No abstract

This paper describes another approach to the photoelectric measurement of h/e which is practicable for the introductory laboratory. The method owes its success to a new way of minimizing photoemission by the anode. A common vacuum photocell (1P39) and slit replace the eyepiece and cross hairs of a student grating spectrometer, illuminated by an ac mercury arc. If the cell is favorably oriented to the incident light, photoemission by the anode at high cathode retarding potential (6 V) can be reduced to less than 0.4% of the maximum photocurrent with zero retarding potential at the 3650-Å line. The 120-Hz component is amplified and rectified for presentation on a 100 μA dc meter. The maximum gain is sufficient to indicate full scale for a photocurrent of 10−11 A. Stopping potentials for six lines between 3650 and 5770 Å are determined by plotting the square root of the photocurrent against retarding potential and by extending the straight portions of these plots to the axis. The slope of a straight line drawn through the stopping potentials, plotted against photon frequency, comes close to the accepted value of h/e. Perhaps the most instructive aspect of this experiment is the experience with electron and photon energies. The student observes directly that electrons ejected by uv photons are much harder to stop than those ejected by photons of yellow light.

The performance of the anechoic room of the Parmly Sound Laboratory has been measured to determine the characteristics of rooms of small volume with wedge-covered walls. The inverse square law holds to within ±1 db to 6.5 feet from 60 to 24,000 c.p.s., although the theoretical cut-off frequency of the room is 115 c.p.s. At 100 c.p.s., the radiation resistance of a loudspeaker placed near the walls was found to be 18 percent above the free-field value, and the radiation reactance was approximately 5 percent above the free-field value. The differences disappeared two feet from the wall. The wall transmission has been measured as a function of frequency, and the wedges themselves are found to act similarly to a thin plate in optics, having a maximum transmission at 110 c.p.s.

An instrument for evaluating loudness of industrial noises of the impact type must be portable, be convenient to use in the field, and yield results which may be interpreted in engineering terms. The development of such a device is described which enables an observer in the field to evaluate the magnitude of impact noise by finding the level of the masking threshold using an appropriate spectrum of masking noise. The instrument consists of a compact random noise generator, amplifier, filter set, attenuator, and mixer, and is normally used with a sound level meter and headphones. Six masking noise spectra of +9, +6, +3, 0, −3, −6 db per octave are available by means of a selector switch. The circuit is arranged so that the attenuator is calibrated in terms of phons. Metering circuits are provided for setting the output of the noise generator and the input from the sound level meter, as well as showing the condition of the batteries. In use, the microphone of a sound level meter is placed in the vicinity of an impact noise and the output of the sound level meter is connected to the masking unit. A spectrum of masking noise is selected which appears in the earphone simultaneously with the impact noise. The level of the masking noise is adjusted in the range of 70 to 110 db until the masking threshold is determined. This is repeated for each spectrum slope until the one with the lowest masking threshold is found. The attenuator dial then shows the level of the masking spectrum which most closely matched that of the impact noise. A demonstration using recorded industrial noises will show the operation of this equipment.