A crookes radiometer is filled with air at a pressure of about
1 Torr. This ionises easily in the RF field and gives a characteristic
pale cyan glow due to the nitrogen and oxygen mixture. The vanes
of the radiometer turn slowly when the glow is initiated, but
the rate diminishes and they come to a standstill after about
a minute. This is because the molecular bombardment that makes
them turn in sunlight is an adiabatic process, whereas RF heating
eventualy brings the system into thermal equilibrium.