multimeter curiousity

[WHR]

I guess ACUV that it kind of comes down to the difference between a science and engineering perspective based on mathematics and theory, or a practical perspective, based on the soldering iron, the test point, the smoking transistor that just got overloaded, the fuse that blew. A machine is both it's physical parts and the principals that allow it to function. To be a good engineer or bench technician, I think you have to see things from both sides. It is hard to work in the practical world without seeing the physical component called a resistor and just simply saying "it has resistance" and confirming the value written on it against a test circuit. It defies common sense to think that the resistor *won't* behave as predicted or marketed or labeled or whatever barring a defect. But that is in itself misleading. If a resistor is frozen to near absolute zero or heated to its maximum operating temp, it will show the limitations of physical characteristics. But if everything is defined mathematically (assuming room temperature or 0 degrees C as the reference point) then all should be in tolerance and happy joy joy. The platinum 100 ohm scale resistance temperature device is a perfect candidate for such a model. It is DESIGNED to be variable with temperature. Resistance value increases as temp increases and vice versa. But the thing that is universal and unchanging (barring defect or operation outside of spec) is the meter that is measuring the transducer, it's stable calibration, and it's reference voltage. Thus, to reliably measure resistance we need a reliable voltage source and a current (albeit the current can be very very small so as to not damage components).

[John Cuthber]

OK, Here's a compromise.

I think that resistance is a genuine intrinsic property of an object. WHR seems to think it's a mathematical abstraction based on the ratio of a voltage to a current and is, therefore undefined when the current is zero.

 

However, in some cases such as tunnel diodes, the word "resistance" is used to represent the first derivative of voltage with respect to current: that's how you get negative resistances.

Here's a picture

 

http://en.wikipedia.org/wiki/File:Voltage_controlled_negative_resistance.svg

 

dV/dI is a better mathematical model than V/I because it includes these negative resistance cases and it's properly defined at zero volts.

 

WHR can have his abstract model- provided that he uses the more representative one and I can say that the resistance is properly defined at V=0 and is (for sensible resistors) the same as it is for other (moderate) voltages.

 

Incidentally, in saying that a bit of platinum wire has a resistance that varies with temperature, you are accepting that the resistance is an intrinsic property of that bit of wire, albeit one that varies with temperature.

[WHR]

Mr Cutter at this point any compromise is welcome, because I know that trying to hash out solid state theory will only lead to ten more pages of bantering. Active devices are just as predictable as passive ones but their operation is contingent upon current in the case of BJT and voltage in the case of vacuum tubes and MOS/CMOS. Just an entire can of worms where points can be made in both cases. But I will accept the compromise because arguing for the sake of arguing is not productive. I do believe in hard science.

[ewmon]

What about this.

 

A resistor is not in a circuit, yet IF its properties of resistance and conductivity do not currently "exist", but are considered — at some time/circumstance — to be attained/acquired/manifested, then they are 0 and/or ∞, but which is which and why, or are they some other values, or do they exist but are "undefined"?

[WHR]

What about this.

 

A resistor is not in a circuit, yet IF its properties of resistance and conductivity do not currently "exist", but are considered — at some time/circumstance — to be attained/acquired/manifested, then they are 0 and/or ∞, but which is which and why, or are they some other values, or do they exist but are "undefined"?

 

I have a better question. You are handed an electronic device that was designed and manufactured on a planet around Alpha Centauri. Assuming that most civilizations would arrive at the same fundamental principles...determine the operation of the circuit, the power required for its operation, and the normal operating impedance of all devices. I contend that accurate measurement techniques would be essential, one component at a time would have to be analyzed. If some components are assumed to be resistors, we can't know the resistance value based on a stamp or color code. We have to determine it with a meter of some sort. Once it is determined, you would have reasonable confidence that it's a stable, predictable value.

 

I do not contend that a resistor laying on a table doesn't have predictable characteristics. If not, tolerances couldn't be claimed, power dissipation couldn't be trusted, operating temperature couldn't be predicted. This value would not be open circuit (infinity) or shorted (0)....But look, even that terminology tells you something. If you place a single resistor in a circuit as the load without other devices, and measure the voltage across it, you will read the applied voltage wether the resistor is faulty and open or operational and 100, 200, 2k, or 5meg. We can't determine the resistance without knowing the current. If we add another resistance in series we can measure the voltage drops, and assuming the value of our reference resistor is accurate as predicted, we can with confidence work out the other resistors value.

 

The exception to the above rule would be a shorted resistor. Assuming such a device could withstand the power that it would be subject to, the voltage drop across it would be below supply voltage and in an ideal short would be a zero voltage drop. Essentially that means that there is no differential between the terminals of the supply when a short is between them. The differential instantaneously tries to correct itself.

 

Maybe a little off the topic but so has the entire thread

 

I actually do have a brain teaser. Lets drop the resistor stuff and have fun.

 

I have a diesel powered generator with a full tank. The engine is set to idle At 3600 rpm, it has a rotor connected to a magnet of known strength and dimensions within a coil with single phase configuration. Just sitting there cold and collecting dust in my garage, does the generator have voltage? All the variables are known, the specifications are set, even frequency.