Why does electrical tester trip the GFI?

I have one of those "Vibrating Solenoid Voltage Testers" (Similar to this)
http://images.lowes.com/product/converted/793950/793950910504xl.jpg
If I put one lead to the HOT outlet terminal and the other lead to the metal box (GROUND), on a GFI outlet, the GFI trips. Why does it trip?
My tester is an oldie, I think they were called "wigglers" back then. But it's basically the same as these newer ones. It vibrates and the little red marker shows the voltage.
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On Jan 8, 9:38 pm, snipped-for-privacy@myplace.com wrote:

GFCIs look for a difference in current flow between the hot and neutral. The GFCI is supposed to trip if the difference in current flow is too large. There is no current flowing in the neutral when you test from the hot terminal directly to ground. There will be a bit of flow through the hot wire since it's activating your wiggly. See what happens if you go from hot to neutral with your leads.
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On 1/8/12 10:38 PM, snipped-for-privacy@myplace.com wrote:

The type of tester has nothing to do with what you see. The GFI is doing its job. That why it is called a GROUND Fault Interrupter. There is normally no current flow from hot to ground.
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Reed wrote:

That makes more sense. I was thinking that the wiggler tripping the Girl Friend Interface was a good thing...
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Then you have a ground fault, exactly as designed. It should not trip hot to neutral. If the current is not equal from hot to neutral, then that means the current must be taking another path, like ground.
Greg
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Sometimes it handy to have a higher current tester to test for bad connections.
According to my calculation, you could easily build vibrating tester drawing less than 1 ma. http://www.piezo.com/prodfan1vac.html
Greg
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On 1/9/2012 9:57 PM, gregz wrote:

Darn, I remember a story in Popular Science or Mechanics about piezoelectric fans of the future. It was years ago and those are the first I've ever seen for sale. I must have missed any consumer devices. o_O
TDD
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The highest z vibrating thing I have seen, is a fan, using piezo electric elements, in series with limiting resistor. These things were popular in the 80's. Probably would still trip.
Greg
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