Abstract: A thermometer suitable for a hot tub, spa, pool, and the like, and including a temperature sensor and indicator and a light source so the indicator can be read at night or in dim light. A tubular configuration which hangs upside down in the water when measuring temperature, and which is manually turned upright for reading, with an orientation sensitive power supply for energizing the bulb when in the upright position. A battery operated power supply which is permanently off or open circuited during shipping and storage and which is activated by the user by simply ramming the cap down on the unit to electrically bridge an open circuit in the lamp supply circuit.