OMG, one of you actually remembers something but the rest are SO far off base I'm going to have a little fun with you. (evil grin with horns)
"OMG 0z4 wasn't that a cold cathode rectifier?"
An octal based cold cathode mercury vapor full wave rectifier.
"I remember the ps on a 57 Chevrolet was located behind the speaker."
I wouldn't dare take the old man's car apart, my first AM DXing radio was a 6V Buick Sonomatic with an internal 8" speaker.
"The European rr's run 16 Hz."
Only one but I don't remember WHICH one. The rest are 50Hz with any of a number of catenary voltages. The Eurostar train set is unique, it runs on them all including 600VDC third rail and Deisel.
"How about the Hazeltine corp."
Hazletine Laboratories actually, the brains behind RCA, Dumont, Electrohome, Hallicrafters, Heathkit and many more.
Re: 25 Hz current;
"I'll take a half educated-guess but I'll probably be wrong........low risk of explosion ignition in flammable gas environment?"
Totally uneducated (;->), a spark is a spark is a spark.
"Subway "3rd" rail current?"
In NY the IND used it for traction and station power. That's why those dim 25W bulbs flickered and drove me bug eyed.
"Bumper Cars sparky pole thing?"
Sparky pole LOL! Frankly I have no idea but I suspect maybe DC since they had DC motors and speed control.
"I'm gonna take a wild guess here, since I never heard the term "coffee pot current". But your other clues give me a hint... DC current."
No, I heard the term used by railroaders but I'm saving the best for last.
"Con Ed still provided DC current in the downtown area (until recently, I think?) for the use of elevators."
Not so recently, it was decades ago DC went out. Previously some scattered neighborhoods had it but the why and wherefore is only a few fragments told to me by some old timers so I can't give you any useful information.
"two words "skin effect"
EEEHHH??? Skin effect only happens at radio frequencies above a few hundred kilohertz or so.
As the talking head remarked just before it exploded, "Get ready for a big surprise!"
I did mention railroaders calling 25Hz "coffee pot current" and here's the reason. When the Pennsylvania Railroad between Boston and Washington was electrified in 1937 they used 25 Hz current supplied by big city generators like Consolidated Edison and Philadelphia Electric. (Con Ed also supplied the IND subway.) Back then low frequency like that 16Hz European system had the advantage, AC traction (induction) motors deliver maximum torque at that frequency so 25Hz was chosen as a bit of a compromise between tractive effort and transmission efficiency. Today's locomotives employ computer controlled variable frequency, variable voltage and variable current power inverters with feedback sensing loops for top efficiency at all load and speed conditions. Another advantage is they'll eat just about anything you feed them so they can run on just about any system anywhere. Today the Amtrak NEC system still operates using the original PRR transmission equipment because 60Hz conversion is not cost effective and gives no particular advantage.
To sum it up, New Jersey Transit shares part of the NEC with Amtrak (New York to Trenton Main) while the rest of the system operates on 60Hz standard current. The locomotives don't even blink when transiting the phase break between the 11KV 25Hz and 25KV 60HZ catenaries although along some older and sagging stretches the pantographs become "sparky poles" as someone tickled my funny bone with previously.
