A field plot of uniform fine sandy loam free of plants was deeply irrigated with water containing 44 me./1. of chloride. The water and chloride content of the soil and the hydraulic head were measured as functions of depth and time for 59 days during summer. The water content, W, of surface layers of soil of thickness from 10 to 50 cm. were functions of time, T, that could be closely represented by equations of the form W = aT-b. This means that the rate of loss of soil water was inversely proportional to time after irrigation. Making use of this equation and its derivative with respect to time, it is possible to calculate the vertical water-flow velocity in the soil at various times and depths. Capillary conductivity values were calculated and agreed closely with published values obtained by steady state laboratory methods for soil from an adjacent area. The evaporation from the soil surface over the 59-day period of record was obtained from the water content of the 0- to 40-cm. soil layer by subtracting the downward drainage calculated from hydraulic gradient and capillary conductivity values. The evaporation thus obtained was 5.6 cm. The evaporation was also independently calculated from the change in water content and chloride content of the 0- to 10-cm. layer of soil. This method gave an evaporation figure of 6.0 cm. Early in the test period a marked decrease in the concentration of chloride in the soil solution developed in the 5- to 10-cm. depth interval. This depth interval apparently corresponds to a zone of condensation of water vapor that moves downward from the overlying and warmer surface layer of soil. However, after taking negative adsorption into account, it was concluded from the vertical distribution of chloride in the soil and the change of this distribution with time that the water transferred in the vapor phase below the 10-cm. depth in the profile was of negligible agricultural significance under the conditions of the test.

The selection of ceramic materials suitable for use as sample retainer plates is discussed. Methods for measuring the permeability, air-bubbling pressure, and air-entry pressure of retainer plates are given. The transfer conductance between retainers and a ceramic suction control surface is satisfactory if the contacting surfaces are reasonably flat and clean. Several tests have consistently indicated, however, that better results are obtained if a capillary contact medium is used when ceramic retainer plates are used on cellulose membranes. The retentivity at high suctions for cores may be significantly different than for air-dried and fragmented soil. For example, the average 15-bar percentage for 7 cores of sandy loam soil was 5.96, whereas the average 15-bar percentage for the soil in these cores after air drying and fragmenting was 5.48, the difference being highly significant. Tests indicate that the water-release curves for cores of a fine sandy loam soil that were wetted in the field by flood irrigation are the same as when the same cores are wetted to zero suction in the laboratory.

A linear high-frequency sweep circuit for the cathode ray oscillograph is described. It employs a linearly charged condenser periodically discharged by a hard tube, the grid of which is biased past cut-off except for short periods when it is driven positive by an auxiliary oscillator. The frequency of this oscillator is made commensurable with that of the unknown wave form to be investigated. The circuit is particularly adaptable for high frequencies, photographs of alternating voltages of frequencies up to twelve megacycles being shown.

When triode vacuum tubes are used as rectifiers approximately constant d.c. voltage can be obtained from a rectifier-filter circuit by having changes in the a.c. line voltage control the grid bias of the triode rectifiers. By using common radio receiving tubes in circuits which are described it is found that a 10 percent change in the a.c. voltage causes from 0.1 to 0.02 percent change in the d.c. voltage output of the filter.

A device has been developed which not only provides voltage regulation for alternators, but short-circuit protection as well. Saturation current from the filament of a thermionic tube forms the control element, while stabilization is accomplished by a feed-back system.

The flow of liquids in unsaturated porous mediums follows the ordinary laws of hydrodynamics, the motion being produced by gravity and the pressure gradient force acting in the liquid. By making use of Darcey's law, that flow is proportional to the forces producing flow, the equation K∇2ψ+∇K·∇ψ+g∂K/∂z=−ρsA∂ψ/∂t may be derived for the capillary conduction of liquids in porous mediums. It is possible experimentally to determine the capillary potential ψ=∫dp/ρ, the capillary conductivity K, which is defined by the flow equation q=K(g−▿ψ), and the capillary capacity A, which is the rate of change of the liquid content of the medium with respect to ψ. These variables are analogous, respectively, to the temperature, thermal conductivity, and thermal capacity in the case of heat flow. Data are presented and application of the equations is made for the capillary conduction of water through soil and clay but the mathematical formulations and the experimental methods developed may be used to express capillary flow for other liquids and mediums. The possible existance of a hysteresis effect between the capillary potential and moisture content of a porous medium is considered.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation L. A. Richards; LOW VACUUM PRESSURE CONTROL APPARATUS. Rev Sci Instrum 1 January 1931; 2 (1): 49–52. https://doi.org/10.1063/1.1748732 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioReview of Scientific Instruments Search Advanced Search |Citation Search

A vacuum tube voltage regulator for ac power units is described, with two novel features. First, saturation current from the filament of a thermionic tube is used as the control element; and second, a feed-back stablization system is employed which makes it possible to obtain stable regulated voltage conditions with high sensitivity. It gives voltage regulation of 1.5% at full load unity power factor as compared to 45% with fixed excitation. The regulator also functions as an overload circuit breaker. Results of tests conducted under steady, transient, and short circuit conditions are given. Finally some further improvements are suggested.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Lorenzo A. Richards, "Temperature Control Apparatus," J. Opt. Soc. Am. 18, 131-137 (1929) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article