Question: Is it permissible to use a thermometer on Shabbat?
Answer: Clearly, there is no room to take one’s temperature with an electronic/digital thermometer. Our discussion will revolve around using a thermometer that is not electronic and contains mercury which expands and rises as it heats up and in this way shows the individual’s temperature in order to ascertain how the patient should proceed medically. (In recent years when society has become aware of the danger and toxicity of mercury and that in the event that the thermometer breaks, the mercury will spill all over the floor and may cause poisoning, many thermometers manufactured today contain an alcoholic substance which resembles the characteristics of mercury from a halachic standpoint as well.)
We must first discuss the prohibition to perform any method of measurement on Shabbat (such as weighing). Regarding this prohibition, this is not a reason to prohibit using a thermometer on Shabbat, for checking an ill person’s temperature is a Mitzvah in order to determine the appropriate treatment. Indeed, the Shulchan Aruch (Chapter 306) rules that one may measure something on Shabbat for the purpose of a Mitzvah, such as measuring something for an ill individual and measuring a Mikveh to see if it contains forty Se’ah of water. Furthermore, Maran zt”l writes in his Responsa Yabia Omer (Volume 9, page 261) that there is another reason for leniency based on the words of Rabbeinu Yaakov Castro who writes that the prohibition of measuring on Shabbat does not apply to a watch, for one is not performing any act of measuring by wearing the watch; rather, the watch functions on its own and tells time. Based on this, the same would apply to a thermometer, for one is not actually measuring the temperature with one’s hands; rather, the thermometer carries out the act of measuring the temperature on its own and it is therefore permissible to use a thermometer on Shabbat. Hagaon Harav Moshe Feinstein zt”l rules likewise in his Responsa Igrot Moshe as does Hagaon Harav Shlomo Zalman Auerbach zt”l in his Sefer Me’orei Esh.
On the other hand, Hagaon Harav Shmuel Ha’Levi Wosner Shlit”a rules in his Responsa Shevet Ha’Levi that even if we assume that it is permissible to use a thermometer on Shabbat, it is nevertheless forbidden to shake the thermometer in order to be able to use it again during Shabbat, for shaking the thermometer causes the mercury to sink back down to the bottom of the thermometer which is essentially “repairing” the thermometer and making it able to be used once again for its intended purpose. This is therefore forbidden because of the prohibition of repairing a vessel on Shabbat.
Nonetheless, Maran zt”l writes that since this action of shaking the thermometer is a simple action carried out all the time, this does not retain the status of repairing a vessel, similar to what the Magen Avraham writes that any vessel which is usually opened and closed all the time is not included in the prohibition of repairing a vessel. Thus, there is room for leniency regarding shaking the thermometer. Hagaon Harav Eliezer Yehuda Waldenberg zt”l rules likewise in his Responsa Tzitz Eliezer as do many other great Poskim.
Summary: One may use a thermometer to take the temperature of an ill individual on Shabbat in order to ascertain the appropriate method of treatment. It is likewise permissible to shake the thermometer afterwards in order to be able to use it again later. There is nevertheless no room for leniency to use an electronic/digital thermometer on Shabbat.