Electricity travels in circuits conducted by the ions in metal, salt, minerals and acid. Batteries harness this conductivity with their composition: most batteries are iron or copper metal tubes filled with a very corrosive acid. When placed in a machine, the batteries complete an electrical circuit and make the machine work. Many types of fruit can do the same thing, though some do it better than others. By conducting fruit clock experiments, students can discover which fruits conduct electricity the best and why.
Making two-fruit clock batteries is very simple with a two-potato clock kit. The kit contains a small, digital clock, with plastic cups protruding from either side, and three connecting wires. Students place a whole fruit in one of each of these cups, insert the red wire into the left-hand fruit, the blue wire into the right-hand fruit, and one end of the yellow wire into each fruit. If the fruits conduct electricity, the clock will turn on. Students should try using two of one kind of fruit first: two lemons, two limes, two strawberries, two tomatoes, two kiwifruits, etc. After recording which fruits make the clock turn on, they can combine them to see if the display shines brighter with two different fruits. Possible combinations include a lime and a kiwi, a lemon and a strawberry, and so on.
Many science projects involve using lemons, limes, oranges and grapefruit to power light bulbs and clocks. The electrical conductivity of citrus fruits is common knowledge; however, other kinds of fruits may conduct electricity as well. Students can test this with a small digital alarm clock, a variety of non-citrus fruits, alligator clip wires and copper and iron rods. Students push one copper and one iron rod into each fruit. Students then attach an alligator clip wire to each of the rods and touch the other ends of the wires to the alarm clock's empty battery terminal. They can then record which fruits do or do not make the clock turn on, and why or why not.
Some clocks require larger batteries than others; this experiment requires a clock that needs D-cell batteries or larger. The class pushes one copper and one iron rod into a piece of fruit; fruit may range from all kinds of citrus, to tomatoes and peppers, to melons. Use anywhere from eight to 16 fruits, connecting them by clipping one end of an alligator clip wire between the iron rod on the first fruit and the copper rod on the second fruit. The next alligator clip wire goes from the iron rod on the second fruit to the copper rod on the third fruit and so on. The wires on either end of the fruit line should each have a free clip that students can touch to the ends of the clock's empty battery terminal. If the clock lights, the circuit is complete and all of the fruits conduct electricity. If not, one of the fruits is a dud. Students can have fun figuring out which fruit to replace.