108 



MEASURING MACHINES. 



59. Measuring Hachines. In measuring the dimen- 

 sions of test bars smaller divisions of the inch than 

 thousandths are rarely required. For the purpose of mak i i ig 

 measurements to ten-thousandths, or even smaller subdivi- 

 sions of the inch, either more precise micrometers, or what 

 are called measuring machines, are employed. Both these 

 depend for their working on the same principle as that 

 adopted in the screw micrometer. The earliest machine of 

 this kind was constructed by the late Sir Joseph Whitworth, 

 and exhibited at the Great Exhibition of 1851. This machine 

 consisted of a rigid bedplate, carrying two headstocks, one 

 at each end. The bars between whose ends the object to 

 be measured was placed, were carried by two head- 

 stocks, somewhat in the same manner as the centre in the 

 loose headstock of a lathe. They received a longitudinal 

 motion by means of fine-pitch screws rotated by hand- 

 wheels. The left-hand headstock had a thread of 20 threads 

 to the inch, and the circumference of the wheel was divided 

 into 250 parts, so that by turning the wheel through one 



FIG. 41. 



division on its circumference, the bar was moved through 

 inj f 2i:v, or TTiW of an inch. In the right-hand headstock 

 the screw had also 20 threads to the inch. It was revolved 

 by a worm-wheel having 200 teeth, and into this wheel 

 geared a single-pitch worm, which was itself rotated by a 

 hand-wheel, having its circumference divided into 250 parts; 

 consequently, if this hand-wheel was turned through one of 

 its divisions only, the bar in the head-stock was advanced 

 through -5 jv x -2^-5 x ^V, which is equivalent to one-millioneth 

 of an inch. But although one millioneth can easily be 



