Magic sensor. Technician monitoring an autonomous sensor on a candle. This long-lived sensor is powered solely by the changes in temperature it measures, requiring no battery. It is only active when there is a change to measure. Here, it is acting as a safety device. Voltage is generated in its pyroelectric materials by changes in tempera- ture. The voltage is converted into a radio signal transmitted by an antenna. This data can be picked up by a receiver up to 20 metres away. Similar sensors can be used to continuously check the pressure of vehicle tyres. The sensor was developed by Siemens, Munich-Perlach, Germany.