MICROMETRY 35 
equivalent to one revolution of the screw which moves it. This system 
has important advantages, especially in the measurements of large 
objects. Unlike the filar micrometer, it does not require the moving 
of the index over the entire length of the object, as a fraction of one 
rotation of the screw is all that is necessary. 
“The scale is 5 mm. long and is divided into twenty spaces; each 
millimeter is marked by a line of double length. As the screw is made 
with a pitch of 4 mm., one revolution of the screw moves the scale 
7+ mm., or one of the intervals. 
Fig. 28.—Spencer Lens Company’s Micrometer Eyepiece. 
“Upon the axis of the screw is placed an adjustable drum which 
may be set to any desired position. This is graduated with one hundred 
divisions, for reading of which an index pointer is fastened on top of 
the micrometer case. 
“In measuring the length of an object, the scale is moved until 
one of the millimeter lines coincides with the margin of the object 
under examination; as for example, line No. 0 in the sketch, then, 
holding the milled head of the screw fast, the drum is turned until 
the index stands at zero. Now turning the screw until the line which 
the other edge of the drum added to the number of full mm. divisions 
covered (nine in the sketch) gives the apparent length of the object 
—its real length depending on magnification used. 
