Difference between variable transformer and adjustable electric voltage regulator

[adrian21f]

such as a variac- http://pages.ebay.com/link/?nav=item.view&alt=web&id=361299617389 (in 1st picture)
and this- http://pages.ebay.com/link/?nav=item.view&alt=web&id=221536413936 (in 2nd picture)
to me they seem the same but extremely different prices and size... im not very familiar with autotransformers so if anyone can please clarify what the differences would be between these two... i just need to regulate voltage for some projects.

 

[ramussons]

The first is a Variable Auto Transformer where you change voltage by moving a sliding contact over the pickup windings. Consider it a transformer with a very large number of tappings to select.

The second is an Electronic device that controls the "average" voltage by using part of the sine wave. If you permit only a part of the sine wave to be used, the "equivalant" voltage will change depending on how much of the sine wave is "used"

All electrical / electronic equipment can be handled by the former, only some by the latter.