FISCHER: LENGTH STANDARDS AND MEASUREMENTS 159 



whose work was so precise that they not only made it possible 

 for American manufacturers to lead in many lines, but they 

 also contributed in no small way to the spreading of uniform 

 manufacturers' standards throughout the world. So accurate 

 is their work that it is very often difficult even with our present 

 facilities to determine with certainty the differences in their 

 product. At the present time steel guages may be obtained 

 that are guaranteed by their maker to be correct to within the 

 1/100,000 part of an inch, and several sets measured at the 

 Bureau of Standards were certainly correct to within twice this 

 error. Roughly speaking the accuracy in their comparison 

 of length standards is about ten times as great as it was forty 

 years ago. At that period, neither the yard nor the meter 

 could be compared with greater accuracy than one part in 

 400,000, while at the present time it is no difficult feat to com- 

 pare them to one part in 5,000,000. The establishment of great 

 national standardizing laboratories with their corps of trained 

 metrologists and special apparatus is largely responsible for 

 this. At the present time practically every country has a labo- 

 ratory whose facilities are available to the manufacturer, while 

 40 years ago he could only secure reliable standards with great 

 difficulty and was compelled to devise his own means for dupli- 

 cating them. 



I have said nothing whatever about the determination of the 

 distances between the planets nor of the units used by astrono- 

 mers in reckoning distances of the stars. Neither have I said 

 anything about the measurement of very small distances or 

 objects by interference methods or by means of the ultraviolet 

 microscope. They form, so to speak, other chapters of the 

 subject which I shall leave to some future ex-president of our 

 Society. I have confined my own discussion to measurements 

 that have been within my own experience. I do not claim to 

 have covered even this range with any completeness; but I 

 have merely attempted to give such facts as would interest 

 you, and in doing this it is possible that I have given more prom- 

 inence to what has been done in the United States and by mem- 

 bers of our Society than is justified by the title of this paper. 



