continuity on a stop tail bulb

[JonE]

Is there a reason that I get continuity not just from both poles to the side earthing surface... but also from pole to pole? (which I thought would be separate circuits)

Happens on two standard bulbs which work fine...

[Derek Sheldon]

Hadn’t thought about it at all before but it would make sense as they are joined via the earth.

[Derek Sheldon]

Hadn’t thought about it at all before but it would make sense as they are joined via the earth.

[JonE]

ha ha. of course! I can't believe I've been thinking about this all afternoon and hadn't twigged.

So is the reason they maintain separate circuitry in practice because the electrical flow is from the dual entry side, and can't "enter" from the earth, so to speak? Still head scratching a bit...

[AustinWood]

The common earth in a stop/tail bulb can cause strange effects.
If the lamp at one side has a bad earth then the stop light can earth through the side light circuit to the lamp the other side. This causes the other side side light to come on unexpectedly.

[JonE]

The common earth in a stop/tail bulb can cause strange effects.
If the lamp at one side has a bad earth then the stop light can earth through the side light circuit to the lamp the other side. This causes the other side side light to come on unexpectedly.

I imagine it could ALSO cause strange effects if I - I mean someone - accidentally used a single pole led which looked otherwise identical and possibly fused across the two terminals. Ahem.

[AustinWood]

It certainly can.
A single pole bulb fitted in the double socket caused a very mysterious problem on an Austin Seven.
At night with the engine running and side lights and handbrake on the engine wouldn't stop if the ignition was turned off.
The two contacts in the bulb holder were shorted together by the single pole bulb. Thus the handbrake being on was causing the brake lights to be on. The sidelights were feeding the brake lights and hence the ignition. Switching the lights off then caused the engine to stop.
That took Ray Walker in Cambridge a long time to work out!

[JonE]

It certainly can.
A single pole bulb fitted in the double socket caused a very mysterious problem on an Austin Seven.
At night with the engine running and side lights and handbrake on the engine wouldn't stop if the ignition was turned off.
The two contacts in the bulb holder were shorted together by the single pole bulb. Thus the handbrake being on was causing the brake lights to be on. The sidelights were feeding the brake lights and hence the ignition. Switching the lights off then caused the engine to stop.
That took Ray Walker in Cambridge a long time to work out!

That might make a good thing for beaulieu or an owners club pub meet - all the cars in the carpark 'attended to' by an official and a prize for the first person to work it out when they got home.