2. DonShirer RE: fishable May 10, 2010 07:06 PM

        I have only examined the movements of a limited number of mechanical watches, but most tended to use non-magnetic materials for their gears. Also ferric materials are most easily magnetized by steady or pulsed (one directional) magnetic fields. Induction heaters have AC (alternating) fields, and unless you yanked the watch away very quickly just as the field was reaching its peak, they probably wouldn't magnetize anything. In fact, placing a magnetized object in an AC field and slowly withdrawing it is one way to DEmagnetize it! Finally, the field around the cooktop usually diminishes rapidly as the distance from it increases, and even a wristwatch usually would not find itself directly on top of the heater.

        1. f
          fishable RE: fishable May 11, 2010 05:45 PM

          Thanks for all the great replies!

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