National Geographic : 1915 Feb
A REMARKABLE SCIENTIFIC APPARATUS, WITH WHICH THE VISITOR CAN SEE THE BENDING OF A HEAVY STEEL BAR BENEATH THE PRESSURE OF ONE FINGER This bar is supported at each end and a small mirror is fixed at the center. Above it is a frame bearing another partially silvered mirror, both of which reflect the light of a sodium burner, the lower mirror showing a series of purple and yellow concentric rings. The slightest pressure on the bar-even the weight of a visiting card or a pin-causes these circles to expand outward, forming, as it were, a series of ripples like those made when a stone is dropped into the center of a still pond. The pressure of one finger on the bar causes the formation of five or six new circles, showing that the bar has been bent about one twenty-thousandth of an inch, as each new circle means a movement of one hundred thousandth of an inch (see illustrations, page 161). THE LIGHT WAVES ARE USED AS UNITS OF MEASURE If all the standards of length in the entire world were by some accident destroyed, the meter could be exactly reproduced from the red line in the spectrum of cadmium, as it is invariable (see page 165). This illustration is a photograph of the spectrum of cadmium vapor, showing the three lines (marked A, B, and C) used to determine the international meter, the world's standard of length. These lines never vary and are exactly 64,384,696, 50,858,219, and 47,999,087 millimicrons long respectively. A millimicron is a thousandth part of a micron; a micron is a millionth part of a meter.