Fig. 1366.
Fig. 1367.
Fig. 1368.
In [Fig. 1363] the points g are those between which the measuring is done, and the slide held by the nut k in position is adjusted by means of inch bars to the distance to be measured; h, the hand-wheel for moving one point, and f the wheel which moves the other. [Fig. 1366] is a cross section of the movable head through the nut k and stud m, by which the movable head is adjusted, and [Fig. 1365] is a cross section through the fixed head. The bars used in setting the machine are shown in [Fig. 1367], and in [Fig. 1368] the points of the measuring screws are shown on a large scale. The other figures show various details of the machine and their method of construction. The vernier, it will be observed, is a double one. This is shown in [Fig. 1364], and is so arranged that the zero is made movable in order to correct the errors of the screw itself. These errors are carefully investigated and a record made of each. Thus, in [Fig. 1363] the arm e is graduated so as to show the true zero for different parts of the screw; d can then be adjusted to a correct reading, and the divisions on the large wheel will then be correct to an exceedingly small fraction. This method of construction enables the machine to be used for indicating very minute variations of length.