APPENDIX NO. 8. TEST OF A TRANSIT MICROMETER. 
459 
and indicates the four whole turns of the screw within which the observations are to be 
made. The diameter of the field of view through the Airy diagonal eyepiece, which has 
an equivalent focus of 12 millimeters, is something over 24 turns of the screw; thus 
giving a space of fully 10 turns of the screw on each side of the four turns in the center 
of the field. 
That portion of the micrometer screw which projects through the box has the 
micrometer head fitted upon it, and secured in position by a clamp nut. The edge of 
this head, graduated at the corner nearest the box to 100 parts (g, Fig. 1), also carries 
at its opposite corner a screw thread of three turns with a pitch of 1 millimeter and a 
diameter of 32 millimeters (t, Fig. 1). Sunk into the outer side of the head and fitted 
concentrically with it is a thin metallic shell, which has fitted upon it a hollow cylinder 
made of ebonite, 6 millimeters long and 26 millimeters diameter (e, Fig. 1). Five strips 
of platinum, o mra .4 thick, and corresponding to the 12.5, 25.0, 50.0, 75.0, and 87.5 divi 
sion points* of the graduation, g, are slotted into the edge of the ebonite cylinder (see 
Figs. 1 and 2) and secured in such manner as to make metallic contact with the microm 
eter head proper, and, through it, with the screw, micrometer box, telescope and telescope 
pivots and the iron uprights of the transit. By releasing the clamp nut within the 
the ebonite ring the graduated head, with its thread, t, can be adjusted, in a rotary 
sense, in relation to, the thread of the screw, and therefore also of the spider thread upon 
the slidei At the same time the position of the platinum contact strips can be set to 
correspond to the zero of the graduation,^, which latter is read by the index, i, Fig. 2. 
A small ebonite plate, />, Fig. 1, secured to the micrometer box, carries upon its 
outer end, mounted in a suitable metal block, the contact spring, s, Fig. 1. It ends in 
a piece of platinum turned over so as to rest radially upon the ebonite cylinder. Its 
width is 4 millimeters, and its thickness that of the contact strips, i. e., o mm .4. A small 
screw, c, Figs. 1 and 2, serves to adjust the pressure of the spring upon the cylinder. 
Against one end of the micrometer box is fastened a small bracket, upon which is cen 
tered a small worm wheel, w, Fig. 1, gearing into 'the screw thread, t, of the micrometer 
head. It has 40 teeth, and moves one tooth for each turn of the micrometer head. The 
rim of a cup-shaped cylinder, r, Fig. 1, which is secured to this worm wheel so that it 
can be turned and clamped in any position relative to the zero point of Die micrometer 
head, has cut into it a notch with sloping ends and of a length corresponding to four 
teeth of the worm wheel, w, or four turns of the micrometer screw. From the end of 
a lever, /, Fig. 1, mounted against the side of the micrometer box, projects a small steel 
pin reaching over the rim of the cylinder, r. The other end of this lever carries a small 
ivory tip, which rests upon the end of a spring, b, Fig. 1, mounted on an ebonite plate 
and pressing at its middle point against a platinum-tipped screw, a. Whenever the 
small steel pin of the lever, /, rests in the notch of the cylinder, r, the spring, b, is in 
contact with the screw, a, and allows the flow of an electric current through the coiled 
wires, m and n, to the contact spring, But when the micrometer has been turned 
two revolutions to either side of its middle or zero position, and its motion is continued, 
the sloped ends of the notch in the cylinder, r, will engage the lever, /, and through it 
force the spring, b, away from the screw, a, thus breaking the current. It will be seen, 
*On the instrument as originally made and tested there were 10 contact strips, corresponding to 
the 10, 20, 30, 40, 50, 60, 70, and 80 division points of the graduation.