The Sages ordained that one may not read by the light of an oil lamp on Shabbat, lest the flame become dim and the reader absent-mindedly tip the lamp so that more oil reaches the wick, thus transgressing the Torah prohibition of Hav’ara. To be sure, the Sages also ordained the kindling of Shabbat lamps, but this serves a different purpose. Shabbat candles are meant to provide light for the meal, not for careful examination of texts, and to allow one to walk around in his home without bumping into furniture. In contrast, the Sages forbade doing things that require careful examination by lamplight, out of concern that in such a case one might end up tipping the lamp in order to see better. One who wishes to study by the light of an oil lamp should ask a friend to keep him company, to ensure that he will not tilt it. Alternatively, he may learn together with a friend so that they can look out for each other (Shabbat 11a; SA 275:1-3).
Many poskim maintain that a lone individual may read by the light of a wax or paraffin candle because there is no concern that he will tilt the candle. Unlike an oil lamp, where the point of tilting is to move oil up the wick to the flame, the wax in a candle is attached to the wick and already close to the flame, so tilting would serve no purpose. Similarly, there is no concern that one might come to adjust the wick, since candles burn well on their own, and there is no need to adjust them at all once they are lit (MB 275:4; Kaf Ha-ĥayim 275:11).
An individual may study by the light of an electric lamp, even if only one of two bulbs in the lamp is turned on, and even when the intensity of the light can be adjusted by means of a dimmer; the ordinance regarding oil lamps was because of concern that if the light grew dim, one might tilt the lamp to restore the light. There was no concern that one might light an additional lamp or add oil to an existing one. Since electric lights do not grow dim, we are not concerned that one might turn on an additional light or turn up an existing one. Nevertheless, le-khatĥila it is advisable to cover the dimmer knob with a note that says “Shabbat” so that no one will accidentally adjust the light (SSK 13:37; Yeĥaveh Da’at 6:20).