The heterodyne beat frequency method of determining dielectric constants has been applied to the measurement of the dielectric constant of air with an applied frequency of field of 900,000 cycles per second. Radio broadcast signals, stripped of modulation, were used as sources of alternating current of constant high frequency. An electron coupled oscillator was used in the circuit containing the test condenser. Both sources of high frequency were isolated from the detector through buffer stages of amplification. Determination of the beat frequencies was made by photographic recording with a two string oscillograph. A dielectric constant of 1.00058986±0.00000050 is found for air at normal pressure and temperature. This result is in close agreement with the results of other investigators for lower frequencies of applied field and for static measurements.

The method used was that of balancing the gas magnetically against an aqueous solution of nickel chloride. By varying the concentration of the solution it could be given a susceptibility approximately the same as that of the gas; then by varying the pressure of the gas or the temperature of the gas and solution, both could be given the same susceptibility. A manometric balance of great sensitivity enabled the observer to tell when the susceptibilities of the gas and the solution were the same. For both paramagnetic and diamagnetic gases, formulas are derived from which the susceptibility may be calculated from pressure and temperature observations on the gas when it is magnetically neutral against the solution. The volume susceptibility under a pressure of one atmosphere at the temperature 20°C was found to be +0.1447 X10 -6 for oxygen; -1.64X10-10 for hydrogen, and -0.81 X10~1 0 for helium. The result found for helium is about 25 times less than Tanzler's value, but when substituted in the formula derived by W. Pauli Jr. for the diamagnetic susceptibility of a monatomic gas, it yields a result bearing on the dimensions of the atom which is compatible with our knowledge obtained from other sources.OEVERAL papers have been published dealing with the magnetic properties of one or more of the gases oxygen, hydrogen and helium, but the results obtained by a considerable number of observers are not in as close agreement as is desirable. It was thought worth while, therefore, when the opportunity 1 presented itself to undertake further research on the magnetic properties of these gases.The method used in the present investigation consists in balancing the gas magnetically against a liquid of known susceptibility by varying either the pressure of the gas or the common temperature. The device used for this purpose was a manometric balance. THE MANOMETRIC BALANCEA diagrammatic sketch of the device is shown in Fig. 1. Here, A is a cylindrical metal container with a cover B. Inside the container is a cylindrical glass vessel C, connected to which is a glass tube D y which passes through the bottom of the container and is then bent as shown. The gas inlet E to the container is shown on the left. Opposite the inlet 1 The appointment of the senior writer as Ernest Kempton Adams fellow enabled him to secure the collaboration of Mr. Hector.

The apparatus described in a'recent paper by Wills and Hector 1 for the measurement of the magnetic susceptibilities of gases, has been rebuilt so as to enable the temperature to be controlled to .02° and the sensitivity of the method increased. The gases used were all carefully purified, and spectroscopic examination showed no oxygen lines. The results obtained for the volume suvsceptibilities at 20°C and 760 mm pressure for helium, neon, argon and nitrogen are -0.780, -2.77, -7.52, and -4.91, respectively, all times 10-10 . They are probably accurate to within 2 per cent. These results are of the order of magnitude required by Pauli's modification of the Langevin theory of diamagnetism and also agree with values predicted by Joos from the electron arrangements.

The dielectric constants of eight gases have been measured by a heterodyne beat frequency method. Special precautions were taken to improve minor defects in the equipment which are believed to have contributed principally to the difference in average values obtained by other observers where these differences were incompatible with the self-consistency of the individuals' data. Data on helium, neon, argon, hydrogen, oxygen, nitrogen, carbon dioxide, and air are given and compared with the results of numerous other observers.

The experience gained in fitting a large number of impaired persons with hearing aids is presented, showing illustrative ranges of impairment between 30 and 100 decibels fitted with three progressively-powered hearing aids. General features of such hearing aids are discussed, showing circuits and arrangements for peak clipping and automatic gain control. The microphone and air and bone receivers are described, considering features developed for providing optimum reliability in service such as moisture proofing, and design arrangements providing miniaturization concomitant with ruggedness.