Magnetomotive force is defined as,

**“The force produced by current through a coil of wire is called magnetomotive force (mmf), as shown in the following figure.**

It is the force by which a magnetic field is produced. Just as an increase in electromotive force produces more current in a circuit, so an increase in the mmf increases the strength of the magnetic field.

**Magnetomotive Force (mmf) Unit**

Magnetomotive force (mmf), represented by ℑ, is measured in ampere- turs (A-t) of the coil shown above.

“One Ampere-turn is the amount of magnetic force or flux produced when one ampere flows through a single turn of an electrical conductor.”

**Magnetomotive force (MMF) Formula**

The magnetomotive force (mmf) formula is expressed by the following relationship.

$\mathfrak{F}=A-t$

A coil having 10 turns and 1A of current produces the same magnetic flux or force as one having 5 turns and 2A current because the ampere-turns are the same.

**Example**

What is the magnetizing force of a coil having 4000 turns and 10 mA current flowing through it?

**Solution**

We have following formula to calculate ampere – turns (magnetizing force):

$\mathfrak{F}=A-t$

So, putting values of current and turns into formula,

$\mathfrak{F}=4000*0.01=40A-t$

Thus, if a large amount of magnetizing force is needed and only a small amount of current is available, it is necessary to use a great number of turns on the coil.

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