506 



SUMMAKY OF ODKKKNT UKNKAHCHKK KKLATINU TO 



slotted sleeve, on which is mounted the units-dial, and also the first 

 pinion-wheel. 



"The connection of the spindle to the registering gear is effected by 

 means of a projecting stop or key fixed on the unthreaded portion of the 

 spindle, which engages with the slotted sleeve, imparting a rotary motion, 

 and at the same time the key is absolutely free to travel transversely in 

 the slot when the screw spindle is rotated, so as to either increase or 

 decrease the gauge. Therefore the pinion-wheel, which is mounted on 

 the sleeve, drives the hundredths dial, operated through the intermediate 

 pinion and cam-wheel, which imparts the step-by-step motion. 



"The pitch of the micrometer screw is -j^ in. The rotating thimble, 

 which is rigidly attached to the spindle and turns with it, is so disposed 

 as to protect the micrometer screw against injury and also to exclude 

 dust or dirt. A knurled head is fitted freely on the outer end of the 



Fig. 132. 



thimble, and when manipulated drives the spindle through the friction 

 of a small spring, which is interposed ; hence it is impossible, with 

 ordinary care, to strain the screw, since as soon as the pressure becomes 

 too great, the spring yields to the resistance and allows the thimble 

 to slip. 



" Fig. 132 shows the gauge when almost closed, with a reading of 

 0*023 inch. 



" In fig. 133 is seen the internal construction of the instrument : — 

 A, micrometer screw-spindle ; B, projecting stop on spindle ; C, first 

 pinion-wheel and slotted sleeve combined ; D, intermediate-wheel con- 

 necting E with C ; E, cam-wheel ; F, projecting lug on cam-wheel E, 

 which gives the step-by-step motion to G ; G-, pinion-wheel, constructed 

 with long and short teeth alternately ; H are the short-teeth on wheel G; 

 J are the long-teeth on wheel G ; K is the thousandths or units dial ; 

 L is the hundredths dial. 



