the bearing when the screw is inserted in a
nut or threaded hole. The cone-shaped point is slightly rounded so that
it will not bear in the bottom of the thread. There is also sufficient
clearance at the bottom of the V-shaped anvil to prevent it from bearing
on top of the thread. The diameter as indicated by this micrometer is
the "pitch diameter" of the thread and is equal to the outside diameter
minus the depth of one thread. This depth may be determined as follows:

Depth of a V-thread = 0.866 / No. of threads per inch;

Depth of a U. S. standard thread = 0.6495 / No. of threads per inch;

Depth of Whitworth thread = 0.6403 / No. of threads per inch.

The movable point measures all pitches, but the fixed anvil is limited
in its capacity, for if made large enough to measure a thread of, say,
1/4-inch pitch, it would be too wide at the top to measure a thread of
1/24-inch pitch, hence each caliper is limited in the range of threads
that the anvil can measure. When measuring the "angle diameter" of a
thread, the micrometer should be passed back and forth across the
thread, in order to make sure that the largest dimension or the actual
diameter is being measured. If the micrometer is placed over what seems
to be the center of the screw and the reading is taken by simply
adjusting in the anvil or point against the thread, without moving the
micrometer back and forth across it, an incorrect reading may be
obtained.

If standard threaded reference gages are available, the size of the
thread being cut can be tested by comparing it with the gage.
Micrometers having small spherical measuring ends (see sketch _A_, Fig.
26) are sometimes used for this purpose. The ball points are small
enough to bear against the sides of the thread and the diameter, as
compared with the reference gage, can be determined with great
accuracy.

[Illustration: Fig. 26. (A) Testing Size of Thread with Ball-point
Micrometer. (B) Testing Size of V-thread by the Three-wire System. (C)
Testing the Size of a U. S. Standard Thread]

=Three-wire System of Measuring Threads.=--A method of measuring threads
by using an ordinary micrometer and three wires of equal diameter is
illustrated at _B_ and _C_, Fig. 26. Two wires are placed between the
threads on one side and one on the opposite side of the screw. The
dimension _M_ over the wires is then measured with an ordinary
micrometer. When the thread is cut to a standard size, the dimension _M_
for dif