The field of applied electricity is such a wide one as to preclude any exhaustive handling of the subject in a book of this size. The aim has been to acquaint the young student with the basic principles of the science, and it is his part to develop these principles along the lines indicated in the preceding pages. But there are some practical applications that may be properly grouped under the heading of this chapter. They may serve as a stimulus to the inventive faculties of the youthful experimenter, and since the pieces of apparatus now to be described are useful in themselves, the time spent in their construction will not be wasted. A Rotary Glass-cutter When making a circle of glass it is generally best to let a glazier cut the disk, otherwise many panes are likely to get broken before the young workman succeeds in getting out a perfect one. But with a rotary glass-cutter the task is a comparatively simple one, and the tool is really an indispensable piece of apparatus in every electrician’s kit. (See Figs. 1 and 2.) The wooden form is turned from pine or white-wood, and is three inches in diameter at the large end, or bottom, one inch in diameter at the top, and two inches high. It is covered with felt held on with glue. Directly in the middle of the top a small hole is bored one-eighth of an inch in diameter, and in this aperture an awl or marker is placed, handle up, as shown in Fig. 2. Notice that the awl is not made fast to the form, but is removable at pleasure. A hard brass strip twelve inches long, five-eighths of an inch wide, and one-eighth of an inch thick is cut at the end to receive a steel-wheel glass-cutter, as shown at the foot of Fig. 1. A number of one-eighth-inch holes are bored along the strip, and half an inch apart, measuring from centre to centre. To cut a disk of glass the form is placed at the centre of the pane, the latter being imposed on a smooth table-top over a piece of cloth. The strip, or arm, is laid on the form, and over a small washer, so that one of the holes lines with that in the form. The awl is passed down through the strip and into the block, and the cutter is arranged in the slot at the end of the arm. Press down lightly on the handle of the awl, to keep the form from slipping; then the cutter is drawn around the glass, describing the circle, and cutting the surface of the glass, as shown by the solid line in Fig. 4. The disk must not be removed from the pane until the margin is broken away. With a straight-edge and a cutter score the glass across the corners, as indicated by the dotted lines in Fig. 4; then tap the glass at the underside along the line and break off the corners. After the corners have been removed tap the glass again, following the line of the circle; then break away the remaining fragments and smooth the edge. [Illustration: FIG. 1 FIG. 2 FIG. 3 FIG. 4 FIG. 5 FIG. 6 GLASS-CUTTING APPLIANCES] To Smooth Glass Edges