Speaker and Solenoid

Parts Needed:


  • 2 Models

  • Straw

  • 1 Battery, C

  • 1 Nail, large

  • Paperclips or small nails to pick up

  • Aluminum foil


  • 2 Models

  • 1 Cup with raised bottom

  • 2 Paper fasteners

  • 1 Rubber band, medium

  • 2 Magnets, rectangle

  • 40' Magnet wire, #30 or a bit larger

  • 2 Palito de paleta

  • 1 Plug, mono

  • 3" PVC tube, 1/2", to make wire coil

String Telephone​

  • 2 Models

  • Toothpick

  • 20' String, kite string works well

  • 1 Plastic cup

  • Nail, to make hole in cup

Extra Tools: 
  • Philips screwdriver (to make holes for paper fastener)

  • Sand paper, small bits

  • Radio

  • Amplifier

  • Black tape

  • Decoration: pipe cleaners

How we build it:


Cut at least 6' of magnet wire (the longer, the stronger it will be). Use sand paper to strip the varnish insulation from both ends of the wire. Leaving a tail of about 4", wrap the wire around the straw near the end.

Leave 4" tail on the other end as well. Twist the two wire ends together so that the coil stays together. Fold tow pieces of aluminum foil around the two stripped ends, so that the wire can make better contact with the battery.



Tape the shorter end tightly onto one end of the battery. Tape the battery onto the straw just above the coil. The other end should just reach to the end of the battery. This is the switch: when it touches the battery, the solenoid will be on.



Unbend one side of a paperclip and "feed" it into the end of the straw while pinching the aluminum foil to the battery. The paperclip should be sucked quickly into the straw and held even when you turn it upside down. The same should happen with a small nail. When you release the battery, the objects should drop out.


Wrap tape around a larger nail until it wedges snuggly into the end of the straw. This is now an electromagnet. You can pick up paperclips, nails, or other small iron objects. When you disconnect the wire from the battery, they should drop off.



Cut 40' of magnet wire and wind it around the PVC. Leave at least 1 foot on each of the ends sticking out. Wrap the ends around the coil so that it holds its shape.


Hot glue the coil securely to the bottom of the paper cup. Sand the varnish insulation off the tips of both ends of the coil.

Take the plastic sheath off the plug. You can thread it on the wires to be screwed back on when the connection is made, or just discard it. Connect one end of the wire to each of the terminals of the plug. Note that each of the terminals is connected to one of the two segments of the plug, which are separated by a black band of insulation.

The wires can't be touching each other where they have been stripped, nor can the terminals touch. Tape each one separately to avoid a short circuit.

This is the coil and the diaphragm for your speaker. The next steps will be adding the magnet. Glue a magnet onto the paint paddle near the end, but not flush. A magnet that fits inside the coil will work best, but any magnet will work. 


Poke paper fasteners into either side of the cup near the bottom. Bend them over inside the cup. Stretch the rubber band over the base of the cup and loop one end of it around the heads of each fastener.

Lift the rubber band and slide the paint paddle with magnet under it. The rubber band should hold the magnet nearby or touching the coil. Turn a radio up loud and plug the speaker in. You should be able to hear the radio if you put your ear close to the cup. If you make the coil and magnet fit together very well, you may be able to hear it across the room.


Plug the speaker into the input of a small amplifier. Now you have a microphone. Speak into the cup, and someone else will hear your voice coming from the amplifier.


String Telephone

Make a hole in the base of a plastic cup. Insert one end of a long string.


Pull the string up from inside the cup and tie a half toothpick onto it. Pull the string from the bottom of the cup until the toothpick is tight.


Fasten the other end of the string to the cup in the same way. Pull the string tight to use it. You can also try: grabbing the string to see if the sound still gets through, talking around corners, tying another string on to make a three-way line.



A Bit More Info:

You can make a simpler electromagnet with just a nail or bolt wrapped with a bit of wire. The solenoid is more interesting because you can see that the magnetic field is strongest in the center of the coil. When you put a nail in the center, the magnetic iron atoms of that nail get aligned, and the nail becomes a magnet.

You can see from the solenoid project that an electromagnet works just like a real magnet with the additional benefit of a switch to turn it on and off. From this project you can visualize the way that most motors work. A spinning shaft has a few magnets mounted on it. Other fixed magnets are mounted in the casing. Some or all of these are electromagnets. One pulls or pushes another one and then shuts off as it waits for the next magnet to come around. Over and over this happens and the motor shaft gets pushed and pulled around. The switches that turn a motor' electromagnets off and on as it turns are called "brushes."

You can trace the sound from the announcer’s voice in the radio studio to the speaker you just made. The announcer’s vocal chords vibrate when she pushes air past them.  This vibration pushes on air and sound waves are set up, moving out in all directions from her mouth and vocal chords.  Some of these sound waves hit the microphone in front of her.  The function of a microphone is to turn sound waves into electrical impulses, called a signal.  This signal then gets sent to the radio station’s transmitter and antenna, where it is converted into radio waves.  All the information from the original sound is contained in these radio waves, which get broadcast out in all directions.  If some of these radio waves make it to your radio before they lose their energy, your radio will receive them, convert that information back into an electrical signal, amplify it and send it to the speaker.

The function of a speaker is exactly opposite that of a microphone:  it turns electrical impulses into sound waves. Speakers do this by means of two magnets pulling and pushing on each other.  One is an electromagnet, whose strength and direction of magnetism depend on the electrical signal that the radio is giving it.  All the information from the original sound is contained in that signal, so the electromagnet pulls and pushes on the permanent magnet to create the same vibration that the microphone originally received.

Either the coil or the permanent magnet is connected to the diaphragm of the speaker.  When the diaphragm starts vibrating, it pushes on the air and sends out a sound wave.  If that sound wave makes it to your ear, you can hear what the announcer is saying. 

The string telephone shows the function of the diaphragm.  When you talk into the cup, the sound waves you create hit the cup and make it vibrate.  The cup in turn makes the string vibrate, and a sound wave takes off down the string.  When the vibration reaches the other end, that cup begins to vibrate.  This vibration pushes on the air around that cup and creates a sound wave in the air.  The shape of the cup directs the sound, and if you put you ear close to it, that sound wave will get in your ear.  When that happens, you will hear the sound.

The string telephone also demonstrates sound traveling in a substance other than air, that is, string.  Playing with the string will show that it really is carrying the sound waves. When the string is tight, it will work the best.  If you let the string go limp, the vibrations will not have any restorative force to sustain them as they travel down the string; you won’t be able to hear anything.  If you grab the string while someone is talking you will stop the wave and very little of the sound can get through.  If you try to go around a corner with it, the same thing will happen.  If you tie on another string, or cross two or more of the string telephones, you will set up a conference call.  A vibration on any string will vibrate all the others. 

Speakers can be made louder by amplifying the signal sent to them, increasing the size of the coil or the permanent magnet or by making them of higher quality, that is, with less distance between the magnet and coil. The best speakers are made so that the coil and magnet are extremely close together. If they are too far apart, the magnetic field decreases in strength and they cannot push and pull as hard on each other. The shapes of the box and diaphragm are also crucial in determining the final sound of a speaker. 


  1. A solenoid is an electromagnet with a hole in the center. When connected, it will suck iron objects inside the hole.

  2. Every common speaker has a coil and a magnet.

  3. The coil becomes an electromagnet when the electricity passes through it. The two magnets then push and pull on each other to make a vibration that is the sound you hear.

  4. The cup in this speaker works like the diaphragm in a normal speaker, or the cup in a string telephone: it takes a small vibration (from the coil and magnet or the string) and transfers it to the air so we can hear it better. 



  1. How could you make the speaker louder?

  2. What happens if the magnet is too far away from the coil of wire?

  3. Does the string telephone work when the string is not tight?

  4. Does the string telephone go around the corners?

© 2020 by Victoria Matelli, Calvin Norwood, Jade Murray

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