Alarm when the
windowpane is
broken
What if an intruder breaks the win-
dow instead of opening it? Or what if he
breaks in a plate glass window to steal
the window display from a store? What
would an alarm system look like that
was designed to set off an alarm in that
kind of situation?
HERE’S HOW
Assemble a circuit like the one shown in
image 1, with battery, light, two-way
switch, and alligator wire.
Cut a narrow, 20 cm-long strip of alumi-
num foil and clamp the free end of each
alligator wire to each end of the strip.
Switch on the alarm system: The red
bulb will light up.
Now imagine that a razor-thin aluminum
strip is attached to a windowpane. Then,
if the pane were broken, the strip would
tear. Try it with your strip — tear it. What
do you notice?
EXPERIMENT 18
WHAT’S HAPPENING
As long as the aluminum strip is intact, the bulb will light up
when the alarm system is turned on, because the circuit is
closed. If it tears, the light goes out — this is the alarm signal
(image 2).
Alarm systems like this are actually in use. In their case,
however, when the aluminum tears it sets off an audible
alarm or a releases a silent call to a security company.
TIP!
You can cut this kind of
thin aluminum strip
from the roll, secretly
install it and connect it
to the alarm system to
secure your windows,
doors, and drawers.
Electricity
21
1
2
Summary of Contents for Electricity and Magnetism 620417
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