Ask the Rov: How do I extend the distance outside of the city that I can walk on Shabbos?
By Rabbi Chaim Hillel Raskin – Rov of Anash in Petach Tikvah
If one wishes to extend the distance he can walk for a mitzva purpose, Chazal allow one to place an eruv techumin. Qualifying purposes include going to the mikva, davening with a minyan, or even taking an enjoyable Shabbos walk or meeting a friend.1
One makes the eruv by placing food or condiment sufficient for two meals in a particular place outside of the city, thereby re-establishing his primary dwelling location at the arrival to Shabbos. This is called an “eruv” (mixing) as he is combining the place where he couldn’t walk with the place he can walk.2
Placing the eruv close to the end of one’s techum allows one to walk an additional 2,000 amos past that point. If the location of the eruv is enclosed with a fence, he can walk throughout that entire area, even if it is larger than 2,000 amos. But if he placed the eruv outside of his techum, it doesn’t count, since the eruv must be in a location where he can permissibly walk to on Shabbos.3
An eruv placed outside the city extends how far one may walk from his home in that specific direction, but limits him in the opposite direction. Is that limit only outside the city or does limit him inside the city as well?
Some rishonim hold that once he establishes his residence in the location of the eruv, this limits how much he can walk in the opposite direction even within the city itself.4 Others maintain that he is still allowed to walk throughout the entire city if he sleeps there overnight, but he may not walk out of the city on the other side.5 The halacha follows the latter opinion.
If in some weeks it is beneficial for him to have the eruv and other weeks he would rather walk in the opposite direction, he may place the eruv with the condition that it should only establish his residence on weeks that he decides to use it. He can even make this decision on Shabbos itself and it can work retroactively (due to the halachic principle of yesh bereira that we rely on in derabonon matters). For a long-term eruv, one should use food items that can last for a long time, such as vinegar, oil, or jelly. (Plain water doesn’t qualify, but saltwater does.)6
See Sources (open PDF)
From The Weekly Farbrengen by Merkaz Anash