Free electrickery

[ZuidWester]

Okay I was bored. I checked the hull of my boat with a voltmeter and found about 0.3 volts coming out of it. It is out of the water on a trailier at the moment. The battery and solar chagrer were both on, so I checked the boat next to mine and read a similar voltage, this time his battery was not attached (he said). Now I'm puzzled. What am I measuring? Both are steel boats of around 9 meters.



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[john_morris_uk]

What are you checking them against? In other words, where are you putting the leads of your test meter?

Wherever you are putting the prods, if you'll pardon the expression, with very high impedance digital test meters its easy to measure something that isn't there in an real sense. A bit of dampness in the air, two dissimilar metals and some salt, and you have a battery - no useful power, but the meter will show a reading.

Try licking your fingers and putting the test prods on them with the meter turned to a low voltage setting. On a high impedance meter, you will show yourself to be generating volts....

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[charles_reed]

Strange things happen in the Netherlands.

I'd guess induction effect from power-lines - especially if you've got 33,000 volts within about 1000 metres.

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[john_morris_uk]

Inductions a possibility if he's measuring AC voltages. Sometimes, the rust and salts in between two metals can make a crude diode to produce DC (the latter used to be a problem one sometimes found when tracing interference from local RF sources in domestic TV and Radio systems: the diode causing harmonics to be formed and thereby interference from a 'clean' source)

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[ZuidWester]

Further research required

I'll do some testing with other boats today. I set the voltmeter to 2 volts DC scale and searched for an earth wherever I could find it (tricky in some places).
I'm concerned about my anodes and wanted to see if they would conduct electricity as a test of correct connection.
Further research is required me thinks!

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[William_H]

John Morris is right the modern electronic voltmeter having extremely high resistance means it needs virtually no power to drive it to measure the voltage generated. The thing that people don't appreciate about voltage generated by galvanic ie dissimilar metals in a conductive fluid is that the voltage generated is a function of the metals involved. Hence a zinc carbon (torch battery) gives 1.5 volts while a lead acid ( lead and lead sulphide) gives 2 volts (don't quote me on the details there) The voltage is fairly constant unless it is disipated or wasted in which case you measure less. The voltage measured is not an indication of the size of the "battery" ie the size of the problem you may have. Electrically that could only be estimated by puting a load on the "battery" and see how much power it will generate. That how ever is totally inpractical and the real indication of an unwanted galvanic cell is the damage it causes in erode the least noble metal.
It is conceivable as already suggested that you are measuring some sort of induction rectified by a dissimilar metal junction sometimes a digitasl voltmeter is like having a very sensitive nose it just smells everything and so is not really so much use. You might be looking to measure 12 volts at a light socket with the light removed. You measure 12 volts only to find eventually that the current for the meter is getting through a corroded switch or wiring which the current for the light would never get through. be aware of the limitations. regards will.

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[Rick]

Will's points are pertinent - infintisimile current at 1.5 V is not really as problem - but 5 amps will whole your boat in next to no time! Look further, and find out what's really going on ......

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