High sensitivity in radio telescopes is achieved by reducing the bandwidth of the receiver; therefore, only with precise foreknowledge of the line frequencies is an astronomical search for the radio spectra of these molecules feasible. To secure precise measurements of these frequencies, a research program in free radical microwave spectroscopy has been started. Since conventional methods are insensitive at the low frequencies of these molecular transitions, the paramagnetic resonance method is being used instead. This involves the application of a strong magnetic field to the radical vapor, which shifts the low-frequency spectra to a conveniently high microwave range, where they may be measured with optimum sensitivity.

The first diatomic hydride investigated by the paramagnetic resonance method was the OH radical. Results of this experiment include the frequencies of the two strong spectral lines by which OH may be identified in interstellar gas; the frequencies are 1665.32 and 1667.36 ** f, with an uncertainty of 0.10 ** f. Success in observing these spectral lines has so far, apparently, been confined to the laboratory; extraterrestrial observations have yet to be reported. Preparations are being made for similar experiments on CH and SH radicals.

The Bureau is pursuing an active program to provide a temperature scale and thermometer calibration services in the range 1.5 to 20 ** f. The efforts and accomplishments fall into three main categories: absolute thermometry based upon the velocity of sound in helium gas, secondary thermometry involving principally studies of the behavior of germanium resistors, and helium 4 vapor-pressure measurements (see p. 144).