As we have seen, the prohibition on using electrical appliances on Shabbat includes microphones and telephones.
Even if a phone is in use before Shabbat, or if a microphone is turned on before Shabbat, one may not speak on the phone or use a microphone on Shabbat, because doing so increases the electric current running through it and violates a Torah or rabbinic prohibition. In addition, the Sages banned hashma’at kol (producing sound) with an object specifically designed for this purpose. Many maintain that one who speaks into a microphone transgresses this prohibition (based on Rema 338:1). Furthermore, using a microphone resembles a weekday activity. It also may seem to belittle Shabbat, as those who hear the amplified sound are likely to think that the microphone was activated on Shabbat (Igrot Moshe, OĤ 3:55).
Nevertheless, one who is hard of hearing may use an electric hearing aid that rests on or inside the ear on Shabbat as long as it is turned on before Shabbat and he does not adjust the volume on Shabbat. When he wants to go to sleep, he should remove the device without shutting it off. In the morning, he may put it back in without turning it on. Although we stated that one may not use a microphone or telephone because speaking into it increases the electric current, this prohibition applies only when someone speaks directly into them. However, when speaking normally, the activation of the device in the wearer’s ear is secondary and therefore a form of grama, which is permitted when truly necessary. Furthermore, there is no problem of producing sound on Shabbat or belittling Shabbat since the sound is heard only by the wearer.
Just as one may not turn on a microphone before Shabbat with the intention of using it on Shabbat, so is leaving on an intercom over Shabbat to hear the voices of people coming to visit, or to hear what is going on in the children’s room. To be sure, if one simply speaks normally in the room and not directly into the intercom, the prohibition is less severe since it is then a case of grama. It is nevertheless prohibited, because it is a weekday activity, it belittles Shabbat, and it produces sound with an object designed for that purpose. However, if the intercom was accidentally left on over Shabbat, as long as one does not intend to use it he may speak in a normal fashion in the room with the intercom.
.If a Jew in the United States who does not observe Shabbat calls a Jewish friend in Israel when Shabbat is over in Israel but while it is still Shabbat in the U.S., the latter may not speak with him, because he is benefiting from his friend’s Shabbat desecration. However, a Jew in Israel may speak with a non-Jew who calls him when it is Shabbat in the U.S., since a non-Jew is not obligated to keep Shabbat (SSK 31:27).
. There is no problem of hashma’at kol since the sound is only heard by the wearer of the device. The only problem is the electrical operations of the hearing aid. Indeed, some prohibit the use of hearing aids (Dovev Meisharim, Levushei Yom Tov §15; R. Yosef Shalom Elyashiv cited in Orĥot Shabbat 26:23). However, in practice many have permitted the hearing-impaired to use hearing aids, including Igrot Moshe, OĤ 4:85. Those who maintain that turning on an electrical appliance without an incandescent filament is rabbinically prohibited because it creates a new flow of electricity in the wires do not view increasing the current as a new creative act. At worst, it might be deemed a weekday activity, in which case one may be lenient in the case of a great need. Similarly, for those who believe that turning on an appliance is prohibited on account of Boneh or because it is creating a kli, there is no problem here; since the hearing aid is already on, increasing the current creates nothing new. Based on these rationales, the following authorities permit the use of a hearing aid: R. Shlomo Zalman Auerbach; Tzitz Eliezer 6:6; SSK 34:28; Yabi’a Omer 1:19:19; Minĥat Yitzĥak 2:17. See Encyclopedia Talmudit vol. 18, pp. 731-732. It is true that Rav Kook in Oraĥ Mishpat §71 prohibits speaking into a microphone, because the speech causes electric current to flow. He explains that this cannot be considered grama since the result is direct and immediate. It seems, though, that since the sound waves resulting from normal speech are only converted to electric current once they reach the hearing aid, this can be considered ko’aĥ sheni (“secondary power”), a form of grama, as explained in San. 77b and Ĥullin 16a. Grama is permitted under pressing circumstances, as explained in Shabbat 120a, SA 334:20, and Rema ad loc.; see above, 9:9.
. One who uses a microphone fully intends to speak into it, and this is the normal way to amplify a voice, so it is not considered grama. In contrast, if one simply speaks normally in a room that has a baby monitor and thereby projects his voice over the intercom, since this is not the normal way to converse and his mouth is far from the intercom, it is considered ko’aĥ sheni and thus grama, as in the case of a hearing aid (though this requires further study; see Harĥavot). However, since the sound can be heard in the room with the receiver, it is a weekday activity and belittles Shabbat. It might also be considered hashma’at kol (Rema 338:1; Oraĥ Mishpat §71; also see Rema 252:5). In contrast, if the intercom was left on accidentally, then since one who speaks in that room is using it only via grama, and he is not interested in making his voice heard, this is a case of psik reisha de-lo niĥa lei in the case of a rabbinic prohibition, where one may be lenient in a case of need. In pressing circumstances, such as when one is sick, one may be lenient and leave an intercom on. See Terumat Ha–goren §79. Yalkut Yosef (vol. 5, pp. 403-405) permits this even le-khatĥila. One should only rely on this leniency under pressing circumstances.