A sensor is a device that can detect specific properties and convert them quantitatively into signals (electrical, mechanical, or thermal). For example, a thermometer uses mercury in a tube as a sensor; its position in the tube tells the viewer the temperature. A thermoelectric cell is another temperature sensor; it produces an electric current proportional to the temperature that can drive a meter. Sensors have been designed to detect most properties that can be identified, from strains in materials, changes in accelerations to changes in velocity or distance. Biosensors have also been developed that can measure organic changes and turn them into electrical signals.