SCREW MICROMETER. 



107 



measurements, is the screw micrometer. An outside view 

 of one of these is shown in Fig. 41. The object to be 

 measured is placed between the two straight cylindrical 

 bars, one of which is fixed and the other movable in a 

 bearing at the other side of the horse-shoe frame. The 

 longitudinal motion of this bar is effected by means of a 

 fine pitch screw which is rotated by a knurled thumb-piece, 

 and whose pitch is ^ in. As the pitch of the screw is 

 in., one complete revolution of the milled head causes a 

 longitudinal movement of the measuring bar equal to this 

 amount, and the outer shell being divided into twenty-five 

 parts, ^5 of a revolution can be read, that is to say, -^ x TO, 

 or ToVir i n - These micrometers are made in different sizes 

 with capacities varying from 1 in. to 2 in. On Fig. 42 is 

 shown another form of screw micrometer designed for 

 inside measurements only, and capable of reading to 

 thousandths of an inch. A useful size is one having a 

 capacity from 2 in. to 6 in. These instruments will be found 

 especially useful for compression tests. 



FIG. 40. 



It may be mentioned that the chief makers of instru- 

 ments of the kinds here mentioned are the two American 

 firms, Messrs. Starrett and Messrs. Brown and Sharpe. These 

 firms also make calipers of larger size, graduated to ten- 

 thousandths of an inch. The agents for these instruments 

 in this country are Messrs. Charles Churchill and Co., 

 Finsbury. 



