65 i SUMMABY OF CURRENT RESEARCHES RELATING TO 



index- wheel being about 1"2 in. in. diameter, it will be seen that the lines 

 on the screw or index-wheel will be over ■ 35 apart, instead of one-tenth 

 (0'035) of that, when the wheel is divided into one hundred parts, in the 

 usual way." 



" I have illustrated two methods of making micrometers, which vary 

 from each other in some respects. One method is shown in figs. 174 and 

 175, and the other in figs. 176 and 177. 



In figs. 174 and 175, is a case with top removed, inclosing the cross- 

 hair frame D. F are wires attached to sliding frame D. These wires may 

 be made of metal or any suitable material, and should be from 1/500 in. 

 to I / 1000 in. in diameter. Glass is a good material to make the wire of, 

 as it can be pulled apart and a square end obtained. The wires can be 

 secured to the frame by wax or any other suitable means. There may be 

 one wire only, or two, as shown in fig. 174, or any number desired, and they 

 may be placed in any position, as shown at A, Z, and Y, fig. 176, or any 

 other preferred. M is a split nut. N is a screw for bringing the two parts 

 of the nut together. J is a screw which passes through nut N, and 

 terminates in a small hardened abutting-end K. O is an index-line. I is 

 a small hardened abutting-piece attached to cross-hair frame D. G is a 

 rod for holding spring H in position. Q is a graduated index-wheel. E is 

 the top to the case C. 



V W X, fig. 176, are adjustable pieces, to which the wires are attached ; 

 T the part of the screw which is 20 pitch ; J, the part that is 25 pitch. 

 The screw Jj passes loosely through the frame C. 



It will be seen that by means of the split nut and screws N all play 

 between the nut and the screw can be prevented. Nut B' may be split at 

 one end, the same as nut M, or made in two parts, with two screws as 

 shown. 



It is evident that with the nuts properly adjusted the frame that carries 

 the wires (cross-hairs), fig. 176, must move with the screw without variation. 

 The arrangement shown in figs. 174 and 175 has the advantage of the 

 split nut, and in addition to that very small abutting-surfaces, so that there 

 will be much less liability for dust or oil to get between the abutting- 

 surfaces K and I than in the usual form. 



There is a great advantage in having several points to aid in adjusting 

 the cross-hairs to a line. If the operator is in doubt whether one point 

 coincides with the line, the other points will help him to decide directly. 



In fig. 174 the nut M may be made in the frame D, as shown at B', 

 fig. 176, instead of being located outside of case C ; but in that case the 

 advantage of the small abutting-surfaces I and K would be lost ; but it 

 would be better than the usual form. 



The index-wheel is divided into ten parts and each part into five 

 fractional parts. Now, with the two pitch-screws 36 and 37*037 pitches, 

 as above described, one division of the wheel will read 1/200,000 in., and 

 each fractional part will read 1/1,000,000 in., for with a Microscope that 

 magnifies fifteen times, one turn of the screw being 3/4000 and one 

 division of the wheel being 3/40,000, and this magnified by fifteen times 

 gives 3/600,000 = 1/200,000, and one-fifth (the fractional parts), will give 

 the 1/1,000,000. 



The advantage in using the end of a wire instead of the side of a 

 spider-web in the usual way, is that the full size of the line is always in 

 view, and, having the wire nearly the size of the line, it is much easier to 

 judge when the two coincide than when the line is covered by the cross- 

 hairs, as in the common way, and when more wires than one are used each 

 one will serve to correct a mistake that might be made with one alone." 



