Hot air ovens are electrical devices that use dry heat to sterilize. They were originally developed by Pasteur. Generally, they use a thermostat to control the temperature. Their double-walled insulation keeps the heat in and conserves energy, the inner layer being a poor conductor and the outer layer being metallic. There is also an air-filled space in between to aid insulation. An air circulating fan helps in the uniform distribution of the heat. These are fitted with the adjustable wire mesh plated trays or aluminium trays and may have an on/off rocker switch, as well as indicators and controls for temperature and holding time. The capacities of these ovens vary. Power supply needs vary from country to country, depending on the voltage and frequency (hertz) used. Temperature-sensitive tapes or biological indicators using bacterial spores can be used as controls, to test for the efficacy of the device during use.