Conductivity of Molten Salt
Many salts can be melted in test tubes, and the conductivity of their melts measured with a low voltage device and either graphite "leads" from pencils, or TIG welding rods.
Conductivity of Salt Solutions
Compare solution conductivity of ionic and covalent compounds. Besides the traditional 120 V light bulb with probes in series (available from scientific suppliers), many conductivity devices have been suggested.