High Frequency Broadcasts.-A new frequency (2.5 MHz) was added to the broadcast of WWVH located in Maui, Hawaii. This frequency provides ground wave coverage of the Hawaiian Islands with improved accuracy capabilities.

WWV, now in Greenbelt, Md., will be relocated at Fort Collins, Colo. From this more central location, it will be able to provide better coverage at higher accuracy for most of its customers. The move is planned for July 1, 1966.

Temperature

The thermodynamic temperature scale is defined by assigning a value to the triple point of water. Then using thermodynamic relations and appropriate techniques, the scale is realized at points in other temperature ranges. To provide a common basis for defining temperatures, IBS works to realize the International Practical Temperature Scale (IPTS), the lower limit of which is defined at 90 °K (-183 °C). Other scales have been established at NBS to extend the practical range down to 2 °K.

Improved Temperature Scale.-A photoelectric pyrometer has been developed with which the International Practical Temperature Scale (IPTS) above the gold point, 1063 °C, is realized about ten times more accurately than with the standard disappearing filament visual pyrometer. The instrument has a temperature resolution of 0.2 °C, about 20 times better than the visual pyrometer.

The significantly greater resolution of the new pyrometer has served to reveal unpredicted sources of inaccuracy that were obscured in the visual pyrometer by the lack of observer precision. Pyrometer lamp instabilities remain the major limitation to accuracy, and their effect is minimized by frequent recalibration.

The IPTS has been realized with an estimated uncertainty (95 percent confidence) of 0.07 °C at 1063 °C, 0.35 °C at 1650 °C, and 1.9 °C at 3730 °C. To correct for lamp drift, the pyrometer is recalibrated at the gold point after each 100 hours of use, and further recalibrated at other points in the low range after every 200 hours of use.

Platinum Resistance Thermometer. In an attempt to provide a more accurately realizable International Practical Temperature Scale, the platinum resistance thermometer is being studied as a possible instrument for interpolating between fixed points in the range 630.5 to 1063 °C. Thermometers have been designed and techniques worked out which show that these instruments should in fact be proposed for international adoption as the specified means of interpolation.

New 4-20 °K Calibration Service.-A calibration service for germanium resistance thermometers in the 4-20 °K range was initiated. The new NBS temperature scale, obtained with the acoustic thermometer, has been transferred to six germanium thermometers which are used to check the customers' units.

Work is continuing to establish the absolute integrity of the new scale and to develop means for more accurately interpolating the resistancetemperature curve between calibration points.

Luminous Intensity

Early research at NBS led to the development of an international standard for luminous intensity based upon the radiation from a black body at the temperature of freezing platinum. This unit the candela-is defined as the visible radiation from 1/60 of a square centimeter of this source.

The development of higher temperature incandescent, fluorescent, and vapor lamps has caused this standard to be less suitable than when it was originally developed. Investigation of alternate methods of realizing this standard are thus under way. One proposal being investigated is to define the standard of luminous intensity by absolute photometry in terms of the radiation in watts from a source multiplied by the response of the average human eye to radiation. The radiation produced by the source would be measured by comparing its thermal effect with the thermal effect of a known electric current passed through a known electrical resistor.

Absolute Photometry.-Two instruments intended for the direct measurement of irradiance in terms of electrical units are being studied. The first is a thermoelectric instrument of the type designed by Guild of the National Physical Laboratory; the second is a resistance-type instrument of IBS design which uses a blackened cone as a receiver. In each instrument the temperature rise produced by the incident radiant energy is duplicated by a temperature rise produced with an internal electric heater. The electric power required to produce this rise is then measured by conventional methods, the incident radiant power being equal to the electric power, subject to correction. This power can then be converted to luminous units. The two instruments have been assembled, and preliminary tests have been completed.

Electric Current

Activities in this area are described along with the other electrical quantities.

Force

MECHANICAL QUANTITIES

Force is a derived mechanical quantity equal to the product of mass times acceleration. In the International System, the unit of force is the Newton, defined as that force which, when acting on a one kilogram mass, will give the mass an acceleration of one meter per second per second. At present in the United States it is customary to use the units of pound force or kilogram force. These are the forces which, acting on masses of one pound or one kilogram, will produce an acceleration of 9.80665 meters per second per second, which is the acceleration due to gravity.

Dead Weight Machines.-The most convenient method of obtaining accurately reproducible forces for the calibration of transfer standards is by means of dead weight machines. In such instruments, the force of the earth's attraction on the dead weight masses is applied to the device being calibrated. It is desirable to adjust the dead weight masses to take ac count of air buoyancy and the difference between the acceleration due to

Deadweights prior to installation in three large-capacity deadweight machines at NBS. The 50,000-lb weights are 10 feet in diameter; the 1,000-lb weights are 3 feet in diameter.

gravity at the site of the dead weight machine and the standard value of 9.80665 meters per second. To provide the needed local value of acceleration due to gravity, an absolute determination, involving the measurement of the time required for quartz rods to fall known distances in a vacuum, has recently been completed. The results, which will be released shortly, provide the local value used in adjusting the masses to an accuracy of about one part in two million.

New Standards for Force Measurements.-Three new deadweight machines with 112,000- 300,000- and 1,000,000-pound force capacities have been manufactured, adjusted, and placed in operation. These machines provide approximately 9 times the deadweight capability previously available for calibrations of force measuring devices. Devices calibrated with these machines will be used as "transfer standards"; They will also be used directly in critical applications such as measuring rocket and jet engine thrusts, and weighing rockets and rocket components. Uses of the

transfer standards will include calibrations of testing machines for determining mechanical properties of materials. They can also be used as laboratory and plant standards for calibrating other force measuring devices.

Forces applied with these machines are derived from the acceleration due to gravity acting on the dead weight masses carefully adjusted to compensate for both gravity and air buoyancy such that the applied loads are in units of pound force. Errors in the applied forces do not exceed 0.002 percent, 10 times better than the older machines, and up to 50 times better than the previously used procedures and equipment for measuring 1,000,000 lb loads. Besides the increased accuracy, substantial savings

in the time required to calibrate the larger devices has been realized. For example, time required to calibrate 300,000 lb-f and 1,000,000 lb-f capacity elastic force measuring devices in compression has been reduced about 30 and 50 percent, respectively.

Pressure

Work has continued on more precise determination of pressure scale reference points. This included preliminary measurements of the transition pressure for change in the crystalline form of bismuth (the Bi I-II transition near 400,000 psi). The work was done with a dead-weightloaded free-piston gage, and is believed to be the highest pressure at which this type of instrument has been successfully used.

At lower pressure, work was initiated on an investigation of airlubricated piston gages for possible use as standards of absolute pressure in the range near one atmosphere. If this work is successful, it may be feasible to supplant the mercury column as a reference device in many cases, thus providing a standard with substantially improved characteristics. Vacuum

A Vacuum Measurement Section has been established in IBS as a separate entity to emphasize effort in a much needed area.

Absolute Instruments for Low Pressure Measurement. In the range down to 10-5 torr, successful preliminary work has been completed on liquid column instruments of two types, with liquid surface displacement for the higher pressures being measured by micrometer driven pointcontacts and for the lower pressures being measured by interferometric means. Work has been completed on a micrometer manometer which employs a silicone oil as the manometric fluid. The measurement uncertainty is 4 x 10-4 torr with a least count of about 1 x 10-4 torr.

Stable Pressure Generator.-The design of a balanced flow pressure facility to obtain stabilities of 1 part in a 1000 over the range 10-3 to 10-8 torr has been completed and construction has begun. The facility will give a stable environment with which a number of absolute procedures can be compared. Design aspects stem from previous molecular flow research.

Calibration Service.-A formal calibration service has been initiated for the range 1 to 1000 millitorr utilizing precision liquid columns, such as the NBS micrometer manometer, as reference standards. A console vacuum calibration facility is nearing completion; it will permit this work to be carried out with proper vacuum environments on a repetitive basis.

Vibration Measurements.-An improved secondary standard for comparison calibration of vibration pickups was built. Inaccuracy in earlier comparison calibrations usually arose because the two pickups being com