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BROKEN ARC STOPS CURRENT.When the increasing distance between tip and flashlight becomes too great for the
arc to continue, the arc breaks. Immediately, the magnetic
field around the coil collapses, and the core releases its electromagnetic hold on the tip. Tension on the flexed metal arm
then snaps the tip back onto the flashlight. And the cycle
begins all over, repeating itself many times each second.
It is the intense heat of the bluish arc that enables the pencil to write. As the pencil moves along, the arc actually scorches the surface of the metal and thereby leaves a lasting trace.
Things You Need
HANDLE: wood dowel, 1 by 5 (the top of an old broomstick would be perfect). CORE: iron bolt, 1/4 by 1-3/4. COIL: spool of #22 or #24 solid hook-up wire. ARM: tin-can strip, 1/2 by 3. TIP: flat-head machine screw and nut, 1/8 by 3/4. BATTERY: 6 volts (12 volts would be even better, but not necessary). MINOR ITEMS: 3/4 outer-diameter washer, 2 small screws, spring clamp.
How to Make the Parts
THE HANDLE AND THE CORE. Start by rounding off one end of the wood rod, if not already round. Drill the core hole in the other end as shown in the drawing. Make it slightly less than l/4 in diameter and about 1 deep. After slipping the washer around the iron bolt. twist the bolt into this hole. A little wax or soap on the threads will help. Leave 1 between the handle and the washer.
For a neat, professional-looking job, it would be nice to have a wire hole extending clear through the handle from the flat end to the round end. Use a 1/8 drill; bore-in half way from both ends. If you cant perform this operation, dont worry. You can run the wire along the top instead, holding it in place with tape or string.