My son has had a fever on and off for a few weeks already. Am I allowed to use an electric thermometer to check his fever on Shabbos?
The rule is that one should not use an electric thermometer unless there is concern of a danger – חשש סכנה. This matter needs to be discussed with your son’s pediatrician.
It is always best to use a non-electric thermometer such as a forehead thermometer strip or an oral glass thermometer etc. Thus there is no need to use an electric thermometer on Shabbos. (Although the digital thermometers are a bit more accurate, nonetheless these thermometers will also detect a fever).
In all cases, it is best to have a non-Jew take the temperature if possible. Nonetheless, as per P’sak of the Alter Rebbe, even a Melacha D’Oraisa with a Shinui would be Muttar in this case. Therefore, it’s always recommended to employ a Shinui.
שו”ע אדה”ז סי’ שכ”ח סעי’ כ”א
ארחות שבת ח”ב עמ’ רמא