Volume 17, Issue 10, 01 October 1946
Index of content:
17(1946); http://dx.doi.org/10.1063/1.1707644View Description Hide Description
According to Einstein, the electrons leave a photo‐electric surface with an initial velocity proportional to the frequency of the impinging radiation. Consequently, when a sawtooth potential is applied to a photoelectric cell, illuminated by a monochromatic light, the current flow starts at a threshold potential corresponding to the wave‐length of the light, and then gradually increases more or less linearly toward a saturation value. When light of assorted wave‐lengths is applied, the total current is the summation of the currents for the individual wave‐lengths, each of which currents has its threshold at a different potential. By double differentiation an indication of the spectral distribution can be observed directly on an oscilloscope which is deflected synchronously with the potential applied to the photoelectric cell. The accuracy of the indication is limited by the indefiniteness of the threshold values and non‐linearity of the currents above the threshold potentials in available photoelectric cells. With S4 commercial surfaces, filters of the three primary colors can be easily differentiated, but with improved surfaces considerably better resolutions may be obtained and further improvement may be had by lowering the temperature of the photoelectric surface. The method lends itself readily to color matching, since two similar devices supplying their signal with opposite polarity will give zero output when the two colors match, while it provides a direct indication of the color and magnitude of mismatch when a signal is obtained. The output signal can be utilized for relays or other control devices in industrial processes.
17(1946); http://dx.doi.org/10.1063/1.1707645View Description Hide Description
The mean square noise current per unit frequency range as a function of frequency, or ``power spectrum,'' of the shot noise produced by a secondary‐emission multiplier tube is calculated. A general expression for it, (3), is set up and then applied to the RCA 931 family of photo‐multiplier tubes. These tubes are of interest as relatively intense sources of random noise of constant intensity up to high frequencies. When an electron enters the collecting space, it initiates a pulse of current in the anode circuit. In the 931 operated with 100 volts per stage, the pulse has a total duration of about 6×10−10 sec. and a half‐width of about 1.5×10−10 sec. The electrons descended from the same photon do not enter the collecting space simultaneously but are spread out over a time of the order of 6×10−10 sec. Because of the non‐zero duration of the pulses and the spread in their time of occurrence, the noise spectrum is essentially flat only up to several hundred Mc, falling rapidly at higher frequencies. The paucity of data and the approximations made in the calculation make our curve of noise output versus frequency significant only as regards order of magnitude and general trend. It does seem safe to predict that the noise intensity ought to be constant from zero up to about 100 Mc, begin to fall off appreciably between 100 Mc and 1000 Mc, and become very weak at higher frequencies. Partial confirmation is provided by unpublished measurements of Jastram at the Radio Research Laboratory, Harvard University, which show that at 30 Mc the noise output is still about the same as in the range 0 to 5 Mc. There is also some experimental indication that the output is about the same at 100 Mc and that it is much smaller at higher frequencies.
17(1946); http://dx.doi.org/10.1063/1.1707646View Description Hide Description
Spectral distributions and powers associated with the output of biased, saturated linear and quadratic rectifiers are determined according to the method of Rice when the incoming disturbance is random noise. Three classes of input spectra are considered: (I), broad band noise, where the central frequency is equal to or less than the spectral width, (II), semi‐broad band noise, for which the central frequency exceeds the width by a moderate amount, and (III), narrow band noise, where the width is much less than the central frequency. Rectification of types I and II yields spectra having roughly the same distribution as that of the incoming waves, but for type III an infinite number of separate noise bands are generated and appear in the output, centered about harmonics of the central frequency. It is found that clipping, whether at the ``top'' or ``bottom'' of the incoming wave, always spreads the spectrum and reduces the output power. Further, it is shown that clipping due to cut‐off alone produces a greater spectral spread than clipping with saturation in addition, for types I and II, but not necessarily for spectra of type III. Symmetrical clipping for classes I and II yields little broadening of the spectrum, even in extreme cases, and for class III waves, the even‐harmonic regions are completely missing except for a d.c. component, and only the odd‐harmonic zones appear. The behavior of linear and quadratic rectifiers is qualitatively similar in most cases. The powers in the d.c. and continuous portions of the output spectrum are shown to be independent of the spectral shape of the incoming noise. Some examples of extreme clipping are considered, numerous curves are included, and a general analysis is outlined.
17(1946); http://dx.doi.org/10.1063/1.1707647View Description Hide Description
Data are presented on the flow of air at low pressures in copper pipes of radius 0.795 and 1.30 cm; iron pipes of radius 2.64, 5.12, and 10.1 cm; and for hydrogen in a copper pipe of radius 1.30 cm. Data are also given for the resistance to flow of air in 1½‐ and 3‐inch nominal pipe size elbows, for 1½‐inch Kinney bellows valves, and for short sections of 1‐inch nominal pipe size iron pipe connecting large gas chambers. The data on straight iron and copper pipes, together with the data from the literature on glass capillaries, are correlated by the introduction of a correction factor F in Poiseuille's equation. F is correlated graphically in terms of a dimensionless group representing the ratio of mean free path to pipe radius, with separate curves for iron pipe and for copper and glass pipes or capillaries.
17(1946); http://dx.doi.org/10.1063/1.1707648View Description Hide Description
The absorption coefficients and dielectric constants of sixteen gases have been measured at the two wave‐lengths λ=1.24 cm and λ=3.18 cm. The gases are H2S, COS, (CH3)2O, C2H4O, C2H5Cl, SO2, NH3, six halogenated methanes and three amines. Certain improvements in technique are described; these improvements permit detection of absorption coefficients as small as 0.2×10−4 cm−1 and measurement of larger coefficients with an accuracy of ± 5 percent. The measureddielectric constants at these wave‐lengths are essentially equal to the static values. A quantitative interpretation of the absorption coefficients in terms of the known structure and spectra of the individual molecules is given. The theory indicates that all non‐planar molecules which possess a permanent dipole moment should show appreciable absorption in the microwave region.
17(1946); http://dx.doi.org/10.1063/1.1707649View Description Hide Description
A method is described for observing the austenite‐martensite transformation in steel by measuringelectrical resistance. Small wire samples are heated and cooled either in vacuum or in air while temperature and resistance are automatically recorded. Temperature throughout the sample is uniform enough to permit a determination of resistance as a function of temperature. It is shown that from these data a complete record of transformation during cooling can be computed. Such transformation data have been obtained on three steels of widely different compositions, representing the types of steel in which transformation is most difficult to observe.
17(1946); http://dx.doi.org/10.1063/1.1707650View Description Hide Description
An analysis of the operation of resonantcavity magnetrons is made without regard to the mechanism of conversion of the d.c. input power into the r‐f output power. The magnetron is represented by its equivalent circuit and is assumed to be essentially in equilibrium at every point of its energy build‐up curve. The last assumption is shown to be reasonable on the basis of measured starting times. From the data of load impedance charts and the rise of the r‐f pulse, the relation between power generated and vane voltage is deduced. By considering the division of the energy between that stored in the resonant system, and that dissipated in the load, the law of build‐up is derived. The dependence of starting time on load is calculated and agrees with experiment when the extra energy stored in the connecting line is taken into account. The starting time is affected slightly by the initial noise level and becomes infinite below a minimum Q. For high Q values the starting time can be varied only by changing the energy stored in the line, which may enable one to design a line for ``mode'' suppression.
17(1946); http://dx.doi.org/10.1063/1.1707652View Description Hide Description
Certain configurations of anode, grid sections, and uniformly emitting cathode produce rectilinear motion of the electrons, both with and without velocity modulation. These systems are (a) plane parallel electrodes and grid sections of unlimited extent, (b) coaxial circular electrodes and grid sections of infinite length along the axis, and (c) concentric spherical electrodes and grid sections. The uni‐dimensional character of the motion permits a complete evaluation of the effect of space charge on the bunching process in the range of drift space where overtaking (crossing of orbits) does not occur.