The weights tested by the Bu- 

 reau range from those used by 

 city and State scalers in testing 

 the weights and measures ol 

 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 

 to be sure that it is not a mere 

 scratch on the surface of the 

 weighing pan. The balance on 

 which this weight is used 

 can weigh with accuracy 

 down to I /50,000,000th 

 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 Avas 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- 

 V a t e d 5 centimeters 

 (about 2 inches) . It was 

 found that the weight 

 was 0.016 milligrams less 

 than when they were 

 side by side. The pre- 

 cision of this instrument 

 can be appreciated by 

 this experiment, for it 

 shows that it can weigh 

 down to I /100,000,000th 

 of the whole. This bal- 

 ance is shown on p. 156. 

 Some of these bal- 



Photos from Bureau of Standards 



te:sting a glass sure^ace; for submarines (see p. 165) 



Figure A shows the concentric rings which appear on a true 



surface when tested by the interferometer (see page 160). In 

 this figure the surface is very nearly, but not quite, true; so that 

 the rings are not perfectly regular. Figure B shows how an 

 untrue surface responds to the interferometer. This instrument 

 is a very valuable one and upon it the lives of men can depend, 

 as, for example, those of the sailors in a submarine who depend 

 entirely upon the periscope for guidance. Should the surfaces 

 in the periscope be untrue, the pilot may easily be led into a 

 situation in which the lives of the crew may be jeopardized. 



161 



