home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: sci.physics.fusion
- Path: sparky!uunet!wupost!gumby!yale!yale.edu!ira.uka.de!sol.ctr.columbia.edu!eff!world!mica
- From: mica@world.std.com (mitchell swartz)
- Subject: silver (and other) wires
- Message-ID: <Bz5zG6.3pI@world.std.com>
- Organization: The World Public Access UNIX, Brookline, MA
- Date: Sat, 12 Dec 1992 20:55:17 GMT
- Lines: 112
-
-
- There has been, and continues to be, much talk on this forum
- regarding nichrome and silver conductivity.
- Some of the calculations (actually or rhetorically) requested
- have been trivial, but the 'debate' continues, and so this table
- and elementary exercise are provided.
-
- For example [sci.physics.fusion:4480; Jed Rothwell; Subject:
- Silver the same everywhere in the world; Date: Wed, 9 Dec 1992] states:
-
- = "the heater is: Ni-Cr two-ply wire (0.1 mm diameter, 40 mm length).
- = Just find out whether it is possible for any silver wire to generate
- = as much heat as it does."
-
- or [sci.physics.fusion:4515; Subject: What Jones said; 11 Dec 1992]:
-
- = "Technical type people know damn well that the only way 6 cm of one wire
- = can dissipate as much heat as 4 cm of another wire is if both
- = have approximately the same resistance. They also know that it would take
- = an impossibly thin silver wire to be as resistant as a 0.5 nichrome wire".
-
- Now that Prof. Droege has vaulted this forum into discussions of
- electrodynamics, an elementary reminder of practical electrostatics
- is in order. Impossibly thin? How thin --->
-
-
- The electrical resistance (lumped parameter) characterizing a
- rod-like material is a function of the electrical resistivity,
- the physical length, and cross-sectional area (presumed circular,
- with total homogeneity of material properties therein) as in
-
- rho * L = Resistance [ohms]
- --------
- A
-
- So the resistivity [rho: ohms-cm] {micro-ohms-cm in the table below
- which has the various materials listed by nichrome type
- (including the constitutional elements composing the various types),
- a common variety, and of course silver. Temperature coefficients
- of the electrical resistance, and the effective temperature of that
- Taylor series simplification are also listed.
-
-
- The nickel-chromiums were discovered in 1905 by Marsh, from which
- the electrical heating industry became generated. A high resistance
- to oxidation, diverse patents based upon slight additions to the mix,
- and a need for such materials have generated diverse alloys.
-
- Notwithstanding the above, the electrical conductivity of a few
- can be compared to silver.
-
- ELECTRICAL CONDUCTIVITY OF A FEW MATERIALS
-
- [based in part upon Chemical Rubber Tables, and Engineering Materials
- Handbook (Mantell) and Rare Metals Handbook (Hampel) ]
-
- ---------------------------------------------------------------------
- |MATERIAL| | microhm-cm | Temp coef | Temp Range |
- ---------------------------------------------------------------------
- Ni | Cr | Fe | | |
- ---------------------------------------------------------------------
- 100 | 0 | 0 | 7.8 | | 20C |
- ---------------------------------------------------------------------
- 0 | 100 | 0 | 13 | | 20C |
- ---------------------------------------------------------------------
- 0 | 0 | 100 | 10 | | 20C |
- ---------------------------------------------------------------------
- 80 | 20 | 0 | 108 | 0.00011 | 20-500C |
- ---------------------------------------------------------------------
- 60 | 16 | 24 | 112 | 0.00015 | 20-500C |
- ---------------------------------------------------------------------
- 35 | 20 | 45 | 100 | 0.00036 | 20-500C |
- ---------------------------------------------------------------------
- Inconel X (annealed) | 122 | [m.p.1399C] | |
- ---------------------------------------------------------------------
- Silver| | | 1.59 | [m.p. 960C] | 20C |
- --------------------------------------------------------------------
-
- In short, in can be seen that the relative electrical resistivity
- for nichrome to silver (in this simple matter) is:
-
-
- rho (Nichrome)
- ------------- = 69.5 (average)
- rho (silver)
-
- with a range of 62.89 to 76.73
-
- for this exercise. Therefore, one can use 70 as an general ratio.
-
- How thin must a silver wire be, to equal the nichrome wire?
-
- Hence a silver wire must be a little less than 1/8th the diameter
- of a nichrome wire to yield a relative area of 1/70th, so as to insure
- an equivalent lumped parameter resistance, for an equal length of wire.
-
- Other length-area-resistivity problems are similarly calculated.
-
- Disclaimer: Of course, this ignores contact potentials, inhomogeneities,
- method of contacting said wire, surface vs. volume conductions, and
- the impact of electric and magnetic fields, &tc.
-
-
- As for melting, the melting temperatures of the respective materials
- are listed in the Table above. The next interested respondent
- is welcome to estimate the thermal barriers surrounding this gendanken
- wire, given any putative applied (thermally dissipative) electrical load.
-
- ===================================================================
- Mitchell R. Swartz JET Technology
- mica@world.std.com (617) 239-8383
-
-