National Geographic : 1915 Feb
The weights tested by the Bu reau range from those used by city and State scalers in testing the weights and measures of commerce down to those used by scientists in most precise and exacting work. To do such testing as this the very finest instruments are required, and weights which vary from great masses as big as a steamer trunk down to tiny atoms which would look lonely on a pin's head. The smallest of these is the I/20th milligram, or I/6oo,oooth of an ounce. Though made of the lightest metal known, aluminum, when set on a balance one has to look carefully in order tobesurethatitisnotamere scratch on the surface of the weighing pan. The balance on which this weight is used can weigh with accuracy down to I/5o,ooo,oooth of an ounce. On one of the balances the weighing is made in a vacuum, so that the weight and buoyancy of the air does not interfere with the result. Another balance is so sensitive that it can de tect the difference due to the diminution of the earth's attraction. To prove this, an experi ment was specially made for the purposes of this article. Two kilograms were weighed side by side, then one kilogram was placed on top of the other, and was thus ele vated 5 centimeters (about 2 inches). It was found that the weight was o.o16 milligrams less than when they were side by side. The pre- TESTI; cision of this instrument Fi can be appreciated by surf c this experiment, for it the ri shows that it can weigh untrue down to i/ioo,ooo.oooth isav of the whole. This bal- as, for ance is shown on p. 156. entire Some of ebl in theuati Some of these bal- situati Photos from Bureau of Standards NG A GLASS SURFACE FOR SUBMARINES (SEE P. 165) gure A shows the concentric rings which appear on a true e when tested by the interferometer (see page 160). In gure the surface is very nearly, but not quite, true; so that ngs are not perfectly regular. Figure B shows how an surface responds to the interferometer. This instrument ery valuable one and upon it the lives of men can depend. example, those of the sailors in a submarine who depend y upon the periscope for guidance. Should the surfaces periscope be untrue, the pilot may easily be led into a on in which the lives of the crew may be jeopardized.