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Nichrome Wire: Nickel and Chromium Fused, Drawn and Annealed
Nichrome is the heating wire of toasters and irons. Albert Marsh patented it in 1906: nickel with less than 25 % chromium, drawn into wire and annealed, with about fifty times the resistance of copper and great resistance to oxidation when hot. Iron, he wrote, will not do. Elwood Haynes (1907) fused pure, carbon-free nickel and chromium in a magnesia crucible under an oxyhydrogen flame. White heat: face shield and gloves.
The picture: Marsh's patent drawing, a rheostat wound with the alloy (US 811,859).
Advanced
3 days
Instructions
1
1
Weigh 85 nickel to 15 chromium
Weigh 85 nickel to 15 chromium
Cut 85 g of nickel sheet into small pieces and weigh 15 g of chromium: Marsh's own example. Both must be pure and free of carbon.
Materials for this step:
Nickel Sheet85 g
Chromium Metal15 gTools needed:
Metal Shears
Balance Scale2
2
Fuse under the oxyhydrogen flame
Fuse under the oxyhydrogen flame
Put the metals in a magnesia crucible, set it in a furnace lined with magnesia, and heat it with the oxyhydrogen blowpipe until both have melted together.
Tools needed:
Magnesia Crucible
Magnesium Oxide
Oxyhydrogen Blowpipe
Crucible Tongs
Face Shield
Leather Gloves3
3
Cast a rod
Cast a rod
Pour the alloy into a narrow mould to get a rod that will start in the largest hole of the drawplate.
Tools needed:
Ingot MouldCrucible TongsFace ShieldLeather Gloves4
4
Draw and anneal
Draw and anneal
Draw the rod down through the drawplate one hole at a time, as in the drawplate blueprint, and anneal it in the furnace whenever it gets hard. Marsh gives no annealing temperature. Stop at 0.41 mm (0.016 in).
Tools needed:
Steel Drawplate
Draw Tongs
Wire Drawing Lubricant
Charcoal Furnace
MicrometerLeather Gloves5
5
Check the resistance
Check the resistance
Measure one metre with the multimeter. Marsh's 15 % wire of this size measured about 2.3 ohms per foot, which is 7.5 ohms per metre.
Tools needed:
MultimeterMaterials
2- 85 gPlaceholder
- 15 gPlaceholder
Tools Required
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