Turning on an electric light or electric heater is included in the rabbinic prohibition on lighting a new fire on Yom Tov. Turning on an electrical appliance without a heating element is prohibited as well.
Just as one may increase a flame to provide light or heat on Yom Tov, so too he may turn up the light of an incandescent bulb with a dimmer switch, or turn up the thermostat on an electric heater, on condition that turning up the light or heat increases the current to the same heating element. However, if it activates an additional filament, it is forbidden, as it is like lighting a new fire.
One may not use a microphone, telephone, or intercom on Yom Tov, as the Sages prohibited the use of sound-producing objects (see Beitza 36b). Moreover, doing so looks like a weekday activity and belittles the holiday. Nevertheless, one who is hearing impaired may use a hearing aid that rests on or inside the ear, since speaking near it increases the flow of the electricity only indirectly (grama). However, a hearing aid may not be turned on or off on Shabbat or Yom Tov (Peninei Halakha: Shabbat 17:3). On Shabbat, the volume of the hearing aid may not be adjusted, but on Yom Tov this is permitted, just as a flame may be raised or lowered when cooking.
On Yom Tov as on Shabbat, one may not open a refrigerator if doing so will turn on a light. All methods used to deal with this issue on Shabbat are relevant to Yom Tov as well (ibid. 17:9).
. When a hearing aid is off at the start of Yom Tov due to circumstances beyond the wearer’s control (ones), some poskim allow it to be turned on, following the reasoning above in n. 1 for lighting a fire. Besides, some permit using electricity on Yom Tov, as explained in the previous note. Additionally, even according to the majority of poskim who do not allow it, some maintain that turning on an electrical appliance without a heating element is only rabbinically prohibited on Shabbat and Yom Tov. It is proper to turn on the hearing aid with a shinui, so that even those who are stringent would agree that there are grounds for leniency.