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Re: Lamp Switch?



wmpool@inetnow.com wrote:

A friend moved and gave me a lamp with a planter around the base of the
lamp. The lamp has no switch. You can touch a leaf, the soil, or both
plastic sides of the lamp base and turn the lamp on and off. I have taken
the plant and soil out of the plastic base and cannot find HOW the circuit
is completed. Anyone have one of these lamps? I'd love to use this lamp at
school, but I have to understand how it works myself. Thanks, Marie

Marie,
The simplest demo for you to give (or at least begin with) is to trot
out two different voltmeters,
one an "old-fashioned" D'arsonval moving coil type and the other a
modern digital voltmeter (DVM) with a millivolt scale. Set each one on
its most sensitive scale, attach leads and just let them lie on an
insulated desktop. Observe that while the moving coil meter reads a
steady zero, the DVM will fluctuate quite wildly with substantial
millivolt readings (if it has an AC millivolt setting, the effect will
be even greater).

Modern DVMs use Field Effect transistors in their input circuitry and so
can have input impedances of tens, hundreds, or more of megohms. This
means that they can measure voltages without drawing appreciable
current; they can readably respond to static fields and behave much like
an antenna. Observe that the DVM reading responds to the touch of your
hand to one of its leads, or just to your motion in its vicinity,
because you are disturbing the ever present electric field to which it
is responding (this field is due mostly to the AC power wiring in your
classroom-take a field trip into the countryside and observe its
behavior, shield it in a Faraday cage, etc.)

There is much more to be said and done (Radio Shack has FETS cheap), but
it all rests on the nearly infinite input impedance made possible by
modern MOSFET circuitry. (a last random thought: make a correlation to
the behavior of a TV set "measuring" the field of a weak signal using a
rabbit ear antenna when you touch [or just move near] its antenna.)

Whatever you do, have Phun!
--
Bob Sciamanda sciamanda@edinboro.edu
Dept of Physics sciamanda@worldnet.att.net
Edinboro Univ of PA http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA (814)838-7185