Part of the regular Minox tank package. Used for pumping chemicals inside the tank, and obviously, taking a reading of its content temperature. Many times missing, when buying the tank, so inquire with the seller.
Fragile, I do not use them. Breaking one of these inside the tank will ruin the negative, and besides, too expensive to replace nowadays :-).
An exact replica of the Minox thermometer, and cheaper replacement too, is the Kaiser Thermometer . Its dimensions are exactly those of a Minox thermometer.