National Geographic : 1951 Dec
© National Geographic Society 762 Kodachrome by Robert F. Sisson S Standard Screw Threads Make the World Run More Smoothly Britain, Canada, and the United States are making nut and bolt threads interchangeable so that machines and military equipment produced in one country can be more easily repaired in another. These gauges for accurate threading of oil-well casings are calibrated at the Bureau. Ektachrome by Willard R. Culver Light of Man-made Mercury Is a Super-accurate Yardstick Wavelength of light of mercury 198, in the tiny tube, is measurable to one part in 100 million, forming the world's most precise standard of length. Precision measurement of the wavelength is achieved with the aid of the targetlike interference pattern, the product of mercury light reflected back and forth between two metal-coated parallel glass plates.