At entrances sensitive from a security point of view, an electronic gate is often installed which can be used to detect the presence of metal. If a person passes through carrying a large enough quantity of metal (such as a gun), a buzzer sounds. People with evil intentions will certainly not be inhibited by the Shabbat desecration involved in going through the gate, but what about an observant Jew? Sometimes a belt buckle or a Tallit ornament is enough to set off the alarm, thereby desecrating Shabbat.
In response, The Zomet Institute has developed a special gateway which can be used on Shabbat. Such devices have been installed, for example, at the entrances to the Western Wall and Shaarei Tzedek Hospital in Jerusalem. In these entrance gates the buzzer has been replaced by a meter, and a small current exists even when no metal is present. A person who passes through the gateway carrying a metallic object changes the level of the current in the device but does not open or close an electrical circuit. A sign at the entrance explains to those passing through that they need not fear desecrating the Shabbat.
These gateways have been approved by the most prominent rabbis, and as noted, they are in routine use in many places where religious people regularly pass through.
The Zomet Institute has also developed a Hand-held Metal Detector.
For details and prices, contact The Zomet Institute.