PEMCO bulb

[Mr. Hemingray]

Forgive me, I know nothing about light bulbs. If you have any questions about glass insulators, then give me a holler!

I hope a picture is worth a thousand words... This was found with a collection of insulators in Mass. The glass part is about 5" long and 3 1/4" diameter. The "tongues" are stamped PEMCO, and there is a PEMCO marked on the porcelain socket as well. Someone guessed it was a bulb for street lights.

Any information about type, dates of use, value, etc. appreciated. Thanks.

[Mónico González]

Welcome to the Forums.
Undoubtely this one is a huge street lighting lamp for series burning fitted with which are known as "voltage valve".
these lamps are intended to operate on constant current circuits in series as said, so, when one lamp burns out it must be any means to shunt it allowing other lamps to still on. This could be get by using a device constituted by a metal-oxide-metal barrier parallel connected with the lamp that normally behaves like an insulator owing the fact that the voltage drop between lamp ends are too low to break these barrier. But when the filament fuses, full line voltage get present at both ends of the lamp and lampholder, and of course, of the short-circuit device. This voltage is enough to "jump" in form of a small arc through the thin oxide layer placed between both metal discs taking them in contact with which the circuit is closed again without affecting the rest of the circuit.
The "voltage valve" must be replaced by another new when changing the fused bulb. To do so, the whole lampholder must be taken off from a socket where the line wires are connected. Those flat leads are the spring sheets that fits the valve element in place and makes contact with the leads in the female socket that supports the lampholder itself.
Lamps for series service were rated for watts and amperes instead volts, being characterized by its low operating voltage and relatively high amperage, so, having a high efficiency and ruggednes because its short and thick filament.
Regards.

[Mr. Hemingray]

M?nico, your description seems to fit. Some one else commented about the "disc" and how it might work. Thanks for the more detailed description.

Do you have any rough dates when this form of street lights in series might have been used?

[Mónico González]

No problem, it's a pleasure to can give you any info.
I guess the mark PEMCO is only related to the socket/lampholder set itself, not being extensive to the lamp. This one seems to be fitted with a Mogul cap. Regrettably its filament is broken as can be seen in the picture.
About the time period of use of incandescence series street lighting, I could say it was in use at least to mid-60's, based upon the info I got from the book "Il sorgenti di luce" (light sources) written in early 60's by italian Carlo Clerici, entitled in Spanish as "Luminotecnia".
In this one, can be found even a table about most usual lamps for series service rated in amperes and watts. As such lamps were intended for European use, these were fitted with E40 "Goliat" caps instead "E39" "Mogul" ones as is usual in US.
Nevertheless, it could probably exist some series lighting installation in service today in any small town, perhaps in some ex-soviet republic or central Europe country. Till I know, there is no such a kind of system in service in any place here in Spain. The main advantage of these systems were the substantial cost savings in copper because the whole circuit can be made of a single wire which could be designed to surround a block, starting from power station to a side and returning to it from the opposite.
Its drawback is the needing of use of special dispersive transformers with abnormally elongated cores into which the secondary winding literally "floats" up and downward adequately counterweighted, above the primary, in response to the current flux (and so, output power) variations caused by variable load conditions on the line, to maintain a virtually constant current in the circuit despite any load variations, even in case of a direct short circuit situation.
These xformers were big, heavy weighted and required some pay of attention and maintenance to be in adequate working condition owing the fact that them had mechanical moving pieces. Also these devices had not a very high efficiency because its operative principle is just based in magnetic losses, so their final efficacy relies on some precise status and values of volt-ampere-resistance characteristics of circuit loading. Furthermore, being them highly inductive apparatus their power factor (cos "phi") is too far from unity, which means more reactive current lost per watt than necessary.
Finally, the secondary voltage can be slightly high than desired to produce an adequate potential fall between lamp leads, even in case of too low rating of them. If the amount of lamps along the circuit grows, transformer output voltage must be raised in accordance, so, it can be some problems of isolation (and safety too) at wires, poles, lighting apparatus, sockets, fixtures, etc. if secondary potential exceeds over some hundred of volts.
Regards.

[Mr. Hemingray]

Again, great info, thanks!

This is from the US, and they may have stopped using the series street lighting sooner. I know the start of that system goes back many years. We have seen other special wiring and configurations to also have the series lighting. However, the socket holder for the bulb seems like something a bit newer, and I doubt it dates way back.

I'd probably say your estimate of the 1960's is probably pretty close. Perhaps 1950's. Seems to fit based on the construction of the socket and the series street light usage.