There is an article on these left hand caps in the newsletter of GE Lamp Components division from 1998, which makes lamp materials for sale to other lighting companies.  They explained that these caps were particularly popular for lighting the tunnels of American subway stations, where bulb theft is apparently particularly rife.  Originally different diameter caps had been used, but not everyone might notice that when removing a bulb.  However when you go to unscrew it and it twists the wrong way, you realise instantly that something is wrong!

In British trains, it was customary for many years to use 3-pin bayonet caps and mushroom shaped bulbs for the same reasons, to discourage pilfering.  Crompton made these kind of special British Rail lamps as late as the mid 1990s, but I think today there are probably no trains left in UK with incandescent lighting.  The 3-pin cap perhaps had another safety reason in this case, because our trains used to use series-wired 65V and 85V lamps, which may explode if accidentally used on the normal 240V mains.

An old thread, but thought I'd revive it, as I just found some LH thread bulbs and the lampholders..! As a hopeless leftie, I love them 

See here for a photo

The only left hand thread I know of here is for truck wheels, the left hand wheels have LH threaded nuts so they don’t come loose on the road, the RH are normal clockwise nuts

The only left hand thread I know of here is for truck wheels, the left hand wheels have LH threaded nuts so they don’t come loose on the road, the RH are normal clockwise nuts

The threads for a lot of things are left handed; the left pedal on bicycles for a start, or it would come off for the same reason as truck wheels. Also the left wheel on a bench grinder, the valve on the acetylene cylinder of an oxy-acetylene torch, probably a few other  things as well, but those are what immediately spring to mind. Oh yes, LPG cylinders I think.

Back in the 1980s my cousin worked at a large shipbuilding facility near Boston, MA. He said there were strings of festoon-like, incandescent work lights strung all over the place. Of interest, he said all of them had left-hand threaded sockets so the workers would not steal the bulbs  for use at home, etc. Made plenty of sense to me. 

Back in the 1980s my cousin worked at a large shipbuilding facility near Boston, MA. He said there were strings of festoon-like, incandescent work lights strung all over the place. Of interest, he said all of them had left-hand threaded sockets so the workers would not steal the bulbs  for use at home, etc. Made plenty of sense to me. 

But then a light bulb enthusiast might come along and permanently borrow one because they have that kind of lampholder.

But then a light bulb enthusiast might come along and permanently borrow one because they have that kind of lampholder.

That's entirely possible, although we would never do such a thing, of course 

That's entirely possible, although we would never do such a thing, of course 

Never!

But then a light bulb enthusiast might come along and permanently borrow one because they have that kind of lampholder.

Come to think about this again... those left-hand thread lamps might have been 240V or 277V since they were in an enormous ship building yard.
It is common to operate area lighting at higher voltages here in the U.S. in industrial environments. My cousin said that they were usually 100W but did not know what voltage they ran at. If they were the higher voltage type as I suspect, then that probably disappointed bulb-snatchers when they got home even if they had a matching socket. 

Interesting, here it's the other way around, certainly on construction sites, I suspect permanent industrial installations are 240V like everything else. Most if not all construction sites here require power tools and on-site lighting to be powered from 240V-110V step-down transformers with the secondary centre-tapped to earth so the maximum voltage to ground is 55V. We have a small 3.3kVA portable one which we use for our power tools and sometimes our 110-120V lamps.

Interesting, here it's the other way around. Most if not all construction sites require power tools and on-site lighting to be powered from 240V-110V step-down transformers with the secondary centre-tapped to earth so the maximum voltage to ground is 55V. We have a small 3.3kVA portable one which we use for our power tools and sometimes our 110-120V lamps.

I am sure there were step-down to 120V transformers and outlets around where he worked for whatever purposes (he was a welder). The majority of all of the other tools around the shipyard were hydraulic operating from the higher-voltage feed; others such as cutting torches and welding equipment was from gas.

I found some more LH thread bulbs, sadly made in China, but they seem to have an E27 cap rather than E26. They're the same length, anyway. If anyone wants any, I had to buy a pack of 12, I only really wanted three or four..!