INVISIBLE WIRE

Evening Post, Volume CXIV, Issue 121, 18 November 1932, Page 3

INVISIBLE WIRE

A TRIUMPH OF FINE DRAWING

Tungsten filament wire used in newly produced sis-watt incandescent lamps is so. fine it is practically invisible. Only four tenrthousandths of an inch in diameter, it is a fifth ' thick as a fine hair. After it is coiled 1390 turns per inch, to form the spring-like lamp filament, the outside diameter of the coil is still less than that of human hair.. ' .

The wire is drawn from a bar of tungsten three-eighths of an inch square, 24 inches long, and weighing 21 ounces. The bar passes through 95 dies, each with an aperture slightly smaller than the preceding one. Nearly two-thirds of these dies are rough-cut diamonds in which tiny holes have been drilled.

A bar of this metal stretches into wire 207 miles long, enough to provide filament for 666,666' lamps. Two weeks of 48 working hours each are necessary to draw one bar into the finished filament wire. In the final drawing the wire moves at'a speed of 105 ft per minute.

One of the biggest problems confronting modern wire drawing is the supply of diamond dies. Trevorx, a little town in Central France, is the centre of the world supply of these dies. Almost, the entire popula ">r. there is engaged in this tedious task, having inherited their skill. All day long they sit at manually operated machines drilling diamond dies of all sizes. Cheap labour and a skill founded on years of experience enable the people 01 Trevoux to maintain their position of prominence in this strange art. When being drilled, the diamonds are placed in revolving spindles, arid hard steel needles, which have been dipped in diamond dust, press against them at the ppint designated for the hole. The hole is conical in shape and is drilled exactly half-way through from each side of the diamond. Where the cones meet, barely enough to break through, the tiny hole forms the die.

Six-watt filament wire in its final drawing passes through a drilled diamond which is about the size of a pin. point. So delicate is the task of drilling a tiny hole in a more speck of a diamond that often as many as 30 attempts are made before a good die results-