Sometimes it is possible to reverse the operation of an electrical device so that it will act in a "negative" and not a "positive" way. This can allow the device to be used on Shabbat, since the Torah prohibits positive action and not the prevention of an action. It must be made clear that the application of this principle is very limited. We call this technique "disconnecting an inactive circuit."
The Zomet Institute applies this "negative" method in various alarm systems, particularly those that are intended for use in a synagogue. Thieves usually are not inhibited by Shabbat prohibitions, and they ply their "trade" on Shabbat just as on weekdays. One way to frustrate them is to install an alarm, for example in a home. But how can the alarm system be activated? And how can the owner enter his home on Shabbat?
In addition, many synagogues would like to install electronic protection devices around the Torah Ark, which usually contains very valuable items. The responsible people usually do not want to use a Shabbat timer for the alarm system, since there are many unplanned occasions when the ark is opened, such as for moving the Torah scrolls to a new position or for use by people who have a family celebration. And if the alarm begins to sound, how will it be turned off on Shabbat?
The Zomet Institute has developed a mechanism which is suitable for passive detectors (magnetic switches, shock detectors, etc) and for alarms where an unsuspecting intruder breaks a trip wire – and a loud alarm sounds, often also actuating a hidden telephone call to a security center.
The principle of the system is the opposite of the usual alarm systems. Without his knowledge, an intruder connects a wire which activates the system. Under normal circumstances, the circuit is turned off.
The way that the Shabbat control works is that a person who has the proper authority turns a key which opens the alarm circuit at another position, in addition to the regular break in the circuit associated with the door. Thus, while opening the door reconnects the alarm wire, no circuit is formed because of the second new opening, linked to the key. After the door is closed, the key is turned back, reconnecting the wire. But the current is still off, because of the regular alarm at the door. The person holding the key merely opens or closes a switch on a line where no current is flowing. The alarm is activated only if the door is opened while the key has not been turned.
When the alarm sounds, it is not continuous but with a ten-second pause every minute, in repeated cycles. This regular pause in the alarm signal provides an opportunity to turn the mechanism off if there was a false alarm. This action is permitted, since it is based on a negative preventive action.
Desecration of Shabbat consists of positive actions such as "burning," "construction," or "creating a new current." None of this type of activity occurs when a person acts in a negative way to prevent possible future electrical activity.
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