Iron filings are very small pieces of iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 that look like a light powder

Powder (substance)

A powder is a dry,thick bulk solid composed of a large number of very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms powder and granular are sometimes used to distinguish separate classes of material...

. They are very often used in science demonstrations to show the direction of a magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. Since iron is a ferromagnetic material, a magnetic field induces each particle to become a tiny bar magnet. The south pole of each particle then attracts the north poles of its neighbors, and this process repeated over a wide area creates chains of filings parallel to the direction of the magnetic field. Iron Filings are used in many places including schools where they test the reaction of the filings to magnets.

History

Filings are mostly a byproduct of the grinding, filing, or milling of finished iron products, so their history largely tracks the development of iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

. For the most part, they have been a waste product.

Iron filings have some utility as a component in primitive gunpowders. In such a fine powdered form, iron can burn, due to its increased surface area.

In modern electronics, some transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s have iron powder cores.

Uses

The primary use of iron filings is in the study and teaching of magnetism and electromagnetic fields. The substance makes impressive demonstrations when sprinkled on a white card placed on top of a permanent magnet. The filings are also found in toys that allow one to draw with a magnetic pen. Additionally, by sprinkling fine iron on a magnetic stripe card

Magnetic stripe card

A magnetic stripe card is a type of card capable of storing data by modifying the magnetism of tiny iron-based magnetic particles on a band of magnetic material on the card...

, it is possible to see the magnetic encoding on the stripe. Another demonstration is to pour onto the exposed platter of a hard drive a semi-viscous fluid in which iron filings are suspended, so that the patterns of bits on the platter are revealed by the alignment of the iron filings.

It is even possible to produce Permanent Black and White Pictures
using magnetised iron filings. The accompanying photograph shows an
experimental artwork entitled "Field of magnetised iron filings"
created in the nineteen eighties as a forerunner of a permanent sand picture of the then Prime Minister Margaret Thatcher (The Iron Lady) which also incorporated magnetised iron filings in Mrs. T's suit and in the background of the composition.

Later the creator portrayed the destruction of the 'Iron Curtain' using magnetised iron filings but also incorporating crushed particles from the Berlin Wall.
The artist is now producing a permanent series of works exploiting
an age-old experiment which never completely fullfilled its potential
and a perfect example of the notion - "Why didn't I think of that
before!"

Iron filings are also used to fortify enriched foods for human consumption.