13. We follow the principle that it is the act of kindling that fulfills
the mitzvah ("Hadlakah oseh mitzvah") (1). Therefore, at the time [the
Chanukah candles] are being kindled, they must be in their correct location
(2) and contain sufficient oil [to burn for the requisite time]. In
contrast, if the candles were lit below three handbreadths ("tefachim" (3))
from the ground or twenty cubits ("amos" (4)) above the ground, and then
moved to their correct location while they were burning, they are not valid
Similarly, if, at the time of kindling there was not enough oil [in the
menorah to burn] for the required time, it does not help ("lo mehane") to
add more after [they are lit]. Based on the same principle, if one
positioned the Chanukah lights in a windy place, where it is likely that
they will be extinguished, one has not fulfilled the mitzvah and is
required to light again. One should not, however, recite a blessing when
lighting the second time.
Conversely, if one put the candles in their correct place (that is, where
they are not likely to be extinguished by the wind), and unexpectedly they
were extinguished, one is considered to have fulfilled the mitzvah (5).
Nevertheless, it is customary that one light them again.
It is customary not to light one Chanukah candle with another, but rather
to use the "shamash" (6) or another ordinary candle.
(1) As opposed to those in the Talmud who rule that it is the act of
placing the already lit menorah in its correct location that fulfills the
mitzvah ("hanacha oseh mitzvah").
(2) For example, next to a window overlooking the street.
(3) Opinions among the authorities as to the exact length of a 'tefach'
('handbreadth') range between 8 and 10cm ( 3 to 4 inches).
(4) Opinions among the authorities as to the exact length of an "Amah"
("cubit") range between 48 and 60cm (20 to 24 inches).
(5) In other words, if one lit them according to all the halachic criteria,
and they were extinguished before the requisite half hour, one is not
obligated to relight them (although the custom is to do so). There are
authorities who rule that even when the menorah was lit in the correct
location, it should not be moved even to another valid location until after
the candles had burned at least a half hour. Others are more stringent,
maintaining that the menorah should not be moved at all while it is burning
(See Mishna Berura 675:6 and Sha'ar Hatziyun 672:12).
(6) People generally set aside an extra candle called the "shamash" with
which to light the other candles.