Build a practical joke lie detector:
1 Jumper Wire 8" (White) # 6SC J3F
1 Jumper Wire 4" (Blue) # 6SC J4
The lie detector build is very simple: replace the photo resistor with the white and blue jumper wires as shown in picture.
Few educated people take polygraphy ("the practice of using a polygraph, or lie detector") seriously nowadays http://antipolygraph.org
. Polygraphy is pseudoscience like ufology or paranormal research. There are, nonetheless, people who actually believe that a lie detector can detect deception. If someone you know believes this nonsense then you can have a little fun at this person's expense.
Explain that you have built a lie detector. It works because when someone is lying they tend to perspire more than when they are not lying. An increase in perspiration will change the resistance between the electrodes which, in turn, will change the pitch of the lie detector. Switch the lie detector on and gently hold the white snap between the the thumb and index finger of your left hand and the blue snap between the the thumb and index finger of your right hand. You will hear clicking from the speakeror perhaps a low picthed tone. Have your friend ask you two questions but inform your firend that you will answer the first one truthfully but you will lie in response to the second question. Continue to gently hold the white and blue snaps while you answer the first question truthfully, but then squeeze both snaps while you answer the second question--squeezing the snap will change the pitch a lot.
By squeezing the snaps, you increase the amount of the skin between your fingers that covers the snaps. This reduces the resistance in the circuit and increases the pitch of the lie detector.
Now have your friend try. At first he or she may be holding the electrodes too tightly changing the pitch. Ask your friend, "why are you so nervous...just relax." Wait until the tone is steady then start asking your questions.
Eventually your friend will catch on that even when he or she is lying that pitch won't change until the electrodes are squeezed.
Since we have replaced the photoresistor with the jumper wires we'll now call this circuit the 555 test circuit, there are a few experiments that can be done to test resistance:
Does adding salt to water decrease it's resistance?
In this video I add Sea Salt to water to see if the resistance of ordinary tap water at room temperature changes. The electrodes from the 555 test circuit are held submerged in the beaker with ordinary clothespins. I apologize for the the loudness of my magnetic stirer/centrifuge--I built it using Erector set pieces. It might have been better to use my K'nex magnetic stirer/centrifuge which is a little quiter but I had to borrow the neodymium magnets from the K'nex model to test the Erector set model. You may be able to hear the change in pitch as I stir the Sea Salt into the 50ml beaker of water (if you change the progress bar back and forth between 10 seconds and 43 seconds it is easier to distinguish the change in pitch). The change in pitch is only a little higher, but yes, adding salt to water does decrease the resistance of water. I also noticed that a lot of the bigger chunks of the Sea Salt did not dissolve, which may account for why Sea Salt seems to taste less salty than ordinary table salt.