| Chronology | Current Month | Current Thread | Current Date |
| [Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
"Bernard G. Cleyet & Nancy Ann Seese" wrote:
People!After a bit of thought, I think I know why there is confusion about "ohmic" conductors.
My ed. of the CRC doesn't define "ohmic." However, Harnwell (2nd Ed. '49) devotes some time to conductors. For ex.: The resistivity of a metal is a constant independent of the current density (i/a) over a very wide range. As an example, Au has no appreciable change in resistivity up to 10E6 A/cm2, and only a few % change @ 10X that, (paraphrased p.93)."Conductors for which this linear relationship exists between the current density and the field strength are said to be linear or ohmic conductors.", (same page, my emphasis).Harnwell then discusses why (theory), and contingent and intrinsic nonlinear elements, (including ballasts -iron alloy in H2, carbon and W fil. lamps, semi-cond's, thermistors (a mixture of semiconductors), and thyrite. Others later discussed as non ohmic conductors include gas discharges and vacuum tubes (before transistors!).The confusion is between conductive circuit elements and conductors. I have always assumed that when one measures a physical quantity, one keeps all variables constant, except those of interest. So if one wishes to measure resistivity, one doesn't shine light on it (important for selenium, aside: prob. first use of a photo conductor was bars of selenium to automatically turn on light house lamps), doesn't heat it or cool it, etc. The independent variable is elec. field, the dependent, current density. If one wishes to measure a circuit element, one measures it in the manner of interest. Don't confuse ohmicity of a material and the material used in a circuit element. A lamp includes a tungsten filament. Tungsten is an ohmic conductor over a wide range of applied electric field, however as used in a lamp, it is contingently non linear!I hope this clears up the confusion.
More:
one of you suggests that one may demonstrate ohm's law. using lamps at approx. 10% of normal "voltage." the idea, of course, is correct, however, I suggest either one must use much lower voltages, or, better, use high power resistors (heat sinked and Ni-Cr wire, or similar low temp. coefficient of resistance). From data I acquired yesterday of two 60w. lamps (std. Sylvania and a NALCO 15 CP -- carbon in vacuum), *R/*V (ohns/volt):
* my draft shows * as delta, but on return becomes an asterix.
60 W. Sylvania Lamp *R/*V (ohms/volt) @ Voltage (V.) = 4 ohms/volt 0.5 ~ 15 V = 1.7 50 = 1.6 75 = 1.0 100 = 1.0 120 = 1.0 135
Carbon data later,
bc