Thanks to Pete, I have a little more info on series street lighting lamps. Quoting freely and adding to what he said (any muddling is mine):
Some street lighting systems were (and some still may be, or were until recently) operated on DC. They began as arc light systems, using high current, where the goal was to maintain a constant current in the system rather than a constant voltage.
The more lamps on a circuit the higher the voltage required to keep the current the same. They used moving core transformers for this. The voltage is less for the smaller wattage lamps, but a 1,000 lumen lamp usually runs at about 10 volts for the proper current rating. Higher lumen lamps need more voltage. The drop across each lamp can be different based on the size, but the regulator keeps the entire circuit at the proper ampere value.
Incandescent lamps were made for use on these systems and consequently are rated in amperes (and candlepower) rather than volts. The most common current rating was 6.6 amps but there were 4, 5, 7.5, 10, 15, and (judging from a lamp I have), 16 amp lamps.
If 60 candlepower seems small for a street light, remember that gas lights did not put out much light so their replacements did not need to either. The small wattage lamps were usually used in alleys.
These current-rated lamps all seem to have mogul bases. Looking through my bulbs I see I also have a tipped 7.5A one with a short coiled filament, a similar tipped 6.6A one, and a tipless 6.6A with the same filament construction. Here's a picture of the 16A one (my interpretation of the marks on the press), which is about 13 inches long and tipped:
[This message has been edited by Chris Kocsis (edited October 10, 2000).]
