A screw gauge consists of a u-shaped metallic frame F. To end of this frame a flat shaft s1 is fixed. On the opposite end of F,a hollow, long cylinder c is fixed. The hollow cylinder has fine threads cut inside it and it serves as the nut. On the outer surface of this hollow cylinder, a cylinder, a line, parallel to its axis, called index line is drawn and is divided into some equal divisions (1mm or 0.5 mm). This serves as the pitch scale(P). a screw with a flat tip s2 having threads exactly identical to the threads cut inside the cylinder, moves through the cylinder c. to the other end of the screw. A milled head M is connected. To this milled head, one end of a barrel B is connected. This barrel B forms a jacket to the first hollow cylinder C. the other end of the barrel is tapered and has 100 or 50 equal divisions on it. This is called the head scale (H). When the two flat shafts s1 and s2 exactly opposite to each other, are in contact, the distance between them is zero. In this position, the edge of the head scale must coincide with the zeroth division of the pitch scale. Further, the index line will be coinciding with the zero o the head scale. When the marble, whose diameter is to be determined, is held between two shafts S and S2 tightly by rotating the milled head M, the readings on both pitch scale and head scale together give the diameter as described below.
