Canepole or any electrician, question on bulb polarity

[Slab]

Question, which part of a typical automotive light bulb is the positive end and which is the negative? I ask because it would seem to me that there is something wrong with this picture:

[pineplace]

The bottom end is usually the positive end and the outside the negative. Same goes for cigarette lighter plug

[Slab]

I guess I should clarify. I said automotive, but this is actually on a trailer. Typical 12 volt system is what I meant. But if I'm not mistaken, it's all the same polarity even on typical household light bulbs as well.

[Slab]

The bottom end is usually the positive end and the outside the negative. Same goes for cigarette lighter plug

OK then, I'm not crazy. Why does the light bulb bottom (right side in this picture) have that screw going through the metal in the fixture (supposedly the positive side), and then it's screwed into the metal of the trailer.

Seems like they are grounding the positive side to the trailer, but in actuality the fixture is wired backwards. And that's why my replacement LED light bulbs won't work. The fixture, by design, seems to be wired backwards. What the heck is with that!

[Will_P]

On dc circuits electricity flows from negative to positive. If the grounding screw was on the other side it would create a short and you wouldn't get any flow through the bulb.

[ozarks ranger]

Most of the time on a trailer the brown wire is the hot to all the lights, green wire is to right turn and yellow is left turn, white is ground maybe the screw on the right has an insulator and is just screwed to rail to hold light on. might start there. good luck

[Clint]

While your trailer frame is the negative side of the circuit and the positive comes in on the wire in actuality if the bulb holder is wired properly it does not matter which way the DC current flows through the bulb. If the positive voltage coming in on the wire goes to the case of the bulb, and the negative connection is the end of the bulb it will still illuminate, as they are not polarity sensitive. However on your LED lights the positive must be connected to the proper terminal as LEDs are polarity sensitive. If you are trying to use the LED replacement bulbs which look similar to the incandescent bulbs they will NOT work in the pictured fixture.

I'm not an electrician, but a retired Electronics Technician and a life long electronics enthusiast. Amateur Radio (Ham) call-sign = W5CPT -

[CatFan]

Like Clint said, you can't wire an incandescent bulb backwards. It was done this way for an easier installation during assembly. Looks like yours actually has a ground wire in addition to the screw, which means you can install a fixture with a pigtail plug and LED light.

[Slab]

Yep, agreed, can't wire incandescent wrong, but still, the stupid fixture grounds the right side to the frame, so this fixture will not work for LED, because they designed it backwards. The engineer should be fired. Just sayin'

I am simply gonna use incandescent bulbs where they are screwed to the frame. At least I can use the LEDs in the 5 light positions at the top front of the trailer where they are screwed into fiberglass. I just flipped the wires and it works fine there.

[scrat]

Bet it reads "made in China" somewhere or maybe ACME.