Cooking The Thermometer - Vayakhel 5770

35:3 ג  לֹא-תְבַעֲרוּ אֵשׁ, בְּכֹל מֹשְׁבֹתֵיכֶם, בְּיוֹם, הַשַּׁבָּת   Parshas Vayakheil starts with the Lav of Loi Sivaru Aish. This is the only one of the 39 Melachos that is mentioned very clearly, although Hoi’tza’a is also mentioned indirectly. Mefarshim give numerous reasons as to why it is mentioned. The most simple reason is the one brought by the Kli Yakar on the Gemara in Maseches Shabbos. The Lav of not lighting fires in your home on Shabbos comes to let us know that the Issur of burning a fire is only in your home, however in the Bais Hamikdash it was permitted to light a fire to consume the Korbanos.

In the Shailos Uteshuvos of Rav Yonason Shteif there is a Shaila regarding using a mercury thermometer on Shabbos. The Issur of Medida which is measuring it wouldn’t be applicable to someone who is sick, so what can be Assur about taking your temperature on Shabbos?

Interestingly enough, the mercury in the thermometer changes and expands as the heat of the body goes through it. If someone were to cook metal on Shabbos, he would Oiver on Havarah even though there is no food being cooked. So is a thermometer that is under your tongue considered like cooking? Rav Shteif says we know that the Issur of cooking is only on fire. The Gemara says that if one cooks in the sun there is no Issur of cooking. It would even be permitted to cook on metal that was heated by the sun. It however would be Assur Midarabanan. If cooking by the sun is Muttar, what would be by cooking the mercury in the thermometer by the heat of the body? Is this included in the Issur of Loi Sivaru Aish? At first glance, it is not fire and therefore should not be included. Interestingly, he brings a Shelah Hakadoish who says that Loi Sivaru Aish is an Issur on anger in a person’s house. That if a person gets himself aggravated and allows himself to become angry because of the aggravation or because of the frustration, he is Oiver on Loi Sivaru Aish Bechol Moishvoisaichem. The type of heat which is in a person’s house is human anger. So Rav Shteif says if a person’s anger which is the body heat is something which is included in Loi Sivaru, maybe it should be Assur in the heating in the mercury of the thermometer as well. As a matter of Halacha, he doesn’t remain with this; however, it is interesting that the Shela says that if someone brings anger into his house on Shabbos that he is Oiver on Loi Sivaru Aish. The extension of Rav Yonason Shteif is rather fascinating to find in a Halacha Sefer.

Agav regarding what level of heat to metal is Assur is actually a Machloikes between the Chazoin Ish and Rav Moshe which is Nogea L’halacha. The Chazoin Ish says anytime you heat metal to Yad Soiledes Boi there is a Bishul and Rav Moshe holds that heating metal to Yad Soiledes doesn’t change the metal, it has to come to a glow. A Nafka Mina would be if a person forgot to put on his Blech before Shabbos. Or if someone would put an empty pot on the Blech in order to warm things on the pot. According to Rav Moshe this is Muttar because the pot doesn’t come to a glow, however, according to the Chazoin Ish it is Assur because the pot will become Yad Soiledes Boi. Rav Pam said that since it is a Shaila of a D’oiraissa one should be Machmir, however, most people are Maikil.

(Igros Moshe 93 pg. 152-153)