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tony12 tony12
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12 years ago
I built a circuit with a 6v battery and it has wire attached to a light bulb and then it leads to a certain material like silver. How would u measure the conductivity of the silver coin with a multimeter?
would the info(conductivity) be for a steel nail, paper clip, small hollow copper tube and Nichrome wire ( used for toasters)
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wrote...
#1
12 years ago
Resistance, which is what the multimeter will measure, of a piece of silver is too low for the multimeter, it will be a small fraction of an ohm.

It also depends on where on the coin you want to measure resistance between. The two faces or one edge to the other.

The only way you can do this is with a large battery and a resistor adjusted to send a large current through the coin, say 10 amps. then measure the voltage drop in millivolts, and calculate the resistance as R = V/I. This will require careful work, as it is not easily done.

Once you have the resistance you can calculate the conductance as G = 1/R in mhos.

Then from the dimensions of the coin and the conductance you can calculate the conductivity of silver.

for your information
resistivity (?) of Ag is 15.9e-9 ?-m

that means the conductivity is 1/? or (1/15.9)e9 mhos/meter or 6.29e7 mhos/meter

let me know if you want to continue with this. As I said measurement of very low resistance requires special equipment and lots of care to get an accurate number.

It would be a lot easier to measure with a long thin silver wire as the resistance would be a lot higher.

.
wrote...
#2
12 years ago
Clamp the coin between two terminals, one against each face of the coin. Set up the circuit and use your multimeter to measure the voltage, V, across the coin. The voltage drop will be small, so you will need a sensitive meter.Then open the circuit, put the multimeter in series and measure the current I in the circuit. Now, since V = IR, you can calculate R, the resistance of the coin in ohms. You may also be able to measure the resistance of the coin directly using the meter's setting for measuring resistance in ohms. It will be a very low number and the multimeter may not be useful for this purpose.

Once you know the resistance, calculate the conductance as 1/R. It is sometimes measured in mhos(mho = ohm backwards, no kidding). However, the official SI unit of measure for electrical conductance is the siemens (S) which is defined as an inverse ohm (ohm^-1). To convert conductance to conductivity, multiply by the area of the coin and divide by its thickness. Conductivity, ?, is the reciprocal (inverse) of electrical resistivity, ?, and has the SI units of siemens per meter (S·m-1) and CGSE units of inverse second (s?1). See:

http://en.wikipedia.org/wiki/Siemens_%28unit%29

If you have an actual silver coin, it will likely contain about 90% silver, so your answer will be less than the conductivity of pure silver, which has the highest conductivity of any metal at 6.3 x 10^7 S-m^-1. For more on conductivity, see:

http://en.wikipedia.org/wiki/Electrical_conductivity

The above reference is a bit technoiud so here are some others:

http://www.newton.dep.anl.gov/askasci/eng99/eng99507.htm

http://www.ehow.com/how_2046943_test-conductivity-metal.html
wrote...
#3
12 years ago Edited: 12 years ago, bio_man
Conductivity is just the reciprocal of resistance, so 1/ohms. The unit is called a Siemen or a Mho. However the conductivity of a material is usually measured per unit volume, so siemens per cubic meter or something appropriate for the measurement. The unit is based on length, because it takes into account the cross sectional area (see the link on resistivity below). It is measured with rods, bars and wires, so siemens per meter etc. The resistivity is in ohm.meters.

Conductivity or resistance are measured by applying ohms law. Thus a known current is passed through the material or object, and the voltage drop across that object is measured. Then R = V/I.

Coming to the coin, as a rod or bar it is very short compared to the cross sectional area, so the measured conductivity would be very high (the resistance is very low). The errors would be very high too, because of the uncertainty in making a good contact on either side. The unknown contact resistance and uncertainty about what area is actually connected would be much more than the actual coin resistance. Even determining the actual length would be an issue, because the faces are not flat. Thinking further, one way would be to make the coin into a long wire of constant cross section and small diameter so the errors could be reduced. If it is pure silver or copper etc, the figure can be determined by looking it up as these figures are well known. The first link below has many materials, and also compares them with copper (100%).

The answer is, it is not really practical to directly measure the coin resistance or conductivity with any certainty at all. If I was obliged to try, I would try to use spring loaded "bed of nails" contacts on each side, with hundreds of point contacts positioned by springs. These would hae to take in the rim even. Demanding gadget to make. The "length" would have to be averaged. That could be done through the displacement volume in water, and the diameter. The actual conductivity meter (ohm meter) would have to be a specialised type for very low resistance, and use a so called Kelvin connection to reduce errors. Still a lot of room for errors, and a correction would need to be made to the actual area (number of pins) contacting, but I think repeatable results could eventually be obtained.

As a method of determining the purity of silver it would not be very helpful. It would make more sense to measure the specific gravity (weight per unit volume) albeit there are problems, or some sort of chemical analysis. Most modern coins have little or no silver.
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