# How to Build a Solenoid

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A solenoid is the name given to a series of current loops, arranged like a spring, that are aligned along a single axis through the center of the loops. When there is a current running through the wire, there is a resulting magnetic field. Thus, a solenoid is a type of electromagnet.

## How to Wind a Homemade Solenoid

Making a solenoid requires winding wire around an insulated or non-conductive cylindrical object, such that the coils can be aligned and the same size. Once the required number of loops are made, the cylindrical support can be removed. The two ends of the solenoid should be left as long tails, which can be used to attach to the positive and negative ends of any electrical component, such as a battery.

The type of wire will depend on the needs of your project. Consider the type of resistance in the circuit, impedance in the wire as well as the overall size of the circuit. Picking the appropriate wire and gauge is the most important part. Once you determine which wire to use, you can begin winding the solenoid!

It is important that the wire be insulated so that as the coils align and are placed adjacent to one another, there is no electrical connection at the places that the wire touches itself. If there were connections, there could be current flowing in those spots which could cause an electrical short, or produce stray or unwanted magnetic fields.

Unlike a permanent magnet, which has a magnetic field due to its inherent properties, an electromagnet can be turned on and off.

## Magnetic Field From a Solenoid

An electromagnetic solenoid has a very simple magnetic field, ​B​. For a solenoid in air with a permeability of air of ​μ​, with ​N​ loops unit length, and a current of ​I​ running through it, the magnetic field

B=\mu NI

Due to how easy it is to make a solenoid, and that a strong solenoid can be made simply by adding a dielectric material or an iron core to the center of the solenoid, to increase its magnetic field, there are many uses for solenoids.

## How to Make a Simple Speaker using a Homemade Solenoid

Have you ever wondered how a speaker works? How does the music turn into a physical vibration, or sound, from a file on your phone or computer?

A speaker consists of a solenoid and a permanent magnet, and some form of amplification. The electrical signal travels through the solenoid as a varying current, changing the magnetic field that the solenoid produces. The permanent magnet is placed at one end of the solenoid, and is resting against a membrane-like surface that can vibrate.

As the solenoidal magnetic field changes, the force between the two magnetic fields causes the membrane to vibrate, producing pressure waves. These waves are actually sound waves, and thus you can hear the music!

To make your own simple speaker, all you need is a permanent magnet, a solenoid, a plastic cup, tape and an AUX cable (to plug into your computer or phone).

## Making a Mini Solenoid

The mini solenoid can be made by using 36-gauge enameled copper wire, wound around a cylindrical object with a 1-inch diameter, to create approximately 100 to 200 current loops. Leave long tails to connect to the AUX cable. If the wire is enameled you will need to sand the ends of the tails to expose to conductive wire.

Secure the mini solenoid to the flat (bottom) end of the cup, and place the small permanent magnet in the center. Using 1 to 3 small, neodymium disk magnets will be sufficient. Secure the magnets gently, so they are able to vibrate against to bottom of the cup. The interior of the cup (where you would normally pour your drink) will act as an amplifier.

Connect the ends of the solenoid to the appropriate wires inside the AUX cable, and plug it into your sound source. Hear the music? Try making more speakers with more solenoid current loops or more permanent magnets to see how the sound quality changes.