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# Ohm's Law: Circuits, Bulbs and Buzzers

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Source:
Author: Jerry Silver

### The Idea

If you have never built a circuit before with your own hands, this is your chance. Like many of the experiments in this book, various levels of complexity exist and you can take the experiment as far as you care to. You start with building a simple circuit, such as making a bell ring. Then, you build a basic telegraph system. You branch out and add series and parallel paths to simple circuits. Next, you measure the current and voltage at various points in the circuit. Finally, you look at how Ohm's law can be applied to more complicated circuits.

### What You Need

• jumper wires
• 6 Christmas tree bulbs (or low-voltage bulbs and sockets)
• various (low-voltage) electrical devices such as bells, buzzers, LEDs
• DC power supply (or battery as in the previous project)
• knife switch
• ammeter (or multimeter set up as an ammeter)
• voltmeter (or multimeter set up as an voltmeter)
• for the telegraph: 5–10 feet of insulated wire, iron nail, two blocks of wood roughly 3 × 6 × ¾ inches, a second block of wood ¼ inch taller than the nail after being nailed into the block, a "tin" can, tin snips, and a few small nails

### Building a circuit

1. Look at the circuit diagram in Figure 100-1 and make the appropriate connections.
2. If you have an adjustable DC power supply, set a voltage of 2–3 volts and keep it constant throughout the test. You may need to adjust this, depending on the circuit you are working with.
3. The circuit diagram should give you all the information you need. Here are a few details that may be helpful:
• Attach a jumper wire to the positive and negative terminals of the DC power supply or battery. (Not that the electrons care, but red is generally used for positive and black is used for negative for clarity in assembling the circuits.)
• There must be a complete path from the positive of the power supply and back to the negative.
• All connections must be metal-to-metal. If insulation is on the wires, you either need to use a bare-metal alligator connector or remove the insulation.

### Making a telegraph

1. Wind 25–50 turns of insulated wire around a large iron nail. Leave the two ends of the wire free and remove about ¾ inch of insulation.
2. Hammer the nail into the wooden block.
3. Cut two strips from your can, roughly 2½ inches long × ½ inch wide. Both pieces should be flexible.
4. Attach one of the metal strips to the wooden block. The height of the wooden block should be ¼ inch higher than the nail.
5. Attach the block with the metal strip to the base block, so the metal is above the head of the nail, but not touching.
6. Build the "key" by nailing the second metal strip to the second block on one side, and then putting a nail underneath the other end of the metal strip. Leave enough of the nail head exposed above the wood surface, so you can wrap wire around it.
7. OK. Let's hook everything up. What you have is a series circuit from the DC power supply through the electromagnet to the key. The key is simply a switch. When it closes, the electromagnet pulls the metal strip down, as shown in Figure 100-2. Short and long durations are the dots and dashes of Morse code. If you have enough wire, you separate the key and the receiver by some distance. (Note: if you use this to cheat on tests in school, please make sure you don't say you got the idea to do it here.)
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