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Q How does saving a picture or file to a hard drive work?

— Ben W., Waunakee, Wis.

A Jignesh Patel, professor in computer sciences at the University of Wisconsin-Madison:

A hard disk drive contains a circular shiny silver disk, similar to a CD or DVD but much smaller.

This disk is coated with a magnetic material, and information is stored by magnetizing very tiny pieces of this surface.

Depending on how each piece is magnetized — either a north-south or south-north orientation — that bit of information is recorded as either a zero or one.

Collections of many zeros and ones represent characters, and strings of characters represent words and so on.

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When the hard disk drive is plugged in, it rotates at a speed of 7,200 to 10,000 revolutions per minute. To magnetize the disk, or write the disk, the device has an arm with a tip at the end, the head. This head moves back and forth across the disk as it spins. This movement either magnetizes the disk, storing information, or does the opposite, reading the information on the disk.

Storing a photo file works the same way. It’s going to get contorted into a sequence of zeros and ones that will get recorded and stored. When you need to read it back, the head will read back those zeros and ones and restore the picture.

A newer technology called flash storage is starting to replace disk drives in many computing systems. This is a totally different technology that has no mechanical parts.

It can pull information out of the device faster, but it’s also more costly to store information in this way.

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Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.

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