Elenco Green Projects - Manual (Page 31)

0.5mA
Liquid Light
0.5mA
Project #33 Make Your Own Parts
Liquid Resistors Project #34
Resistance = Voltage / Current, so you can use the
battery voltage (3.6V) and the current you measure
to find the resistance of your puddles and drawings.
Long narrow shapes have more resistance than
short wide ones. The black core of pencils is
graphite, the same material used in the resistors in
the pivot stand.
Build the circuit, set the
meter (M6) to the 0.5mA
setting, and set the slide
switch (S5) to position B.
Make your parts using either
the water puddles method
(A), the drawn parts method
(B), or the pencil parts
method (C). Touch the metal
in the jumper wires to your
parts and read the current.
Method A (easy): Spread some water on the
table into puddles of different shapes, perhaps
like the ones shown here. Touch the jumper
wires to points at the ends of the puddles.
Method B (challenging): Use a SHARP pencil (No. 2 lead is best)
and draw shapes, such as the ones here. Draw them on a hard, flat
surface. Press hard and fill in several times until you have a thick,
even layer of pencil lead. Touch the jumper wires to points at the ends
of the drawings. You may get better electrical contact if you wet the
metal with a few drops of water. Wash your hands when finished.
Method C (adult supervision and permission required): Change
the setting on the meter to the 50mA scale. Use some double-sided
pencils if available, or VERY CAREFULLY break a pencil in half. Touch
the jumper wires to the black core of the pencil at both ends.
Project #35
Replace the meter with the
red LED (D1, + on top).
Place the jumper wires back
into water, into salt water, or
on the shapes you drew.
Pure water has very
high resistance because
its electrons are tightly
held in place. Impurities
(such as dissolved dirt,
minerals, or salt)
decrease the resistance
because they have
loose electrons, which
disrupt the structure and
make it easier for other
electrons to move
through. Build the circuit, set the meter (M6) to the 0.5mA setting,
and set the slide switch (S5) to position B. Add about 1/4 inch
of water to a cup or bowl. Connect the jumper wires to the
circuit as shown and place the loose ends in the water, make
sure the metal parts aren’t touching each other. Measure the
current through the water.
Add salt to the water and stir to dissolve it. The current
should be higher now, since salt water has less resistance
than plain water. If the current is too high to measure on the
0.5mA scale, switch to the 50mA scale.
Now add more water to the cup and watch the current.
If you have some distilled water, place the jumper wires in
it and measure the current. You should measure close to
zero current, since distilled (pure) water has very high
resistance. Normal water has impurities, which lower its
resistance. Now add salt to the distilled water and watch the
current increase as the salt dissolves!
You can also measure the current through other liquids.
Don’t drink any water or liquids used here.
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See projects 1 & 3 if you need to
recharge the battery (B4).