be

It was recently found that the number of reference points that may used (previously believed to be limited to only a few) is unlimited. Thus, a continuum of points may be approached and all details of the wave front obtained.

Standards of Mass and Weighing Techniques. An arresting mechanism was developed for knife-edge balances which requires only one moving part and permits all knives and flats in the system to remain in contact during the unloading and reloading process. The arrestment error associated with this device is a few parts in one billion, at the one-kilogram level.

No practical method has been available for hydrostatically weighing frac tional-gram weights, due to uncertainties in the meniscus correction. However, the Cartesian diver, operating completely submerged, offered a possible solution. Hence, a crude closed-body Pyrex diver was blown and allowed to fall under constant pressure. It was loaded first with one, then with a second, 150-milligram weight, the two differing by about 15 micrograms in weight. The difference in fall rate, clearly visible to the naked eye, demonstrated the practicability of such a device for the hydrostatic weighing of small weights.

Experiments conducted in NBS shops showed that Hull's method of burnishing stainless steel to a high finish with a diamond tool was satisfactory for standard weights. The use of this process eliminates much of the timeconsuming and costly polishing which weight manufacturers previously performed in order that weights would meet acceptable standards.

For a given material the surface-to-mass ratio of weights increases as their size decreases. It is desirable to minimize this ratio; however, fractional gram weights are usually constructed from sheet metal or wire which tends to increase this ratio. A method of melting fine wire and letting the surface tension of the liquid metal form a sphere-which has the most favorable ratio was developed, and several platinum weights were constructed.

Recent experiments demonstrated that the passage of small amounts of a-c power through a knife-flat contact did not result in a measureable change in balance performance. A condenser plate was therefore attached to the beam of the NBS one-kilogram balance and energized through the knife edge. This device permitted mechanical manipulation of the beam by a small externally controlled variable electrostatic force, and eliminated most of the uncertainties of operation associated with mechanical members being brought into contact with the beam.

As a part of a continuing program for improving the precision of mass measurements and mass standards, a 200-milligram torsion fiber balance was put into service for calibrating weights 200 milligrams and smaller. It is an equal-arm balance in which torsion fibers (instead of the conventional fulcrum knife edge and plane) and suspension fibers (instead of the terminal knife edges and planes) are used. These fibers, made of fused silica, are fused to the beam, which is also made of fused-silica fibers fused together to form a one-piece unit. The free end of one of the torsion fibers is fixed to the case and the free end of the other torsion fiber is attached

etched trapezoidal pattern. Slight deviations from the basic waveforms of these patterns recently caused difficulty in instrument calibration, so a reevaluation of the calibration techniques used was made.

The measuring force on the stylus was reduced to a point where no visible permanent deformation of the standard occurred, and the traversing velocity was reduced enough so that dynamic tracking effects were eliminated. Interferometric measurements on gage steps permitted investigation of the instrument magnification, linearity, and hysteresis.

A simple method was devised for the measurement of radius of curvature of the tracing stylus. Measurements of the average curvatures in the "corners" of the samples were made by microinterferometry, providing corrections for the differences between the locus of the stylus motion and the true waveform of the sample. Measurements were also made of the uncertainty in using a planimeter for the evaluation of roughness, and one of the triangular-wave-type standards was calibrated by the planimeter method for use as a master for calibration by comparison.

A series of comparisons of measurements on surface roughness standards was undertaken. A British manufacturer of surface roughness measuring equipment supplied samples, and measurements were compared with those of an Atomic Energy Commission laboratory. Average deviations from the mean values on four samples were very small, and further work with other laboratories is contemplated.

2.1.2. MECHANICS

The Bureau's work in mechanics is primarily in the development and improvement of methods of measurement of mechanical phenomena in solids, liquids, and gases; the establishment of required standards in mechanics and the relation of such standards to the prototype standards; the support of these activities by theoretical and experimental researches into mechanical phenomena; the determination of physical constants of particular importance in mechanics; and provision of assistance to other laboratories in relating their measurements to a common basis (or to established standards) by transfer standards, calibration services, and other means. Measurement areas include sound pressure and intensity, shock, vibration, force, strain, pressure, vacuum, viscosity, and rate of gas and liquid flow.

These measurement areas are of vital importance in the missile and space programs, which require great accuracies over widely extended ranges under extreme temperature environments. Special emphasis therefore is given to research directed toward meeting these needs.

Because of the increasing requirements for measuring mechanical quantities in defense industries and in government laboratories, and because of the requirements of missile and space projects, requests for calibration services continued to increase.

Considerable progress was made in the design of the special-purpose equipment to be housed in the Engineering Mechanics Laboratory now under

construction at Gaithersburg, Md. Detailed data necessary for the planning of the Sound Laboratory and the Fluid Mechanics Laboratory were developed for use in preliminary work on the design of these buildings. The availability of these facilities will make it possible to provide more comprehensive services in several of the measurement areas in mechanics.

Infrasonic Waves in the Atmosphere and in the Earth. Infrasonic waves having fairly constant periods of about 6 seconds pass through the atmosphere of the Washington area. These waves-usually called microbaroms come mainly from easterly directions and travel parallel to the ground. A mathematical analysis of the influence of ocean waves on the atmosphere shows that these might be the source of the infrasound. The impact of waves on a beach produces the familiar audible sounds of surf, and, in addition, the periodic arrival of the waves can generate infrasound of the same period in the atmosphere. Experiments are being planned to check the validity of this hypothesis.

The infrasound generated by geomagnetic storms comes mainly from northerly directions, possibly from the auroral zone in the Arctic. A new infrasonics station is being installed at the Bureau's Boulder (Colo.) Laboratories to obtain data that will supplement data from the Washington station. The two stations will show a directional effect that may make possible a decision as to whether or not these waves originate in the auroral zone.

The microbaroms in the atmosphere have their counterpart in the microseismic waves in the earth's crust, as these waves also have periods near 6 seconds. Three seismometers of a pattern developed at the Bureau are being installed at sites spaced far enough apart to allow determination of the propagation speed and direction of the microseisms. It seems unlikely either that the microbaroms cause microseisms, or vice versa. Whether or not they have a common origin is one question to be studied with the combined infrasonics and seismics stations.

Elastic Changes Caused by Static Loads. The nature even the tion of the existence of the so-called Fitzgerald "resonances" in various crystalline materials has engendered much discussion and controversy in recent years. According to Fitzgerald's theory, certain static stresses, either residual (built-in) or due to external load, cause severe changes to occur in the dynamic elastic constants of these materials. Characteristic of these changes are sharp resonances and high internal friction at various frequencies in the range 100 to 5000 cycles per second (c/s). There is also a general loss of elastic stiffness over this range, particularly at the lower frequencies. The implications of such effects in the fields of fatigue failure of machine and structural elements, particularly those associated with aircraft, are serious, as are also the fundamental solid-state problems raised. For this reason, the Bureau undertook measurements on quartz, fused silica, and polycrystalline aluminum. The technique used was designed to allow easy detection of these effects, provided that they actually are associated with the theory proposed to explain them or with any similar theory. No trace of the predicted effects on the elastic constants was found. This negative

Stress-strain characteristics of metals at high temperatures (up to 1,460 °F) were obtained by impacting cylindrical specimens in a furnace (barely seen at right) with projectiles impelled by an air gun (foreground). (See p. 35.)

result lends support to the conception that either the theoretical background employed by Fitzgerald to explain his results is not relevant to them, or that the observed resonances are artifactitious.

Acoustical Repulsion of Birds at Airports. The Fish and Wildlife Service is studying the use of acoustical stimuli to repel birds from airports. At airports near the ocean, the presence of sea gulls on or about the runways presents a great hazard to aircraft. In order that the acoustical signals used in this program may be accurately controlled, NBS is working with the Fish and Wildlife Service-giving technical advice on equipment and on its operation and calibration. As part of this study, scientists from these organizations made two field trips to a nearby area frequented by these birds. The scientists played at various sound levels previously recorded tape of the distress cries of sea gulls. At most of the sound levels, 10 seconds of signal was enough to repel the gulls out of sight. Generally, within an hour or two gulls would again be feeding at the site. It is not known at this time whether the same gulls returned or a new group of gulls flew in.

Reverberation-Chamber Technique for Calibration of StandardType Noise Sources. A new reverberation-chamber method of test was developed for the measurement of sound power output of noise sources in both 1 and 13 octave bands covering the frequency range 40 to 10,000 c/s. The development of the method required specialized calibration of the reverberation chamber and of all instrumentation over an extended frequency range.