Measuring electric conductivity My daughter is doing a science experiment on which metal (e.g., copper, silver, aluminum, iron) has maximum electric conductivity. We are assuming we can accomplish this by using different metal, 3 to 6 volt electrical circuits, and a basic digital multimeter.
How we can do it? Do we have to measure resistance of this metal? Or is there a different way. Please guide us.
 A: Conductivity is given by $$ \sigma=\frac{l}{AR} $$ where where $l$ is length, $A$ is cross sectional area and $R$ is resistance.
Therefore if you can measure these three things for each metal you can calculate their conductivities.
A: You have to use the equation for resistivity: $$\rho=R\frac{A}{l}$$ where $R$ is resistance, $\rho$ is electrical resistivity, and $a$ is area. So, for all the metals, I would use the same length and area in order to keep $l$ and $a$ constant.
In order to find the resistance, $R$, for the metals, you need to use a potentiometer, or variable resistor, and plot a graph of voltage vs. current in order to determine the resistance of the metals. In order to accomplish this though, you need to have a voltmeter attached parallel to the potentiometer and an ammeter in series with the circuit. Since resistance is voltage divided by current, just divide the voltage by the current on any point on the graph.  
Now that you have $l$, $a$, and $R$, you can solve for $\rho$, electrical resistivity. In order to find conductivity, you simply take the inverse of the resistivity, $\rho$, value.
A: Apart from the mathematics already given, and considering more practical issues- you will not see much difference between all the metals you listed by measuring with a cheap multimeter. If you throw in carbon and silicon - then yes you will see a difference.
All the metals you listed will all read zero ohms resistance on a cheap meter. And conductivity is just the reciprocal of resistance often written in units of mhos. So infinite conductivity! (But not really, there is a difference you just can't detect it). You need to either get very long (tens of meters), thin specimens of the metals (coils of insulated metal wire for example) or you need a more sensitive (and expensive) ohmmeter that resolves in milliohms (1/1000's of an ohm). 
In any event you'll either need to make all the samples the same so it's not the geometry determining the resistance as noted in the other answers, but rather the resistivity of the particular metal, $\rho$.
A: This adds to the other answers that explain the basic concept behind the measurement. 
Consider that the conductivity of all these materials is pretty high.
To give you a starting point, 1 meter of 26 AWG copper wire will be about 0.13 ohms. This is a reasonable value to measure, but does require some care to measure accurately (for example, accurately enough to distinguish copper from silver). Depending on the samples you have available of your different materials you might need to measure even lower resistance values than that. 
If you do a naive measurement with a two-probe ohmmeter, the contact resistance between the probes and your samples is likely to disturb the measurement. 
In that case, you will want to do a four-wire resistance measurement to prevent the probe wires and contact resistances from being measured as if they are part of your samples.
Another thing to be aware of when interpreting your results is that the resistivity of these metals can depend on how they are alloyed and how they are treated. For example, annealed copper and work-hardened copper will have different resistivity values. This difference could be enough to overwhelm the difference between the materials themselves, for example if you compared a sample of silver wire from a jewelry-maker's supply store with a sample of copper wire intended for electrical use.
