Metal Detector
Overview
Alexander Graham Bell kept working on telephone design even as phones were being installed across the US and spread throughout Europe. He read scientific journals that recorded observations, issues, and improvements. One journal passage caught his interest: The earpiece would make a high pitch ringing noise under certain circumstances.
At once he went to the laboratory and started experiments to investigate this idea! In this exploration students use two coils, an "interrupter" (small DC motor), and a earphone to observe interactions.
Preparing Materials
Students can make the same observations that Bell did in a simple way. Find the following materials:
• Copper enameled wire (any gauge will work, but #24-#30 is recommended) See length calculation here.
• A tapered plastic container (yogurt, cottage cheese) or plastic bottle that has a diameter of about 4-5 inches.
• Some tiny strips of duct tape
• Sand paper
• A hobby motor
• A power source (batteries)
• An earphone
Read the first section of the Assassination of Garfield Story
Exploration
Have students review aspects of the telephone: a microphone and a speaker connected with a wire and battery. A microphone (coil and magnet) are connected to another coil and magnet (earphone). Bell's simple design below shows "A" the sender and "F", the receiver. When these two coils were near one another, the telephone would make a high pitched noise.
Step 1. Make two coils out of copper enameled wire by wrapping around a plastic container (cottage cheese or something similar). 20 turns should be sufficient.
Step 2. Carefully tape the coil in four places to secure it from unravelling. Leave about 6 inches on each end sticking out.
Step 3. Sandpaper the ends of the wire so you can see the bare copper under the enamel. Carefully wrap each wire end onto the earphone jack so that the two wires do not touch each other. Secure with tape. (See diagram below)
Step 4. Prepare the other coil the same way by sanding the ends. Connect this coil to a battery and a motor in series (making a loop). You can add a switch in the circuit loop if you desire. See below:
Step 5. Place both coils near one another as shown. Confirm that you can hear a tone in the earphone that corresponds to the motor's noise.
Step 6. Move one coil away from the other until the noise stops and then starts again. Find the place where there is the least noise.
Step 7. Once you have found the "null" or quiet place, try placing a piece of metal into the spot of the overlap.
Try different spots, metals and variables of your system. How could this be used by Bell in his search for Garfield's bullet?
Concluding the Lesson
Bell made observations, came to some preliminary conclusions, and then hypothesized he could design something that could detect the bullet in Garfield's back. The actual apparatus is shown in diagram below and similar to what you have just made:
Read the conclusion of the story. Have students discuss what variables might be important and why the bed springs were a problem. Bell had to put his noise source in a distant room when he did the experiment. Can you tell why that would have been important? How could you improve upon your design?
