Temperature control on the nanometer scale is a challenging task in many physical, chemical, and material science applications where small experimental volumes with high temperature gradients are used. The crucial difficulty is reducing the size of temperature sensors while keeping their sensitivity, working temperature range, and, most importantly, their simplicity and accuracy of temperature reading. In this work, we demonstrate the ultimate miniaturization of the classic thermometer using an expanding column of liquid gallium inside a multi-walled C nanotube for precise temperature measurements. We report that electrical conductivity through unfilled nanotube regions is diffusive with a resistance per unit length of approximately 10 kOmega microm(-1), whereas Ga-filled segments of the nanotube show metallic behavior with a low resistance of approximately 100 Omega microm(-1). No noticeable Schottky barrier exists between the nanotube carbon shell and the inner Ga filling. Based on these findings, an individual carbon nanotube partially filled with liquid Ga is used as a temperature sensor and/or switch. The nanotube's electrical resistance decreases linearly with increasing temperature as the metallic Ga column expands inside the tube channel. In addition, the tube resistance drops sharply when two encapsulated Ga columns approaching each other meet inside the nanotube, producing a switching action that can occur at any predetermined temperature, as the Ga column position inside the nanotube can be effectively pre-adjusted by nanoindentation using an atomic force microscope.