Resistance wire
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Resistance wire is wire intended for making electrical resistors (which are used to control the amount of current in a circuit).<ref>"Resistance Wire".</ref> It is better if the alloy used has a high resistivity, since a shorter wire can then be used. In many situations, the stability of the resistor is of primary importance, and thus the alloy's temperature coefficient of resistivity and corrosion resistance play a large part in material selection.
When resistance wire is used for heating elements (in electric heaters, toasters, and the like), high resistivity and oxidation resistance is important.
Sometimes resistance wire is insulated by ceramic powder and sheathed in a tube of another alloy. Such heating elements are used in electric ovens and water heaters, and in specialized forms for cooktops.
Types
Nichrome, a non-magnetic 80/20 alloy of nickel and chromium, is the most common resistance wire for heating purposes because it has a high resistivity and resistance to oxidation at high temperatures, up to 1,400 °C (2,550 °F). When used as a heating element, resistance wire is usually wound into coils.
Kanthal (Alloy 875/815), a family of iron-chromium-aluminium (FeCrAl) alloys, is used in a wide range of high-temperature applications, up to about 1,425 °C (2,597 °F). One difficulty in using nichrome or kanthal wire is that common tin-based electrical solder will not bond with it, so the connections to the electrical power must be made using other methods such as crimp connectors or screw terminals.
Constantan (Cu55Ni45) has a low temperature coefficient of resistivity; and as a copper alloy, it is easily soldered. Other constant-resistance alloys include manganin (Cu86Mn12Ni2), Cupron (Cu53Ni44Mn3)<ref>https://www.cartech.com/ssalloysprod.aspx?id=1940 Archived 2016-07-20 at the Wayback Machine Carpenter Technology, CBX Cupron® Alloy</ref> and Evanohm. Melts at about 1,220 °C (2,230 °F).
Balco (Ni70Fe30) and similar alloys have very high, but more linear, temperature coefficients of resistivity, making them suitable for sensing elements.
Many elements and alloys have been used as resistance wire for special purposes. The table below lists the resistivity of some common materials. The resistivity of amorphous carbon actually has a range of 3.8 – 4.1 × 10−6 Ω m.
| Material | Resistivity | |
|---|---|---|
| (ohm-cmil/ft) | (10−6 ohm·cm) | |
| Aluminum | 15.94 | 2.650 |
| Brass | 42.1 | 7.0 |
| Carbon (amorphous) | ≃23.8 | ≃3.95 |
| Constantan | 272.97 | 45.38 |
| Copper | 10.09 | 1.678 |
| Iron | 57.81 | 9.61 |
| Manganin | 290 | 48.21 |
| Molybdenum | 32.12 | 5.34 |
| Nichrome | 675 | 112.2 |
| Nichrome V | 650 | 108.1 |
| Nickel | 41.69 | 6.93 |
| Platinum | 63.16 | 10.5 |
| Stainless steel (304) | 541 | 90 |
| Steel (0.5% carbon) | 100 | 16.62 |
| Zinc | 35.49 | 5.90 |
Trade names
Trade names include:<ref>"Resistance Wire".</ref>
| MWS Wire Ind. | Carpenter Tech. | Driver-Harris | Harrison | Hoskins | Jelliff | Kanthal |
|---|---|---|---|---|---|---|
| MWS-875 | Alchrome 875 | HAI-FeCr AI 25 | Alloy 875 | Kanthal A-1 | ||
| MWS-800 | Evanohm | Karma | HAI-431 | Chromel R | Alloy 800 | Nikrothal L |
| MWS-675 | Tophet C | Nichrome | HAI-NiCr 60 | Chromel C | Alloy C | Nikrothal 6 |
| MWS-650 | Tophet A | Nichrome V | HAI-NiCr 80 | Chromel A | Alloy A | Nikrothal 8 |
| MWS-294 | Cupron | Advance | HAI-CuNi 102 | Copel | Alloy 45 | Cuprothal 294 |
| MWS-180 | 180 Alloy | Midohm | HAI-180 | Alloy 380 | Alloy 180 | Cuprothal 180 |
| MWS-120 | Balco | Hytemco | HAI-380 | Alloy 120 | ||
| MWS-90 | 90 Alloy | #95 Alloy | HAI-90 | Alloy 290 | Alloy 90 | Cuprothal 90 |
| MWS-60 | 60 Alloy | Lohm | HAI-60 | Alloy 260 | Alloy 60 | Cuprothal 60 |
| MWS-30 | 30 Alloy | #30 Alloy | HAI-30 | Alloy 230 | Alloy 30 | Cuprothal 30 |
