This pen prevents ruining shirts by retracting the tip automatically
when the pen is clipped into your shirt pocket. The catch that keeps the
tip out is actually in the pocket clip. When you pullout on the clip to
put it on your shirt, the pin that engages the catch is released causing
the tip to retract.
Many Elevators are equipped with an electric eye to prevent doors from
shutting on people. They also are equipped with sensors and alarms to prevent
operation when overloaded.
Some brake systems utilize permanent magnets to generate power-off braking
force. Current supplied to magnetic coils counter permanent magnetic
force to keep the brakes from engaging during normal operation.
Many smoke detectors are designed so they cannot be mounted on the wall
until a battery is installed.
Retail stores and libraries use "electronic article surveillance" systems
to prevent removal of merchandise and books that have not been paid for
or checked out.
This device continuously monitors the water level. If the level is too
low, a valve opens to add water to the pool.
This Stove burner turns off automatically when pot or pan is removed.
The manufacturer has rigged these wallets to sound an audible beep when
a card is absent from the wallet. While this device would clearly
be annoying to some users, for others it would be a big help in remember
to replace credit cards after use.
The manufacturer has rigged this drill to stop immediately when the
bit contacts any grounded or electrically live materials so you can't drill
through hidden pipes or wiring.
In China, where electric power has been scarce, hotel rooms are equipped
with a room key holder which turns off the power when the key is removed.
This insures no electricity flows to the room while it is vacant.
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