There is very little traditional halakhic literature written about the ner tamid. There is not, like other customs, a reference to this in the Shulkhan Arukh, the most prolific Jewish legal code. In searching for further information, I came across an article by Dr. Alexander Klein (in Hebrew). There, he points to two interesting pieces of information which speak to the development of the custom.  The first, from the Kol Bo (late 15^th or early 16^th century), tells of the custom to “light a candle in the synagogue before someone enters to pray.” The Kol Bo connects this action to replicating actions in Temple as well as the idea that the Shechina appears with a minyan. In this respect, the candle is then a reminder of the Temple as well as an indication that one’s actions have immediate repercussions for the Divine. Klein notes that the Kol Bo’s account does not use the language of Ner Tamid, rather it is simply a candle.

     

       A more recent source, R’ Menachem haCohen, a 20^th century Israeli rabbi, notes the following:

He then continues, describing how the ner tamid was also known as the ner ma’arvi or western candle, because in the Temple it had been placed on the western side, though now we generally put it on the eastern side (presumably intending the side closest Jerusalem), In any case, what we learn is that this particular custom has not developed a highly structured “halalkha” around it.

The Ner Tamid, or eternal light, is at its origin a reminder of the Jerusalem Temple.  The westernmost branch of the Candelabrum was kept lit in perpetuity.  Early synagogues followed suit by positioning the Ner Tamid on the western side (Babylonian Talmud, Megillah 29a).   Interestingly, Medieval synagogues only lit the Ner Tamid morning and evening when prayers were recited (Minhagei Yeshurun 2).  From the historical record we learn that both the placement and use of the Ner Tamid have changed over time.  While the practice in most synagogues today is to hang a Ner Tamid above and slightly forward of the Holy Ark, there is no requirement that it be so.  Some synagogues have affixed the Ner Tamid on the Aron Kodesh itself.  Some European synagogues have placed the Ner Tamid inside the Aron Kodesh.  Other synagogues (like the Spanish Potuguese Synagogue in Amsterdam) have dedicated one branch of a hanging candelabra situated between the central bimah and the Aron Kodesh as its Ner Tamid.   The variants are legion.  I recently visited the synagogue in Lisbon.  Its hanging Ner Tamid was oil fueled and lit just before Kabbalat Shabbat.  As much as it remains a link to the Jerusalem Temple, the Ner Tamid comes to symbolize the eternality of Torah and the enlightenment we gain from its study as well as the call for Israel to be a beacon of light to the world. 

A Ner Tamid (Eternal Light) is an essential element of every synagogue sanctuary and is traditionally suspended or placed above the Aron Kodesh (Holy Ark).  According to the rabbis, the light of the Ner Tamid represents the continuous presence of G-d in our midst.  G-d is associated with the image of fire throughout the book of Exodus, and the early lamps were fueled with oil and produced flame.  The practice of placing one in every sanctuary is derived from Exodus 27:20-21 which contains the commandment to keep a light continually burning outside the curtain that is placed over the ark.  There may also be a connection with Leviticus chapter 6:5-6 where we are commanded to keep a fire burning on the altar for all times.  Josephus recorded that a day was set aside each year during which all of the people would bring wood to the Second Temple, so that the flame would not go out.  Thus, the Ner Tamid may also remind us that we must actively work to keep G-d at the center of our community.

 

The only real requirement is to keep the Ner Tamid lit at all times.  It can be fashioned from virtually any material, in a wide range of sizes and styles.  It can be suspended from the ceiling or placed over the ark on a stand.  It can be fueled by oil and wick, but today we generally use electricity.  A few synagogues have recently moved towards solar power to symbolically connect the presence of G-d to the use of renewable energy and resources.