Masers are atomic clocks which have outstanding short-term stability.
The drift is specified at initially less than 1 part in 10 to the 15th per day. These devices are 46 x 76 x 109 cm in size with batteries weighing 227 kg. A small storage bottle supplies molecular hydrogen to the gas discharge bulb. In the discharge bulb, molecules of hydrogen are dissociated into atomic hydrogen. They then pass through a source collimator and a magnetic state selector which filters atoms at the desired atomic state and passes them to the resonance cavity.