The eruv allows observant Jews to carry needed things in public on the Sabbath.
A designated space may be considered a private domain only when several conditions are met: It must cover an area of at least 12 square feet and must be somehow demarcated from its surroundings--either by a wall of some kind or by its actual topography (it must be all lower or all higher than its surroundings). If a space is frequented by more than 600,000 people each day it is considered too big to turn "private" with an eruv (which is why a large city like New York cannot have a single eruv constructed around it, though individual neighborhoods do). And an eruv cannot encompass two separate public spaces, so for instance, it may not cross over a river that cuts through town.
When eruvim (the plural of eruv) or parts of eruvim are constructed (as opposed to using pre-existing structures), they generally consist of a wire surrounding the designated area. The eruv, therefore, is unlikely to be noticed except by those looking for it, which fulfills a talmudic guideline that the eruvbe an integral part of the city, as unobtrusive and unnoticeable as possible.
Despite its symbolic nature, the eruv is intended to mimic in some way the form of walls, which need doorways--defined as two posts with a crossbeam over them, strong enough to withstand an ordinary wind. The eruv likewise needs openings, consisting of crossbeams resting or passing directly over the top of the doorpost (lehi). This is how modern rabbis arrived at the solution of having the eruv be made of a wire: The poles holding up the wire represent the "doorposts," and the wire itself represents the "crossbeam."
Many communities construct their eruvim by using lighting (or utility) poles to fulfill the requirement of doorposts and a continuous cable, string, or wire to represent the crossbeam. In order for this arrangement to be acceptable, the "beam" must rest directly above the top of the doorposts. Since this is not typically the case with utility poles--where the cable is attached either to the side or to an element that is held away from the pole--communities often attach a thin rod to the pole to serve as a substitute for the doorpost.
In areas where poles and lines do not exist, new arrangements must be made. The design of an eruv can make use of existing fences, overhead wires, hillsides, buildings, bridges, and other means that may serve as indicators of the eruv boundaries. This often requires communities to obtain permission from the appropriate authorities and property owners, and to work with local governments, power, telephone, and cable companies--a lengthy and often difficult process.
Those who use an eruv are obliged to ensure that it is intact before carrying on Shabbat. In most cases there is a group that is responsible for maintaining the eruv, providing information regarding its status, conducting weekly inspections to ensure that it hasn't been damaged, and dispensing repair crews when necessary. Many communities provide current information regarding their eruv by phone (eruv hotlines) and the web. They also provide maps delineating the exact boundaries of the eruv. (While Orthodox and Conservative understandings of Jewish law recognizes the restrictions that make an eruv necessary, other denominations generally do not.)
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