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- [Voiceover] Most solids
offer some amount of resistance

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to the flow of current through them.

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This allows us to define
things like the resistivity,

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or the conductivity, but the
same is true for liquids.

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Consider this container full of a liquid.

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We can measure its resistivity.

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Now, if I took a battery,
and I put one lead here,

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and one lead here, if there
is a voltage and this liquid

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is able to conduct electricity,
then this current should

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be able to flow through
the intervening liquid

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over to the other side and then back up.

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Sometimes, this is done
with AC current, otherwise,

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you might get electrolysis and
then you get bubbles in here

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and that changes the liquid in some way.

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We want to measure the
resistance and the resistivity

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of the liquid, not of some altered liquid.

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So, sometimes you use AC, but
this is the general principal.

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Send in a voltage, a certain
amount of current will flow.

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How can we use that, to
determine the resistivity?

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Well, we know resistivity
is equal to the resistance

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that we measure times the
area divided by the length,

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and now you see there's
kinda of a problem length.

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I can imagine getting that.

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This length in here would
just be this distance.

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There's my length, because my "resistor,"

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is this liquid in here.

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But, what's my area?

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So, this would be a bad experiment to do.

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If we want to measure the
resistivity, what we really want,

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is something where we
have a well defined area.

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Let me get rid of this.

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Imagine you had two plates.

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Take these two plates.

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You put them in the solution
you want to measure the

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resistivity of, so, we put them in here.

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Stick them into there.

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They have a well defined area.

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We've got those.

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We can measure those if we want.

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We set them apart some known
distance between them, L,

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and you hook them up to battery.

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So, take this one, hook
it up to a known voltage,

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hook the other side up to the other plate,

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and if this solution, if this
electrolytic solution in here

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can conduct electricity,
current will flow from this side

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to the other side, and you
can measure these quantities.

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You measure the length, that's easy.

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You measure the area.

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You got that.

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How do we measure the resistance?

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Well, we know the voltage.

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We can have a known voltage
of the battery up here,

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and you can stick ammeter in
here to measure the current.

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If I stick an ammeter,
ammeters measure the current.

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Now, I can just use Ohm's
law, and I know that

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the resistance is just going
to be the voltage divided

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by the current, and if plug
all these values into here,

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I can get an experimental value for the

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resistivity of this liquid,
sometimes it's called

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the electrolytic resistivity.

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Or, one over the electrolytic
resistivity would be

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the electrolytic conductivity.

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So, this would be the
electrolytic conductivity.

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So, this is an experimental way to do it.

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Honestly, you don't
even have to go through

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all that much trouble.

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You can just take a solution.

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First, put a solution in here
that has a known resistivity.

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That way, you can just do
this: R equals Rho L over A.

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If you know the resistivity,
and you can easily

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measure the resistance,
then you can just figure

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out what this constant is,
and this will stay the same.

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You just leave those
same plates in there with

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the same length and the same area.

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Put a new solution in there,
and that gives you this number,

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and this number staying the same.

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So, technically speaking, you
don't have to go in there,

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measure the area each time, and the length

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between these each time,
if you have some calibrated

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electrolytic solution where
it has a known resistivity.

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Or, you could use it the other way.

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If you had a solution
with a known resistivity,

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but there may be impurities
in there, or there

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may be dissolved salts,
or something, and you

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want to know what the concentration is.

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Well, that is going to
directly affect how much

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current will flow, and
it will directly affect

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the measured electrolytic conductivity.

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So, if you measure this,
and it comes out different

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from what you would expect
from a baseline solution.

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You can figure out what the concentration

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is of the conductive impurities
within this solution.