Datasheet-1

Project 11
Make Your Own Parts
Build the circuit shown, and set the meter (M5) to the 1mA setting. Make your parts using either the water puddles
method (A), the drawn parts method (B), or the pencil parts method (C), and turn on the slide switch (S1). Touch
the metal in the jumper wires to your parts and read the current in milliamps.
Part B:
Place the ends of the wires in a cup of water, making sure the metal parts aren’t touching each other. Turn on the
slide switch and read the current on the meter.
Add salt to the water and stir to dissolve it. The current should be higher now (if not already at full scale), since salt
water has less resistance than plain water.
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.
Note: Depending on your local water supply, your current measurement may exceed the 1mA scale. You can switch
the meter to the 5V scale to get a better comparison, though it isn’t really a voltage measurement. (In the 5V setting,
the water resistance is compared to the internal resistance of the meter. A low reading means the water has relatively
high resistance, and a high reading means the water has relatively low resistance.)
Which gave a higher reading on the meter,
long narrow shapes or short wide shapes?
You can use Ohm’s Law to measure the resistance of
your puddles and drawings. The voltage is about 4.5V,
and use the current measured on the meter.
The black core of pencils is graphite, the same mate-
rial used in resistor components throughout the elec-
tronics industry.
Pure water has very high resistance because
its atoms hold their electrons tightly and have
no room for more. Impurities (such as dis-
solved dirt, minerals, or salt) decrease the re-
sistance because their atoms have loose
electrons, which make it easier for other elec-
trons to move through.
1mA
Voltage
Current
Resistance =
Method A (easy): Spread
some water on the table
into puddles of different
shapes, perhaps like the
ones shown below. Touch
the jumper wires to points
at the ends of the puddles.
Method C (adult supervision and permission required): Change
the setting on the meter to the 1A scale. Use some double-sided pencils
if available, or VERY CAREFULLY break some pencils in half. Touch
the jumper wires to the black core of the pencil at both ends.
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.
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