Electrical conductivity is the ability for an item to conduct electricity. Things that conduct electricity are called conductors; those that don't are called insulators. Metals are what people usually think of as conductors, since that is what wires are made of. Pure water isn't a conductor, but dissolve some salts or other inorganic solids in it and it becomes one due to the ions in the dissolved salt.

Measurement

Conductivity is measured by the presence of resistance. The more resistance an item has, the less conductive it is. In wiring, the thicker the wire, the less resistance it has to current passing through it--this is why things that draw more power need thicker cords. Resistance is measured by an ohmmeter. Multimeters usually have an ohmmeter included.

Ohm's Law

Resistance is measured in ohms. Ohm's Law states that 1 ohm is the resistance through which 1 volt will maintain 1 ampere of current. Current is the energy flowing through a conductor, measured in amps, or amperes. Voltage is what pushes the current through the conductor, measured in volts. Power is volts times amps, measured in watts. If you know the volts and the amps or watts, calculating the ohms is a simple math formula. Ohms equal volts divided by amps, or volts squared divided by watts. Ohmmeters and multimeters have this formula built in. The meters work by sending a small current through what you are testing and measuring the voltage produced, then applying the formula and giving you a reading.

Basic Technique

Start by setting the meter to the highest level of resistance. Touch the leads together and verify that the reading is zero ohms, indicating no resistance. Touch or clamp the leads to the item that you want to test. If the reading is close to zero, then select a smaller range to get better precision. If you get no reading, then you have an open circuit and the item has no conductivity.

Testing Liquids

To test a liquid, attach the clamp extensions to the ends of your test leads, or use leads with clamp ends. Then put a wire strip into each of the clamps. This protects your leads in case the liquid is corrosive. Touch the wire strips together to verify that you have a good connection and no resistance before putting the strips into the liquid you are testing. Conductivity testing is one of the ways that water is tested for purity. The more dissolved inorganic solids there are in the water, the higher the conductivity. The temperature of the water also affects the conductivity--the warmer the water, the more conductive it is.

Source:

All About Circuits

EPA.gov

Resource:

Ohm's Law