1. It is accepted amongst the authorities that it is Biblically prohibited to light an incandescent light bulb on Shabbos. This view was expressed by Harav Yitzchak Schmelkes zt”l (Beis Yitzchak Y.D. 120), Harav Chaim Ozer Grodziensky zt”l (Achiezer 3:60), Harav Isser Zalman Meltzer zt”l (Haskama to Sefer Chelkas Yaakov), amongst others. The question that must be addressed is which Melocho does one transgress when turning on these light bulbs.
2. The Rambam (Shabbos chapter 12) writes, “A person who heats iron in order to strengthen it by submerging it in water is liable for [performing] a derivative [of the forbidden labor] of kindling.” Based on this teaching of the Rambam many poskim maintain that turning on incandescent light bulbs on Shabbos, which heats up a metal filament, transgresses the Melocho of Maavir (kindling). (Achiezer ibid., Harav Shlomo Zalman Auerbach zt”l in Meorei Eish, Keren L’David 80, Even Yikra Third Edition 168, Pnei Meivin 57, Divrei Chizkiyah vol. 2 page 89 and Yaskil Avdi 4:16)
3. The Merkeves Hamishnah feels that according to the Rambam one only transgresses the prohibition if he heated the metal for the intent to strengthen it. If he heated the metal in order to produce light the prohibition is only Rabbinic. A similar notion was expressed by the Avnei Nezer (Orach Chaim 229). However, Harav Shlomo Zalman Auerbach zt”l explains at great length that making metal glow red hot is considered creating a fire, according to the Rambam, it makes no difference whether he intended to do so in order to create light or in order to strengthen the metal.
4. The Maharsham (2:247) questioned whether turning on a light bulb on Shabbos involves the Melocho of Maavir since the “flame in the light bulb is not consuming.”
Following the line of reasoning of the Maharsham when the Rambam rules that heating metals is considered kindling on Shabbos he is discussing a situation where the fire consumes the metal. However, many poskim disagree with his assertion on two fronts. (A) His assertion that a fire must be consuming on Shabbos does not seem to be the same conclusion of the Shulchan Aruch Harav (495 Kuntres Achron 2) who indicates the destructive element of fire is not significant. Creating flame is all that matters, even if it is the type of flame that is not destructive by nature. (B) In addition the Tzitz Eliezer (1:20:7) points out that filaments in a light bulb are destructive by nature. If that filament would be exposed to oxygen and kindling there it would create a flame. It is normally constructed in a safe protective way, however, the filament itself absolutely is capable of creating a fire.
5. The Raaved disagrees with the Rambam and writes that heating metal is not considered kindling (Maavir) but cooking (Bishul). The Chazon Ish (50:9) therefore writes that turning on a light bulb would constitute the Biblical prohibition of cooking on Shabbos.
6. Harav Shlomo Zalman Auerbach zt”l (Minchas Shlomo Kama 12 note 6) questions this ruling on the following grounds. The law is that one only transgresses Shabbos on a Biblical level if one cooks an item using fire (Eish) or an item heated in fire (Toldos Eish. The Rabbis extended this prohibition to include cooking food in a pan heated by the sun (Toldos Chama). Therefore, argues Harav Auerbach zt”l, in this case one is cooking the metal using electric currents which is neither fire or an item heated by fire and therefore one would not transgress a Biblical prohibition. (Parenthetically, the Chazon Ish himself feels that one cannot cook in an electric current for reasons beyond the scope of this article.)
7. It must be noted that the Halacha may be different regarding Led and fluorescent light bulbs and a Rav should be consulted.