Make Electronics

Experiencing Electricity 7

Experiment 1: Taste the Power!

Procedure

We’re going to use the meter to discover the electrical resistance of your
tongue. First, set your meter to measure resistance. If it has autoranging, look
to see whether it is displaying a K, meaning kilohms, or M, meaning megohms.
If you have to set the range manually, begin with no less than 100,000 ohms
(100K). See Figures 1-23 through 1-25.

Touch the probes to your tongue, about an inch apart. Note the reading,
which should be around 50K. Now put aside the probes, stick out your tongue,
and use a tissue to dry it very carefully and thoroughly. Without allowing your
tongue to become moist again, repeat the test, and the reading should be
higher. Finally, press the probes against the skin of your hand or arm: you may
get no reading at all, until you moisten your skin.

When your skin is moist (for instance, if you perspire), its electrical resistance de-

creases. This principle is used in lie detectors, because someone who knowingly tells

a lie, under conditions of stress, tends to perspire.

A 9-volt battery contains chemicals that liberate electrons (particles of electric-
ity), which want to flow from one terminal to the other as a result of a chemical
reaction inside it. Think of the cells inside a battery as being like two water
tanks—one of them full, the other empty. If they are connected with a pipe,
water flows between them until their levels are equal. Figure 1-26 may help
you visualize this. Similarly, when you open up an electrical pathway between
the two sides of a battery, electrons flow between them, even if the pathway
consists only of the moisture on your tongue.

Electrons flow more easily through some substances (such as a moist tongue)
than others (such as a dry tongue).

Figure 1-26. Think of the cells in a battery as being like two cylinders: one full of water, the
other empty. Open a connection between the cylinders, and the water will flow until the
levels are equal on both sides. The less resistance in the connection, the faster the flow
will be.

Figure 1-23

Figure 1-24

Figure 1-25. To measure ohms, turn the

dial to the ohm (omega) symbol. On an

autoranging meter, you can then press

the Range button repeatedly to display

different ranges of resistance, or simply

touch the probes to a resistance and wait

for the meter to choose a range auto-

matically. A manual meter requires you to

select the range with the dial (you should

set it to 100K or higher, to measure skin

resistance). If you don’t get a meaningful

reading, try a different range.