Another method is to lay the glass on a table, upon a piece of felt or cloth, and allow the edge to project over the table for two or three inches. Hold the glass down with one hand to prevent its slipping; then, with a piece of corundum, or a rough whetstone and glycerine, work down the edge until it is smooth, turning the glass continually so that the edge you are working on hangs over the table. This process of grinding is somewhat tedious, but perseverance and patience will win out.
To Cut Holes in Glass
Holes may be cut in glass in several ways by an expert, but the boy who is a novice in this line should stick to slow and sure methods and take no chances. Fortunately, glass is little used in voltaic electricity, but it is indispensable in the construction of the frictional machines, Leyden-jars, and condensers, where glass is used as the dielectric, also for the covering-plates to instruments.
The simplest method is that of rotating a copper tube forward and backward over the glass, using fine emery dust for the cutting medium and oil of turpentine as a lubricant. The copper tube must be held in a rack, so that its location will not shift during the rotating or cutting motion. The rack in which the tube is held may be of any size, but to take a disk or square of glass, twenty inches across, the frame should be twenty-two inches long, ten inches wide, and twelve inches high, as shown in [Fig. 3].
The side-plates are eleven inches high and ten inches wide, the top is twenty-two inches long and ten inches wide, while the under ledge is twenty and a quarter inches long by ten inches wide. This frame is put together with glue and screws. Across the back, from the corners down to the middle of the under ledge, battens or braces are made fast to prevent the frame from racking. A hole is made through the middle of the top and under ledge for the copper tube to pass through. If different-sized tubes are to be used, blocks to fit the top and under board are to be cut and bored, so that they may be held in place with screws when in use. To cut a hole in glass, place the disk or pane on a felt or cloth-covered table, and over it arrange the frame, so that the tube will rest on the spot to be drilled. Drop the copper tube down through the hole, having first spread the bottom of the tube slightly, so that it will not split the glass. Now put some emery inside the tube so that it will fall on the glass; then place a wooden plug in the top of the tube and arrange an awl, or hand-plate, so that the tube may be pressed down. Take one turn about the tube with a linen line, or gut-thong, and make the ends fast to a bow, so that it will draw the string taut but not too tight. Lubricate the foot of the tube with oil of turpentine, and draw the bow back and forth. At first the motion will cause the copper to scratch the glass, and then cut it, until finally a perfectly drilled hole is formed. During the operation both glass and frame must be held securely, and the bow drawn evenly and without any jerking motion. Holes of different sizes may be cut with tubes of various diameters. Small holes may be cut with a highly tempered steel-drill and glycerine, the drill being held in a hand-drilling tool or in a brace.
Anti-hum Device for Metallic Lines
In overhead wires, where galvanized or hard copper wire is used, the hum due to the tension of the wires, and the wind blowing through them, causes a musical vibration which becomes most annoying at times. This can be overcome by a simple device known as an “anti-hum.” It consists of a knob made of wood or rubber, through which a hole is bored, and around which a groove is cut. One end of the wire is passed through the hole and a loop formed, the loose end being wrapped about the incoming wire. The other end of the line is passed around the knob in the groove, and the end twisted about the line-wire. The knob is then an insulator and a sound-deadener at the same time. To complete the metallic circuit a loop of wire is passed under the knob, the ends of which are made fast to the line-wires, as shown at [Fig. 7].
