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I've tried to create this quite simple circuit.

  • V1: 1.5 v (AA battery)
  • LAMP1: 1.5 v, 0.2 a (E10 incandescent light bulb)
  • "Salt water": 10 ml regular water + 1 g salt

When I connect the wires together, the lamp lights up. When I connect the wires to various conductive materials (a spoon, tin foil, a penny,...), it works as well. But if I put the wires into a tiny bucket containing (salt) water, it does not.

Could you please explain why? (I guess that it is related to the water resistance/conductance but how exactly and how do I calculate it?)

Thank you.

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    \$\begingroup\$ Salted water electrolysis an interesting experiment, but don't build and use a system based on it. Salted water electrolysis generates chlorine gas. \$\endgroup\$ – martinm Mar 18 '14 at 4:47
  • \$\begingroup\$ Thank you for your comment. I really doubt that the current delivered by an AA battery (1.5V, 2.2 Ah) in 10mL of salt water would generate any amount of gas likely to be armful. Even though it's more a chemistry question, if you know the equation to calculate the amount of gas produce by such electrolysis, feel free to share it. \$\endgroup\$ – Umpa Mar 18 '14 at 11:31
  • \$\begingroup\$ I learnt how to calculate that at school a long time ago, but I forgot :-) however what I meant is that a one shot experiment is probably OK, but producing even small amount of hydrogen and chlorine in a continuous may have unpleasant consequences. \$\endgroup\$ – martinm Mar 19 '14 at 4:42
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The spoon, tin foil, and penny are all metals, and will have a very low resistance - probably well under 1 ohm, so will allow ample current to flow to light the lamp.

The salt water will have a much higher resistance, allowing very little current to flow - this low current will not be sufficient to cause the lamp to light. I just measured the resistance of some salty water - it was about 50,000 ohms (50Kohms) which would only allow 30 uA (.03 mA) to flow in your circuit - not nearly enough to light a lamp.

If you connect an ammeter in series with the lamp, you will be able to measure the current with different test materials.

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  • \$\begingroup\$ Thank you for your answer, as a matter of example how much voltage would be needed in order to be able to use salt water as a conductor (to light the lamp)? \$\endgroup\$ – Umpa Mar 18 '14 at 1:57
  • \$\begingroup\$ @Umpa: that would depend on how much current the lamp requires to produce visible light (and on how much salt you put in the water). For a sensitive LED that might produce some light at 1 mA, you would need about 50 volts with the salt concentration I had. \$\endgroup\$ – Peter Bennett Mar 18 '14 at 2:04
  • \$\begingroup\$ Peter, what was the distance between the electrodes, when you've measured 50kΩ? Just curious. \$\endgroup\$ – Nick Alexeev Mar 18 '14 at 2:21
  • \$\begingroup\$ The meter probes were an inch or so apart - in a small glass, with an inch or so of water, and a few shakes of salt. (not a very scientific experiment!). \$\endgroup\$ – Peter Bennett Mar 18 '14 at 2:41
  • \$\begingroup\$ @PeterBennett: it's a regular E10 1.5V / 0.2A \$\endgroup\$ – Umpa Mar 18 '14 at 2:53

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