288 



REPORT 1884. 



Proposed Small Screw Gauge. 



6. It is evident tbat by taking the exact successive powers of 0-9 for 

 the pitch, complex numbers would soon be arrived at. Such dimensions 

 would, however, involve a degree of accuracy which is hardly attain- 

 able iu practice, and it may be shown that, with two significant figures 

 employed throughout to express the pitch, the degree of accuracy likely to 

 be attained in screws of the kind under consideration is reached. Relying 

 on this fact, the series of pitches given in Column VI. 1 is arrived at for 

 screws ranging from '236-inch to the smallest in use, O01-inch in diameter, 

 in place of the mathematically exact series obtained by raising - 9 to 

 successively higher powers. 2 And this is the series which the Committee 

 recommends for adoption. 



7. Viewing the numbers thus obtained in the first place merely as a 

 graduated series of pitches, and ignoring the unit of measurement, it 

 may be admitted that the series of powers of 0'9 from which they are 

 deduced is perhaps as good a one as can be suggested for the purpose, and 

 it is found, to very closely correspond with experience. Thus Column VI., 

 which gives the nearest approximation to this series that is practically 

 required, is well adapted for such a system of screws. It is to be observed 

 that in selecting a series of pitches there are three simple alternatives 



1 It may be incidentally pointed out here that this series comprises two screws, 

 with pitches of 1 mm. and 0-25 mm., which would be serviceable for micrometers. 



2 Sir Joseph Whitworttrs gauge, in general use, ends at Jin., where this commences. 



