Weird question, what causes wires to spark when connected to 12v?

[cgseanp]

ok, title not exactly accurate but hard to describe.

I've been working on/testing my 12v wiring for my 7 way plug for a while now and noticed that certain wires will spark when I put 12v of power on them using a jumper (not when powered from the TV). I'm just connecting a wire straight from a 12v source inside trailer to the wire where it comes into the trailer from the umbilical plug.

The wires that spark are the running lights, brakes, reverse. The ones that don't spark are the left and right brake lights/blinkers. It seems the ones that spark are the circuits that split off to multiple locations vs just going to one side..

It's not a crazy spark, just when I initially hit it with 12v it sparks, or if the alligator clip isn't secure, it might spark a little.. it does not spark when connected to the TV.

I just want to verify nothing weird is going on! thanks for the help

[Mollysdad]

A spark or arc is an indication that current is flowing.
WHY it's flowing is another question.
You can get lights to light by jumping certain wires in the 7-way.

[Bill M.]

The ones that are drawing more current. One really small signal light may not draw enough to see the spark. Multiple outline lights might.