A variety of discoveries, inventions and new experimental and theoretical results characteristic of high-precision devices are discussed in connection with the Canterbury ring laser and with optical supercavities in general. Cavity losses of a few ppm define one fundamental limit for precision measurements. We report a ringing phenomenon in the responses both of the Canterbury ring cavity and of commercial (Newport SR-130) supercavities under excitation by an external laser, whenever the laser frequency and the cavity resonance cross in time. The temporal waveform or ringing profile for any cavity is defined by two parameters. We invent an efficient, inexpensive and competitive "ringing" method for measuring losses of supercavities, and demonstrate its accuracy of 1 ppm. It was found that the asymmetry in the output profile of a scanning Fably-Perot interferometer is much more significant than formerly realised, despite the long history of such interferometers. For the output profile of a scanning Fabry-Perot cavity to have an asymmetry of less than 5%, for instance, the cavity resonance scanning time should be more than 100 times the cavity decay time. Another method for measuring the cavity loss by simply scanning the cavity is presented with a resolution of and system bias of 1 ppm and 4 ppm respectively. Backscattering of a few ppm at the supermirrors induces significant beat frequency pulling in a cavity. A totally novel method based on the measurement and Fourier analysis of the instantaneous beat frequency is presented to determine the frequency pulling and related fundamental parameters for our ring laser. The origins of the beat frequency shifts induced by the dispersion of the laser medium and by mode competition are discussed. A new method is presented to stabilize the beat frequency by using a transverse magnetic field, in view of the empirical observation of such an effect. Its theoretical origin is shown here to be a significant puzzle, since such an effect seems to be forbidden by time-reversal and parity selection rules for any matter-radiation interaction effect for any polarisation state of the ring, in any multipole of interaction and even with allowance for the possible partial orientation of the plasma by the applied field. A detailed account of the construction and operation of the Canterbury ring laser facility is given. Our results from the Canterbury ring laser itself demonstrate a precision of frequency resolution of 11 µHz (a fraction 2.32 x 10-20 of the He-Ne laser frequency, corresponding to a sensitivity of 1.6 x 10-7 ΩE in earth rotation measurement). This is of the same order as the quantum limit for beat frequency linewidths, given the present losses of our ring laser, and is at least two orders of magnitude greater than the sensitivities of existing laser gyroscopes.