Index of content:
Volume 20, Issue 9, September 1949
20(1949); http://dx.doi.org/10.1063/1.1741638View Description Hide Description
The performance parameters of a voltage stabilizer consisting of a voltage‐stabilizing element (such as a VR tube) connected across the output of a current‐stabilized power supply are determined and a particular parameter (the range‐regulation factor Q) suggested as a figure of merit. The advantages of a current‐stabilized over an ordinary supply from the standpoints of voltage stabilization and suitability for variable load are emphasized. Stability criteria for the case when the stabilizing element has a negative‐resistance characteristic and a graphical analysis for the case of non‐linear elements are given. Application to the special case of VR tubes is made with experimental results.
20(1949); http://dx.doi.org/10.1063/1.1741639View Description Hide Description
The relations existing between source dimensions, beam divergence, luminosity, and resolving power of a 180° homogeneous field spectrometer are established. From these the conditions for maximum luminosity for a given resolving power are derived. By increasing the source height and slightly decreasing the width, the luminosity may be increased without affecting the resolving power.
20(1949); http://dx.doi.org/10.1063/1.1741640View Description Hide Description
Measurements are reported for the dependence on energy of photographic darkening due to electrons of energies between 4.4 and 45 kev. For Ilford B‐2 plates, in the linear density exposure region, the results can be expressed by the empirical formula D/Q/A=KEx. D is the photometric density, Q/A the exposure in electrons/mm2, E the energy in electron‐volts; K and x are constants with the values (0.30±0.03)×10−12 and 1.45±0.05, respectively. The relationship for two other emulsions tested is complicated by the presence of surface coatings on the emulsions. Reproducibility of results is discussed.
Results are given on density fluctuations due to granularity of Ilford and x‐ray emulsions. The limit of sensitivity, using the L. and N. recording microphotometer with scanning beam 1.0 by 1.3 mm, is estimated at 450 electrons/mm2 of 40 kev energy for the x‐ray emulsion and 680 electrons/mm2 for the Ilford plates. The latter is limited by photometric sensitivity.
20(1949); http://dx.doi.org/10.1063/1.1741641View Description Hide Description
The construction and operation of a dynamic electron multiplier, of the type sometimes called a multipactor, is described. Briefly, the multipactor consists of two parallel secondary emissive plates having an alternating voltage across them. The magnitude of the voltage is adjusted so that the transit time across the gap for electrons starting with zero velocity from one of the plates when the field is passing through zero shall be just one‐half the period of the a.c. voltage. In this way, electrons and their secondary descendents can be made to bounce back and forth between the plates and multiplication takes place. The necessary phase relationships between the electrons and the field are discussed.
The multipactor described here uses 10 cm microwave power to get the a.c. voltage. It has been used as a gamma‐ray detector in which case the cavity is ``broken down'' by the multiplication of a Compton electron ejected from the cavity wall by a gamma‐ray. This breakdown gives rise to a microwave pulse several millivolts high at a crystal rectifier. The efficiency of this process is about 1.5×10−5 count/gamma‐ray. The rise time of the pulse is calculated to be about 5×10−10 sec. and is measured to be less than 10−7 sec. The deadtime is presumed to be about 5 microseconds.
20(1949); http://dx.doi.org/10.1063/1.1741642View Description Hide Description
A recent investigation has resulted in the development of a multiple pressure tube anemometer, which measures both wind speed and direction without moving parts exposed to the wind.
From the expression for the air flow past a quadric surface, it is shown that, by the use of one system of suitably spaced impact holes, it is possible to measure the rectangular components of wind velocity in one quadrant. Another arrangement of holes is presented that takes advantage of the breakdown in flow pattern past 90° to measure wind components in all quadrants without ambiguity.
The components of wind velocity so measured are combined in an electromechanical system which provides indications of speed and direction.
Finally, the experimental instrument embodying these principles is evaluated and performance data taken from it are presented.
20(1949); http://dx.doi.org/10.1063/1.1741643View Description Hide Description
The density response of photographic emulsions to the radiations of Ca45, I131, P32 and Zn65 has been determined. Characteristic curves for all four isotopes against Eastman no‐screen x‐ray film and Eastman type M stripping film are presented, together with certain additional studies on Eastman NTB stripping film and Eastman type K x‐ray film. The relative photographic effectiveness of the isotopes examined increases in the order: Zn65, P32, I131, Ca45, C14. Applications of the data for quantitative radioautography are discussed.
20(1949); http://dx.doi.org/10.1063/1.1741644View Description Hide Description
Two types of inductance apparatus have been applied to the measurement of the susceptibility of diamagnetic compounds. With an alternating current apparatus, an accuracy of 0.1 percent of the diamagnetic effect, Δκ=0.7·10−8, has been obtained. The time of measurement was about 8 minutes. Liquids could be easily measured; solids would give no difficulties. In an apparatus with varying coefficient of inductance, the effect of a moving substance in a magnetostatic field was investigated. To date, its precision is a factor of 10 smaller than that obtained with the first apparatus.
20(1949); http://dx.doi.org/10.1063/1.1741645View Description Hide Description
The theory and operation of a simple tachometer are described, in which a direct electrical e.m.f. is generated by attaching a magnet to the rotating member. It is shown how the position and orientation of the magnet affect the magnitude of the generated e.m.f., and suggestions are made for further increasing the e.m.f. generated per rate of revolution.
20(1949); http://dx.doi.org/10.1063/1.1741646View Description Hide Description
It is desired to determine the feasibility of measuring wet bulb temperatures without a wick. The sensing element is a thermistor bead of small mass and rapid response, which, when dipped into distilled water, retains water in sufficient quantity to register a wet bulb temperature when exposed to the air. The experimental apparatus and procedures are described in detail, and the data presented. It is concluded that wickless wet bulb measurements are feasible, with an accuracy of the order of ±0.1°C.
20(1949); http://dx.doi.org/10.1063/1.1741647View Description Hide Description
A thermistor for use with alternating current and having a half‐temperature at 300°K of 7.2°C is easily constructed by immersing platinumelectrodes in a viscous solution of waterglass. It is three times as sensitive as commercial thermistors, withstands a.c. potentials of more than 240 volts, and its cold resistance and sensitivity are easily controlled. Other electrolytic solutions have been tested, but these, although withstanding high a.c. potentials, are less sensitive.
- PHYSICAL INSTRUMENTS FOR THE BIOLOGIST
20(1949); http://dx.doi.org/10.1063/1.1741648View Description Hide Description
A screw‐driven microinjection syringe capable of delivering volumes as small as 0.1 mm3 with statistically good reproducibility is described. It features interchangeable impellers mounted on a sliding way for simultaneous availability of coarse and fine adjustment, and a mechanical positioner permitting known and reproducible fractional revolutions of the control knob.
20(1949); http://dx.doi.org/10.1063/1.1741649View Description Hide Description
A copper cylinder was constructed and to it four layers of oaktanned leather were applied. The thermal gradient through the leather was measured by means of thermocouples on the cylinder and between the layers of leather. By suitable extrapolation of this gradient the temperature of the outside leather surface was determined accurately and constituted the standard surface temperature to which the readings of the surface temperaturemeasuring devices were compared.
Several types of surface temperaturemeasuring devices were compared and evaluated on the basis of:
(1) accuracy as determined by agreement with the standard surface temperature,
(2) adaptability to use in a variety of experimentally produced environmental conditions similar to naturally occurring environmental conditions.
It was concluded that:
(1) The radiometric type of skinthermometer was the most dependably accurate instrument of those tested, although requiring corrections when used in ``sunlight.''
(2) Under normal conditions, the #40 gauge bare wire thermocouple was of the same order of accuracy as the radiometer and was only slightly affected by environmental changes. The accuracy of the coarser bare wire thermocouple was considerably more impaired by such changes.
The thermocouple glued to the surface, or embedded in solder and taped to the surface, or mounted on copper mesh and tied to the surface was less accurate than the bare wire thermocouple. In general, increasing the effective mass of the thermocouple decreased the accuracy of the instrument under normal conditions as well as under the changed environmental conditions.
(3) The Dermalor resistance thermometer was next in order. Errors were not greater than two percent under certain conditions but increased to four or five percent during radiation.
(4) The errors in the disk thermistor readings ranged from −1.2°C to −1.9°C under the conditions studied. The accuracy of this instrument could be increased by calibrationin situ against a radiometer. However, this would not obviate the error produced by infra‐red radiation, for instance. This error amounted to −0.7°C.
Other types of thermistors having much smaller contact surfaces are available and it is reasonable to expect that these instruments would yield readings as accurate as those of a thermocouple. However, they too would be subject to the errors exhibited by thermocouples or any other small instruments in actual contact with the surface. The feasibility of employing a thermistor as a micro‐radiometer is under consideration. It presents special advantages to use in this manner in that thermistors can be constructed with a rapid response time, and as they have a high resistance they are particularly well adapted for electronic recording.
(5) The Roll and Mold pyrometers were entirely unsuited to this measurement in the temperature range studied, showing errors of from 3°C to 7°C depending upon the experimental conditions.
(6) Mercury‐in‐glass thermometers were unsuited to this type of temperature measurement because of the long contact time required which affects the temperature itself, and because of the marked impairment of their accuracy by environmental changes.
- Laboratory and Shop Notes
20(1949); http://dx.doi.org/10.1063/1.1741650View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1741652View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1741653View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1741654View Description Hide Description
- Regular Features
- New Instruments
- New Materials