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Neat 1891 Cornell University book

Jeff Lumsden

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It’s interesting that they offered “Constant Potential” and “Constant Current”.

“Potential” is an old word for “voltage”.

Today, all electric companies provide “Constant Potential”, 120v in the US, 220 in Europe, 100 in Japan, etc. 

For “Constant Current”, the service is one loop that passes through the bulbs in series. To turn off a bulb, its switch shorts out the bulb, as opening the circuit would turn off all the bulbs. But, just like the old Christmas tree bulbs, if one burns out, none of them light. 

if you look closely at the miniature Christmas bulbs from 10 years ago (before LED), you’ll see a shorting ring that is loosely fitted around the two wire posts that support the filament. If the filament of one bulb burns out, the two posts spring out and the ring shorts them together, letting the current pass through so the other bulbs stay lit.  If the string had 100 bulbs and the wall voltage is 120 volts, each bulb sees 120/100 = 1.2 volts.  If one bulb burns out, the 99 remaining bulbs see 120/99 = 1.21 volts, not enough to matter.  But, if more than maybe 20 bulbs burn out, all of a sudden all the bulbs burn out. A customer asked me to fix their string last month and almost all of the bulbs were burnt out before the fuse popped.

“Constant Current” bulbs basically set their own voltage, depending on their resistance. Opposite of our bulbs, where a low resistance draws more current and therefore more wattage (Wattage = Volts x Amps), in a “Constant Current” circuit, every bulb sees the same current, so a high resistance drops more voltage and therefore more wattage. 

The old carbon filament bulbs have high resistance when cold. In a “Constant Potential” system like a modern system, the high resistance lets a small current pass through the bulb when it is first turned on. As the filament heats up in the next fraction of a second, its resistance decreases and the current ramps up. Watch a carbon bulb as it turns on and you’ll see this. The gradual turn on avoids thermal shock and is why these bulbs last so long. Most 110 year old bulbs still work. 

Now, if you run a carbon bulb in a “Constant Current” system, the bulb’s high cold resistance works against it, causing a thermal shock that tended to burn out the bulb. 

I’m not sure, but a “Constant Current” system might work just fine with carbon arc lamps. 

When I used to work for a defense contractor, there was a situation where we needed to run a light bulb at 2.3 Amps, even though the bulb would burn out at 2.4 Amps. So, we drove each bulb with a precision constant current source. This let the bulbs turn on softly, making them basically last forever. 

Early computer data communication used “4-20mA Current Loop”.  For digital communications, the “1” and “0” bits were sent as either 4 mA or 20 mA. No matter how long the data cable is, if 20 mA is transmitted, 20 mA is received. The voltage will drop on a long cable, but the current won’t.  

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