The atomic hydrogen maser. Is it the best clock for gravity and relativity measurements?

The atomic hydrogen maser currently offers the best stability for averaging intervals, tau, between 1 sec and 1 hr. For tau approximately 1 hr the available fractional frequency stability of 6 x 1016 makes it very useful for astrophysics and relativity measurements. An outline of the theory of operation and the major frequency perturbing influences is given. These are discussed in two categories: random, thermal effects and systematic effects. The methods to minimize the perturbations are described. Data are given related to stability and effects of environment on recently built equipment. These results closely approach theoretical limitations imposed by room temperature operation. The significance of high stability oscillators to the measurement of angle, velocity, distance and time for gravitation and relativity experiments are described in terms of Allan variance statistics. (Auth.)