The Shulhan Aruch, in discussing the laws of Shabbat, codifies the prohibition against reading by the light of a candle on Shabbat. The Sages enacted this provision out of concern that one may tilt the oil lamp to improve the light. The flame in an oil lamp often has a tendency to flicker, and the Sages feared that a person involved in reading may forget it is Shabbat and tilt the lamp, which would constitute a Shabbat violation. They therefore forbade reading by candlelight on Shabbat.

With the invention of the light bulb, the Halachic authorities ruled that this prohibition would not apply to reading by the light of a bulb. Quite obviously, the light of a bulb does not flicker, nor is there any way of tinkering with the bulb to improve the light. Therefore, we have no reason to apply the prohibition that was enacted with regard to oil lamps. One might have argued that we should still forbid reading by a bulb because of the concern that the bulb may be burn out, and the person might then forgetfully change the bulb on Shabbat. But such an argument is incorrect. Changing a light bulb would be analogous to adding more oil to a lamp whose oil had run out. When the Talmud speaks of the enactment forbidding reading by candlelight, it mentions only the concern of tilting the lamp, and not the concern of adding more oil to an empty lamp. Apparently, the Rabbis were not concerned about the unlikely situation that a lamp will run out of oil on Shabbat and one will then refill it; they feared only that one may tilt the lamp while reading. Hence, we have no reason to forbid reading by the light of bulb out of concern that one may change the bulb.

This applies to all kinds of bulbs, both fluorescent and incandescent.

A more complex question, however, arises regarding lights with dimmers, which can be adjusted to increase or decrease the amount of light produced by the bulb. Should we forbid reading by such a light, given the concern that one may unwittingly adjust the dimmer to increase the light?

Hacham Ben Sion Abba Shaul (Israel, 1923-1998), in his work Or Le’sion, indeed ruled that reading by such a light is forbidden, just as one may not read by candle light out of concern that he may tilt the lamp. He adds, however, that when it comes to learning Torah one may be lenient and read by the light of a lamp with a dimmer.

Hacham Ovadia Yosef, in both Yehave Da’at and Yabia Omer, disagrees. He contends that turning the switch of a dimmer, which increases the flow of electricity into the lamp, is analogous not to tilting an oil lamp, but rather to adding more oil. And nowhere do we find that the Sages were concerned that one may add oil to the lamp. As we saw, the concern was only that one might tilt the lamp. Hacham Ovadia thus claims that we cannot introduce a new provision out of this concern, and it is therefore permissible to read by the light of a lamp with a dimmer on Shabbat. He adds, however, that one should preferably attach before Shabbat a small note near the switch with the word "Shabbat" as a reminder not to turn the switch.

Summary: It is permissible to read by an electric light on Shabbat, even if it has a dimmer, as we are not concerned that one might adjust the dimmer. Preferably, as an added precaution, one should place a note on the switch of the dimmer to remind him not to adjust it while he reads.