Elenco Snaptricity&reg - Manual (Page 26)

-25-
Educational Corner:
Hydro-Resistors Project #16
Build the circuit shown. Set the meter (M5) to the 1mA setting. Add
about 1/4 inch of water to a cup or bowl. Connect the jumper wires
and place them in the water, make sure the metal parts aren’t
touching each other. Set the slide switch (S5) to position B to turn the
circuit on.
Assembly
Measure the current through the water.
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.
Operation
1mA
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. A
high reading of 4V or more means the
water has relatively low resistance.
1mA
Snappy says: Pure water has very high
resistance because its atoms hold their
electrons tightly and have no room for more.
Impurities (such as dissolved dirt, minerals, or
salt) decrease the resistance because their
atoms have loose electrons, which make it
easier for other electrons to move through.