According to mm :
On Fri, 07 Mar 2008 13:14:33 -0600, (DT)
wrote:

When you plug it in it mates with dimpled tab to make secure contacts.
If in double break apart a receptacle(only couple dollars a piece) and
see for yourself.

The Straight Dope addressed this question, and there seems to be no one simple
answer:

http://www.straightdope.com/classics/a2_389.html

The straight dope piece seems to have gotten it right - about
it _used_ to be necessary with old outlets that didn't grip so well,
the pins could interlock into buttons on the tabs. So, it used to have
a reason, and now it's probably they can't figure out why they're
there, so it's easier to stay that way ;-)


I think it's because putting holes in things make them stronger, like
the guy said. Look at the George Washington Bridge in NYC. It has
loads of holes and look at how strong it is.

Nope.

Punching holes in metal structures does not make them stronger. In
fact, it makes them somewhat weaker than equivalently sized solid
structures. The reason punching holes (or leaving it out altogether
such as in trusses) it is to make it _lighter_.

They're aiming for the rigidity implied by the larger cross-section,
without some of the added weight.

Just for the record, I just dismantled two outlets, one inexpensive one
and the
other a high end commercial quality Decora... no dimpled tab in either, just
flat copper strips.

I have a related question. Why are some prongs solid, others are
folded over with each layer totally flat against the other, and still
others are foled over but have one side freely attached and springy.

The third kinds seems the best.

The only ones of the third kind I've seen have been folded types that
wore so badly that the join (at the tip) is gone. Or were clipped.
The temptation for one to go sideways is too high.

As to why some are folded and some are solid - different manufacturing
processes.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.