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One wire light bulb?

ConnorL

Feb 7, 2018
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Hello to all, i am very interested in electronics but have little education in this area. I recently built a display of vintage electronics with a plasma ball at the center. There are many wires going from circuit board to switches, etc. Only the power supply wires to plasma ball and two small light bulbs are carrying any current. When I went to hook up another vintage light bulb it lit with only one of the two wires connected. I don't understand how this can be. Does it not take a complete circuit to light a bulb? Please explain if you can.
Thanks
 

Alec_t

Jul 7, 2015
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Welcome to EP!
The path for the current would have been completed via your body.
 

Minder

Apr 24, 2015
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Similar in effect to the neon screwdriver testers.
You can stand on a milk crate and place the screwdriver tip in the socket and the neon lights!
M.
 

WHONOES

May 20, 2017
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Hmm, a demonstration of the impedance of space????
 

ConnorL

Feb 7, 2018
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Thank you for the input. I should have been more clear in my description and I will include pictures soon.
The light bulb in question is from the late 1950s to 1960s. Total length is about 1 1/4". It is incased in a plastic material. It has 2 wires protruding from it. It was removed from a piece of test equipment called a signal generator. (There's more to the name I think...).
Holding the insulated casing and touching 1 wire to a connection on an installed bulb makes this one light up. Other wire is not touching anything.
I kind of understand the neon idea, like holding a fluorescent tube next to the plasma ball, but this is an old and small bulb. Were neon bulbs made that small?
Unfortunately the plastic covers the actual bulb so I cannot verify the presence of a filament. Impedance of space????
Thanks again for any input.
 

Minder

Apr 24, 2015
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They came small,
Measure the resistance across the leads!
M.
 

ConnorL

Feb 7, 2018
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I will try that this evening (pst) and post results. Thank you.
 

ConnorL

Feb 7, 2018
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So I used an ohm meter which read infinite resistance. As in the two wires are not connected. This raises another question, why two wires? Also, does this mean it is a "gaseous bulb"( for lack of a better term)?20180207_201513.jpg
 

ConnorL

Feb 7, 2018
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The only writing
 

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ConnorL

Feb 7, 2018
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I think I just answered my question. Holding the bulb against the plasma ball lights the bulb. Which means it is gaseous. Correct?20180207_202954.jpg 1518064394872166598739.jpg 20180207_202954.jpg 20180207_202954.jpg 1518064394872166598739.jpg
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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This is probably a neon lamp. It may or may not have a current-limiting resistor inside. Neon lamps always require a current limiting resistance when connected to a low-impedance voltage source, which is what mains wiring is. If the lamp does have such a resistor, you can usually connect the two wires directly across 110 - 120 V AC mains voltage to light the lamp. But read on before trying this...

OTOH, if the two wire leads connect directly to the neon bulb inside the plastic housing without a series current-limiting resistor, then connecting the two leads to the mains will result in (possibly spectacular) destruction of the lamp. To be safe, first find a small resistor rated at a quarter-watt or more with a value of at least 47,000 ohms and wire it in series with one of the two lamp leads. It should now be safe to connect the free end of the resistor and the other lamp lead directly to the 110 - 120 V AC mains to light the lamp. For added credit and to increase your understanding, you can use a multimeter to measure the AC voltage drops across the two components of this simple series circuit.

Be careful working with mains voltages: they can be lethal. If you do this experiment, please report back with the voltages you measure: the mains voltage, the voltage across the resistor, and the voltage across the two lamp leads. Remember, voltage is always measured between two points in a circuit. Connect the multimeter leads before applying mains voltage. Turn off the mains voltage before moving the leads to a new measurement location. Have fun, but be safe.
 

AnalogKid

Jun 10, 2015
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Yes, that is a small neon indicator for a control panel of some kind.
Yes, it has a current limiting resistor built in.
Yes, it will measure an infinite impedance unless the impedance tester imposes over 60-80 V across the leads.

ak
 

ConnorL

Feb 7, 2018
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Thank you. Very interesting stuff. I played with it some more and found that if I touch the two wires to the outside of the plasma ball it lights the bulb according to the amount of arc going towards the wires. I'm thinking of attaching a small copper plate to the end of each wire and then mounting the plates to either side of the ball strictly for visual appeal. I will probably try the resistor experiment first. How would I identify such a resistor?

ak[/QUOTE]
 

WHONOES

May 20, 2017
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Yes, free space does have an impedance 376.73 Ohms. Look it up on Wikipedia under the title "Impedance of free space".
 

Minder

Apr 24, 2015
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and found that if I touch the two wires to the outside of the plasma ball it lights the bulb according to the amount of arc going towards the wires.
ak
[/QUOTE]

An old circus trick of holding and illuminating a fluorescent tube in one hand and the other hand near a plasma or H.V. source, lighting the tube.
BTW the one you have is traditionally used in mains appliances, freezers etc.
The Neon screwdriver version uses a path through the body to earth ground.
M.,
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I will probably try the resistor experiment first. How would I identify such a resistor?
Have you made friends with Google yet? Here is the result of using this search string: "47k ohm resistor". However, based on your responses so far, it is clearly too dangerous for you to be messin' around with mains voltages at this time. You are safely insulated from the voltages that are used to produce the plasma ball by its glass envelope. Not so if you go poking around a convenience outlet energized with mains power. Best to stick with "vintage electronics art" and stay away from any direct contact with electricity until you know a LOT more about it.

When I went to hook up another vintage light bulb it lit with only one of the two wires connected. I don't understand how this can be. Does it not take a complete circuit to light a bulb? Please explain if you can.
Yes, it does take a complete circuit to light the bulb. Sometimes the circuit path is not obvious though. For example, when atmospheric lightning comes down from the sky to strike something on the ground, where is the "return path" to complete the circuit?

For your lamp, the circuit is from the central "plasma ball" electrode through the ionized gas inside the globe, then through the glass wall of the globe via capacitive coupling, then back to "ground" through whatever is touching the outside of the globe. By "ground" I mean the other terminal of the high-voltage power supply that is energizing the plasma ball. It is probably not obvious where that is, but the metal framework supporting the plasma display globe and its power supply should be a "gud enuf ground" for your purposes.

The resistance of this path is very high, millions of ohms, because of the insulating glass sphere which is always in between the plasma ball in the center of the globe and "ground" on the outside. That is why you see random discharges of "lightning streamers" inside the globe, and why those streamers appear to be attracted to fingers placed on the outside of the globe. Your body (or other objects) represent a low-resistance connection to the ground-return that completes the circuit. When you bring your fingers (or other objects) close to the glass globe, their low resistance appears in parallel with all other paths to ground.

Placing copper electrodes on the two wire leads of your lamp should not cause it to light up unless one of the leads, or the surrounding plastic case, is connected to ground, typically by you holding the plastic case or touching one of the two leads with your fingers. Alternatively, you can wrap aluminum or copper foil around the outside of the plastic case and then connect that foil to ground. Or connect just one, but not both, of the two leads to ground. Try visiting this Internet page for more information on plasma globes.

Yes, free space does have an impedance 376.73 Ohms. Look it up on Wikipedia under the title "Impedance of free space".
This is the radiation impedance of free space, for purposes of coupling electromagnetic energy into it, typically by means of an impedance-matched antenna. It has nothing to do with how a plasma globe works.
 
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