Ancient Jewish History: The Menorah
One of the oldest symbols of the Jewish faith is the menorah, a seven-branched candelabrum used in the Temple.
It has been said that the menorah is a symbol of the nation of Israel and its mission to be "a light unto the nations." (Isaiah 42:6). The sages emphasize that light is not a violent force; Israel is to accomplish its mission by setting an example, not by using force. This idea is highlighted in the vision of the Prophet Zechariah who sees a menorah, and G-d explains: "Not by might, nor by power, but by My spirit." (Zechariah 4:1-6)
The kohanim lit the menorah in the Sanctuary every evening and cleaned it out every morning, replacing the wicks and putting fresh olive oil into the cups. The illustration feautred on this page is based on instructions for construction of the menorah found in Exodus 25:31-40.
The lamp stand in today's synagogues, called the ner tamid (lit. the continual lamp; usually translated as the eternal flame), symbolizes the menorah.
The nine-branched menorah used on Chanukah is commonly patterned after this menorah, because Chanukkah commemorates the miracle that a day's worth of oil for this menorah lasted eight days.
The menorah in the First and Second Temples had seven branches. After the Temples were destroyed, a tradition developed not to duplicate anything from the Temple and therefore menorah's no longer had seven branches. The use of six-branched menoras became popular, but, in modern times, some rabbis have gone back to the seven-branched menoras, arguing that they are not the same as those used in the Temple because today's are electrified.
Sources: Judaism 101; The Jewish Book of Why