First, get a medium sized potato and two nails. Have some small gauge wire available. Obttain a drinking glass that will hold the potato in an upright position. Stick the nails into the potato at one end and in the middle. Get a reading then from the wires that are attached to the nails sticking into the potato. H.H.H.
Another way is to connect two different materials, copper, aluminum, iron and place them into water (a glass of fresh water). For example you can take an iron nail and connect it to a piece of sheet of aluminum with a wire. You will see that aluminum corrode while the iron will not corrode. To increase the effect you can add some salt (NaCl) to the water.