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Constantan Resistance Wire from Copper and Nickel
Constantan is a copper-nickel alloy, usually 55 % copper and 45 % nickel, whose electrical resistance stays nearly the same as it warms; that is why strain gauges, thermocouples and resistance coils are wound with it. Copper and nickel melt together in any proportion, so the alloy is made by fusing the two, casting small sticks and drawing them into wire.
The melt follows C. Morfit's copper-nickel alloy as Brannt gives it in *The Metallic Alloys* (1889): nickel and an equal weight of copper fused with borax in a graphite crucible, the rest of the copper stirred in. Hiorns (1912) adds that the nickel must be as pure as possible, which is what the Mond process gives.
高级
Foundry day, then drawing
说明
1
1
Weigh the charge
Weigh the charge
For 1 kg of 55/45 constantan weigh 450 g nickel and 550 g copper, and set aside 100 g of the copper for the second addition. Use nickel as pure as you can get: Hiorns warns that silicon, carbon, iron and sulphur in it greatly reduce how well the alloy works.
此步骤所需材料：
铜锭550 克
羰基法镍450 克所需工具：
天平2
2
Fuse nickel with an equal weight of copper
Fuse nickel with an equal weight of copper
Put the nickel and an equal weight of copper (450 g of each) into a graphite crucible with some borax, and fuse them in the furnace. This is Morfit's first stage: nickel melts far hotter than copper, and melting it among copper brings it down sooner. Constantan itself melts at about 1,220–1,300 °C.
此步骤所需材料：
硼砂粉1 as required所需工具：
石墨黏土坩埚
木炭炉
坩埚钳
耐热手套
皮革围裙3
3
Stir in the rest of the copper
Stir in the rest of the copper
Add the remaining 100 g of copper to the melted mass, stirring constantly with an iron rod until it has gone in and the metal is one even liquid.
所需工具：
铁制搅拌棒木炭炉坩埚钳耐热手套4
4
Cast small sticks
Cast small sticks
Pour the alloy into a warmed ingot mould as small sticks, as Morfit did. Pour in one run; the alloy chills quickly.
所需工具：
锭模坩埚钳耐热手套皮革围裙5
5
Anneal the sticks
Anneal the sticks
Anneal the cast sticks before working them: cast copper-nickel is crystalline, and Hiorns gives careful annealing at a moderate temperature as the cure.
所需工具：
退火炉坩埚钳耐热手套6
6
Draw it like copper wire
Draw it like copper wire
Forge a stick into a round rod, file a taper and draw it through the drawplate hole by hole, as copper wire is drawn. Never take the size down too fast: Hiorns warns that metal drawn without annealing breaks unless it rests five to ten minutes between passes. Anneal it as the copper wire is annealed. Simmons's strain gauges used No. 40 gauge, about 0.08 mm.
7
7
Measure the resistance
Measure the resistance
Measure the diameter in several places, cut a measured length and find its resistance on a Wheatstone bridge. Compare the result with the ohms per metre in the next step. A wire that reads much higher than expected has a thin place or a crack in it.
所需工具：
千分尺
惠斯通电桥8
8
Charge and resistance figures
Charge and resistance figures
正在加载 Jupyter 笔记本…
9
9
Troubleshooting
Troubleshooting
Constantan wire troubleshooting.
Flow
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10
10
History and honest limits
History and honest limits
In 1887 Edward Weston found an alloy whose resistance falls as it warms, a property, he wrote, of no other metallic conductor he knew. Wikipedia says his "Alloy No. 2" was produced in Germany and renamed Konstantan. By 1912 Hiorns listed Constantan, Eureka and Heckenham metal among the copper-nickel alloys used for resistances.
Morfit's alloy was 33 nickel to 67 copper; we keep his two-stage method and scale it to 55/45, the usual constantan and Simmons's wire. That scaling is ours. The 55/45 figure and the resistivity are from Wikipedia's Constantan article. Very fine wire like No. 40 gauge needs drawplate holes that fine.
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