NOTES FROM THE U. S. BUREAU OF STANDARDS.* H E I G H T OF KENNELLY-HEAVISIDE LAYER.
A paper to be published in the November number of the Bureau of Standards Journal of Research will present experimental results obtained in measurements of the virtual height of the Kennelly°Heaviside layer by the radio echo method. The procedure is essentially that first used by Breit and Tuve and consists of the receiving and oscillographic recording of signals from a high-powered transmitter which is sending out pulses of extremely short duration with sufficient intervals of no emission between pulses to record the echoes. The time interval between the arrival of the ground wave and the first echo is used to ~calculate the virtual height. The report covers daytime observations made each week between January I6 and June I9, I93O. The frequencies used were 4045 and 8650 kc. Two evening tests are also described. Curves are given comparing heights with sunspot numbers and magnetic character. Records taken on April 28, I93O, during a solar eclipse, are shown. NATIONAL STANDARD F O R TESTING P O W E R LINE INSULATORS.
A further step in the nation-wide protection of high voltage electrical transmission lines which have been put into service throughout the country during the past several years to carry power at voltages as high as 220,000 was taken on October 15 with the approval by the American Standards Association of a national standard for testing power line insulators. This standard, prepared through the co6peration of the American Institute of Electrical Engineers, the National Electrical Manufacturers Association, the National Electric Light Association, the National Bureau of Standards, and other organizations concerned with high voltage transmission, assures against insulator failure the lines carrying an increasing percentage of the nation's power supply. The standard will enable laboratories in New York and in * Communicated by the Director. 82i
BUREAU OF STANDARDS NOTES.
[J. F. I.
California to use the same authoritative tests to determine the reliability of insulators. In several of the laboratory tests, voltage applied to the insulator is increased at the rate of Io,ooo volts every fifteen seconds until failure occurs. The final voltage must be far above the greatest voltage which the insulator either singly or in series with other insulators will be required to withstand. It is pointed out t h a t the suspension insulators used for cross country transmission lines must be tested not only for their resistance to high voltages but also to the tremendous stresses set up by wind and by the ice which m a y cover the lines in winter, especially since the transmission towers from which the lines are suspended by means of the insulators are in some places as mtich as a mile apart. ULTRA-VIOLET TRANSMISSION OF SEE SHELLS.
W h a t is known as "Philippine Window Shell" or " m o t h e r of pearl" is obtained from a mollusk (Placuna placenta) and is extensively used for decorative lamp shades. This shell is composed of calcium carbonate, is translucent, and has a light pink color when not weathered. Several years ago the bureau conducted some tests on the ultra-violet transmission of this material to determine whether it prevented the passage of rays injurious to the eyes. It was found that thin lamina of the material are transparent to light waves as short as 250 millimicrons, but the complete shell, which is c.6 to 1.5 mm. in thickness, is too inhomogeneous to transmit rectilinearly in any one direction, an appreciable a m o u n t of ultra-violet radiation. Moreover, ordinary incandescent lamps with which such shades were being used, do not emit sufficient ultra-violet radiation to injure the eye. Recently, the subject has arisen of using these sea shells in the same way as the special window glasses, for transmitting ultra-violet solar radiation for healing purposes. An examination was, therefore, made a short time ago of new and of partially weathered material. Measurements were made of the total amounts of wave-lengths 290 to 313 millimicrons, diffusely transmitted in all directions, and also rectilinearly through the material, which in this instance was only 0. 7 millimeters in thickness.