Both the proton and electron of a hydrogen atom have spins. The atom has a higher energy if both are spinning in the same direction, and a lower energy if they spin in opposite directions. The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of 1,420,405,752 Hz. This 1420 MHz frequency is important in radio astronomy because is corresponds to the 21 cm line of interstellar hydrogen.
Hydrogen masers are very complex devices and sell for as much as US$235,000. They are made in two types, active and passive.
In both types, a small storage bottle of molecular hydrogen leaks a controlled amount of gas into a discharge bulb. Molecular hydrogen consists of pairs of atoms bound together. The molecules are disassociated in the discharge bulb into individual hyrogen atoms by an arc. They are now called atomic hydrogen and pass through a collimator and a magnetic state selector. The atoms are thereby selected for the desired state and passed on to a storage bulb. The storage bulb is roughly 20 cm high and 10 cm in diameter and made of quartz. Its inside is coated with teflon. This coating slows the recombination of the hydrogen atoms into hydrogen molecules. The storage bulb is in turn inside a microwave cavity made from a precisely machined copper cylinder. This cavity is tuned to the 1420 MHz resonance frequency of the atoms.
In the active hydrogen maser, the cavity oscillates by itself. This requires a higher hydrogen atom density and higher Q for the cavity. The active maser is more complex and more expensive but has better long term frequency stability. E.g., the CH1 75 model made by PTF weighs 90 kg and consumes about 100 Watts. Its long term frequency accuracy is about +/- 5*10-16 over five years. When used as a clock, it will gain or lose a second in about 63 million years.
In the passive hydrogen maser, the cavity is fed from an external 1420 MHz frequency. The external frequency is tuned to produce a maximum output in the cavity. This allows the use of lower hydrogen atom density and lower cavity Q which reduces the cost. The CH1 76 model also made by PTF weighs 55 kg and consumes about 90 Watts. Its long term frequency accuracy is much worse than the active maser at about +/- 1.5*10-12 per year.