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If I switch on my electronic kitchen scale while it's on the table, it reads 0 grams. If I pick it up, it reads around -500 grams. Conversely if I switch it on in my hand it tares to 0g and then reads about 500 grams when I put it down.

What's the difference, physically, between resting on my (not moving) hand, and on the table?

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Its most likely due to one or more of the physical design, the location of the sensor, the way you hold it and the compliance of the surface your hand. Your hand is always moving a little, that might be a factor. – RedGrittyBrick Jul 6 '14 at 11:54
How are you holding the scales? Are they resting flat on the palm of your hand? – John Rennie Jul 6 '14 at 12:02
@JohnRennie: yes, originally it was resting on my palm, but I get the same effect resting it on my fingertips or holding it between my fingers and thumb. – Max Jul 6 '14 at 13:21
@RedGrittyBrick: I would expect some fluctuation because of my hand moving, but I expect it to fluctuate around 0 rather than around -500. – Max Jul 6 '14 at 13:23
Given the answer, this seems to be a question about the design of the scales, rather than the physics of the situation, which turns out to be trivial. – David Richerby Jul 6 '14 at 19:37

It turns out that the scale has little feet, and if I hold it up by these feet it reads 0 g. If I rest it on a small object so the feet are hanging off, the reading is negative. So it seems that the "base" of the scale is actually directly attached to the pan rather than to the feet. So I'm effectively holding the pan up while the feet hang down, causing the negative weight.

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This is a very common design as there is no reason to have more than two parts that are free to move relative one another. – dmckee Jul 6 '14 at 14:01

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