Chazal prohibited any form of measuring on Shabbos or on Yom Tov since it is a degradation of Shabbos and is similar to weekday activity, and also because it may cause a person to write. Some examples include: Measuring ingredients for a salad dressing, determining the size of a room (even just by counting the tiles), checking one’s weight or measuring height, or timing how long an activity takes. Only precise measuring is prohibited (even if later used in an imprecise manner), however approximate measuring (e.g. a cupful of baby cereal) is permissible. this also prohibits the use of even a non-electric thermometer.
[1],
Chazal permitted measuring
mitzva objects (e.g. a
mikva, techum Shabbos), since it doesn’t appear mundane.
12 However, if it isn’t obviously
mitzva-related, it is not permitted.
13 Likewise, some say that if the
mitzva object could have been measured before Shabbos (e.g.
kazayis of
matza,
revi’is for
kiddush), it may not be measured precisely on Shabbos.
14 In practice, one may rely on the lenient opinion in case of need.
15
halacha permits measuring for health needs since this is also a
mitzva. One may therefore measure fever with a non-electric thermometer, use a blood pressure device, or measure medicine for the exact dose.
[2]
Electric thermometer
Regarding the use of electricity itself on Shabbos – independent of other possible
melachos such as turning on lights or writing – there is dispute whether closing a circuit constitutes the
melacha d’oraisa of
boneh (building) or is prohibited
mid’rabanan due to
molid (birthing a new energy). Yet, the majority of contemporary
poskim hold that it is
d’rabanan.
[3]
If an electric thermometer is
d’rabanan, then having a non-Jew use it would be a
shevus d’shevus, a double
d’rabanan, which is allowed for great need or a
mitzva purpose. A Jew may not perform the act himself, even a double
d’rabanan for the sake of a
mitzva.
[4]
Is having one’s temperature taken considered taking part in the act?
Shluchan Aruch rules that if a tooth is causing you great pain, you can have a non-Jewish dentist remove it on Shabbos.
[5] Some explain that opening your mouth is only minimal assistance (
mesayei’a), and the
melacha is still being done entirely by the non-Jew.
[6] Others argue that even such a level of assistance forbidden; it is only permitted in cases that the Jew does not help at all.
[7] The Alter Rebbe follows the first opinion and permits any procedure that the non-Jew could do himself without the Jew preventing him.
[8]
Although
mesayei’a is permissible on a biblical level, Chazal prohibit it out of concern that one would come to do the
melacha on his own. However, they lifted their decree in
mitzva and health situations.
[9]
Examples of qualifying
mitzva purposes (for
mesayei’a and
shevus d’shevus) include to enable the performance of timely
mitzvos like
shofar,
lulav or to transport important components of the Shabbos meal.
11
In conclusion , having one’s temperature taken by a non-Jew can be allowed when it is truly needed or for a
mitzva purpose (and when visiting a sick person is needed that will also be included) .
[10] 