The Present Status of Micrometry. By 31. D. Ewell. 683 



equals 39*37012 in. For some time previously lie had adopted 

 the relation 39*37015. The United States yard, established by 

 the relation, 1 yd. equals \%%^ m., is therefore about 2*6 fi longer 

 than the present accepted value of the British yard, a difference, if 

 it really exists, of no consequence whatever, except in measure- 

 ments of the greatest precision. It is, however, uncertain whether 

 any such difference actually exists, for the reason that successive 

 comparisons between the British yard and its authentic copies 

 usually vary more than the amount above stated. 



Since the death of Professor Powers, to whom the science and 

 art of metrology and micrometry are very greatly indebted, very 

 little attention, outside of the National Bureau of Standards, 

 appears to have been paid, at least in this country, to either 

 metrology or micrometry, and the making of micrometers has 

 become almost wholly a commercial matter, with what results in 

 point of accuracy it is the principal object of this paper to disclose. 

 By Professor Eogers metrology and micrometry were most ardently 

 and arduously pursued from mere love of the subject, and at great 

 pecuniary loss to himself ; and to-day the micrometers ruled by 

 him are far superior to any others manufactured and on sale in the 

 United States. 



In pursuance of the object above stated, the writer has, for the 

 last year, been collecting micrometers from all available sources, 

 and subjecting them to measurements, the results of which are 

 recorded in the following tables. The scales for sale at the present 

 time, both here and abroad, are almost universally on glass. 

 Nearly all of them are engine ruled, but some are photographic 

 copies of ruled scales ; and for use with low powers are very good 

 indeed. The lines of these, however, are altogether too coarse and 

 rough for use with medium or high powers. A few scales appear 

 to have been etched on glass, and these also have lines so coarse as 

 to be of no use with others than low powers. The lines of those 

 ruled with a diamond on glass in many instances have greatly 

 deteriorated, a very common experience, so far as the writer's 

 observation goes. A few were ruled on speculum metal, silver or 

 platin-iridium, and the lines on these are in a state of perfect 

 preservation, as good in fact as when first ruled. In the writer's 

 experience with' micrometers, extending over a period of about 

 twenty-three years, the only scales that can always be relied upon 

 are ruled upon metal surfaces. These, whether the surface be 

 silver, nickel, speculum metal, platin-iridium, or steel, are, with 

 reasonable care, entirely free from deterioration by lapse of time, 

 and are so far superior to glass that no one accustomed to their use 

 would be satisfied with anything else. The only difficulty lies in 

 the illumination, and that difficulty is trifling. A good line on 

 metal would stand any degree of amplification. The writer has 

 often measured spaces ruled on speculum metal and platin-iridium 

 with a power of from 1000 to 1500 diameters, the lines under even 



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