We took a family trip with our grandsons to the New York Transit Museum in Brooklyn, NY. Retired subway cars were on display, some of them seemingly not that old while others dated way, way back. Visitors could freely roam in and out. I was in this one car that had been in service in 1903 which meant it predated the advent of electronics. Even the vacuum tube had not yet been invented by then.
I noticed the passenger area’s bare light bulbs and got really close to one (Figure 1). It was rated at 56 watts and 120 volts. A question came to mind as to how did that car use 120-volt light bulbs when the third rail voltage was (and still is) 600 volts DC?
Figure 1 A subway car light bulb up close showing 56-W and 120-V rating.
When we got home, I tried looking up subway car technical data, but when I came to a wiring schematic, I couldn’t read it. The symbols were indecipherable to me. Only then did it dawn on me that five such bulbs connected in series would be operable from 120 x 5 = 600 volts. If any one of the five were to burn out, all five would go dark, but then maintenance would change all five and discard four good bulbs with the one blown out bulb. It sounded wasteful, but it would have been a practical approach.
Is that the actual truth? I don’t know, and there was nobody on hand to ask, even if I had been quick enough of wit to inquire. Also, I just wasn’t smart enough on site to see if the total number of bulbs in the car was a multiple of five. Maybe one day, I can do that.
Another point about those subway bulbs is that they had left-handed threads on their bases, while household bulbs use right-handed threads. This was to discourage light bulb thefts. Stolen bulbs would not fit into light bulb sockets in households, only into the sockets of subway cars.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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