Re: What makes heat focus at the tip of a soldering gun?

Hi Joel,
The answer to your question lies in the field of metallurgy (the science of
metals). Metallurgy tells us that whenever a metal is bent certain stresses
are generated within it. These stresses cause an increase in the resistance
of the metal at the point of bending.

Lets see what happens with Copper. When you bend it the wire needs to
elongate on the outer edge of the bend and compress along the inner side of
the bend. This causes stresses to be built up along the wire. These
stresses in turn lead to an increase in electrical resistance. When current
is passed through such a wire, the heat generated is maximum at the point
of bending since the resistance to the flow of current is maximum there
(heat generated due to electrical current is directly proportional to the
resistance of the material). 

Thus your soldering gun will heat up maximum at the bend.

When you straighten that bend later, the stresses minimise, allowing more
current and hence less heat to be generated there.

A few interesting tips:
Try to bend the wire along a radius (in a semicircle). The temperature all
along its length should be the same. Reduce this radius and you will find
that the temperature at the bending point will be maximum.

Also measure the temperature of the wire at the point at which it had been
bent before. If youre measurements are accurate, you will still find that
the temperature there will be slightly higher than that of the rest of the
wire. Reason: The stresses generated earlier when you bent the wire, have
not completely disappeared.

Cheers,
Nauzad