Halacha Corner: Using a Thermometer on Shabbat

Question: Is it permissible to use a thermometer on Shabbat?

Answer: Regarding measuring temperature with a digital thermometer, it is clearly not permissible, and any discussion on this matter pertains only to measuring temperature with a non-electric thermometer containing mercury.

Initially, one might consider this prohibited because measuring actions are forbidden on Shabbat (such as weighing items). However, for this reason, using a thermometer on Shabbat should not be prohibited, as measuring temperature for a sick person serves a mitzvah purpose, allowing proper care for the ill person. The Shulchan Aruch (Section 306) states that measuring for the purpose of a mitzvah is permitted on Shabbat, such as measuring for the sake of a sick person, or measuring a mikveh to ensure it contains forty se'ah.Additionally, Maran HaRav Shlita wrote in Responsa Yabia Omer Vol. 9 (p. 261) that there is another reason for leniency, based on the Maharikash's ruling regarding clocks operating on Shabbat, which states there is no prohibition of measuring because we are not actively performing the measurement with the clock—it operates independently and measures time on its own. Similarly, with a thermometer, we are not directly measuring the temperature ourselves; rather, the thermometer operates independently to perform the measurement, so it should be permissible to use on Shabbat. He provides additional reasoning for leniency. Therefore, it appears permissible to measure temperature using a thermometer on Shabbat. This was also ruled by the great rabbis Rabbi Moshe Feinstein in Responsa Igrot Moshe and Rabbi Shlomo Zalman Auerbach in the book Me'orei Aish.

However, the Gaon Rabbi Shmuel Halevi Wosner Shlita wrote that even if we permit using a thermometer on Shabbat, it is still forbidden to shake the thermometer on Shabbat to enable reuse, because the shaking action lowers the mercury back to the bottom of the thermometer, effectively fixing the device for reuse, which would be prohibited as repairing a utensil on Shabbat.

However, on this issue as well, Maran HaRav Shlita wrote that since the action of shaking the thermometer is a simple action routinely done at any time, such an action does not have the significance of repairing a utensil, as the Magen Avraham wrote that anything routinely opened and closed does not violate the prohibition of repairing a utensil. Therefore, it seems permissible to shake the thermometer as well. The Gaon Rabbi Eliezer Yehuda Waldenberg in Responsa Tzitz Eliezer and other leading poskim also ruled this way.

In summary: It is permissible to use a thermometer on Shabbat to measure a sick person's temperature to determine appropriate care, and it is also permissible to shake the thermometer afterward so it can be reused. However, using an electric thermometer on Shabbat is certainly not permitted.

Rulings of HaGaon Rabbi Ovadia Yosef Shlita courtesy of "Daily Halacha" website