On an Arrangement for dividing Inch- and Metre-Sea les. 217 



appears that, so far as the numerical value of the resistance 

 can be obtained, it is at all events not glaringly incompatible 

 with the values of the resistance to external magnetomotive 

 force which have been obtained in an entirely different 

 manner. 



XXIX. On an Arrangement for dividing Inch- and Metre-Scales. 

 By R. H. M. Bosanquet, St. John's College, Oxford. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



THE screw of the slide-rest of the Royal Society's lathe in 

 my laboratory has a pitch of -J- inch. It was therefore 

 an obvious arrangement for dividing decimal-inch scales, to 

 fit this screw with a micrometer-wheel of 25 holes. 



20 holes then correspond to -j 3 ^ inch. 

 4 „ „ -gL i ncn . 



2 „ „ Tooi ncn - 



The holes are worked with pins, and a V falling over the pins 

 in the same way as in the micrometer of the same lathe de- 

 scribed in Phil. Mag. x. p. 220. 



Before this was completed it occurred to me that a metre- 

 scale could be divided by the same arrangement, if there was 

 any moderate number which would serve as a factor. 



The common equivalent is 1 inch = 2*5400 centim. Ac- 

 cording to Everett's book of units the error of this is about 

 2 units in the fifth place of decimals; and therefore it is 

 negligible for practical purposes. 



o 



Hence 1 centim. = tttt eighths of an inch. 

 2-54 & 



400 

 = 127 m v 



It was only necessary therefore to set a division of 127 holes 

 round the micrometer-wheel, and 40 of these holes give a 

 millimetre. If it were desired to divide to tenths of millim., 

 of course 4 of the holes would be used. This division of 127 

 holes and the one of 25 for the inch-scale are executed on the 

 same wheel, by means of which therefore both metre- and 

 inch-scales can be divided without any shifting whatever. 



In dividing millimetres a little piece of brass is used, 

 which reaches from the pin in use to the hole at distance 40, 

 so as to avoid having to count every time. 



The division of 127 was executed with the micrometer by 

 means of the following approximation depending on the 67 

 wheel. 



Phil. Mag. S. 5. Vol. 15. No. 93. March 1883. Q 



