I carried a microwaveable hard plastic drinking cup which I would fill with water, set the timer to 30 seconds, and if the water was not 'hot to the touch', it was 'not working'. Depending on how large or small a cup you use, the water will likely boil in less than 60 seconds - I found that with the size cup I used, 30 seconds would make it 'hot to the touch'.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )