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If you do not read voltage by testing as in above diagram ... YOU DO NOT HAVE WIRE LINE VOLTAGE!!!!!!!
We've been over this before, but I'm sticking to my guns. I wrote this a bit different from previously, but I know that there will always be some "push back", or "reluctance to accept" on this article, as the historically acceptable method has been in use for so long.
As an owner of several commercial salmon trollers, for decades I always tested my stainless trolling wire voltage by lowering a wire (cannon ball) into the water and connecting the positive probe of my multimeter to that wire above water. The negative probe contacts the rudder stock, bonding wire, engine block or negative battery post, etc. With the meter set on dc volts, I will then see a reading of somewhere usually in the .5 to .7 volts positive (+). This is the norm as I knew it ... we knew it.
Unfortunately, after developing my Lurecharge "voltage tuned" products and loosing many hours of sleep, I came to the conclusion that the voltage reading I was seeing was only a "potential" wire voltage. By a potential, I mean that the voltage you read is only an available voltage and once you remove the meter probes, there is no circuit. This is the same as testing ones car battery. The voltage is available for use, but no current is flowing, except through the test meter while it is connected or if you turn something on. What you do read on your boat is the voltage field radiating from the (bonded) hull's underwater metal components, but not a wire voltage. If you have, and use a "black box", you should be good to go as is. If you do not, this will apply to you. For the following test, disconnect any black box or other wires from your trolling wires. Please do the following test away from dock to avoid interference from other boats or power sources. Downrigger weights should be insulated from the downrigger wire with a plastic clip or gangion twine. I attach alligator clips to my meter probes to make things easier ($2 at Radio Shack).
I would not be convinced myself if I had not challenged myself on ABSOLUTELY everything that I do with my lure voltages. The simplest confirmation test that I do (I have 3) is a "residual voltage" test. Any time that voltage is applied to, or is present in a metal, that metal retains some of that voltage for around 15 minutes. Basically, the metal becomes a capacitor and retains voltage that slowly dissipates. I test the subject metals quickly in a separate container with unused water to confirm the presence of voltage.
For sports boats, you mostly have stainless wire and no brass markers or stoppers. The voltage (fish sees) should be close to zero. For commercial trollers using brass markers, you will show some voltage because of the 2 dissimilar metals (brass and stainless) which create their own galvanic corrosion cell (this depends on the grade of brass used by the wire manufacturer. If they used "high brass", there may be no significant corrosion and resulting voltage). The brass will become the sacrificial anode for the more noble stainless. A by-product of the corrosion is electricity. Just like the zincs on your boat. For those using monel (wire) markers, stainless and monel are pretty close galvanically and the result will be minimal voltage. Take note of any other metals in contact with your stainless wire. This only applies to wire that is underwater. Your brass/bronze gurdies will only become a part of the cell if there is a lot of salt water to make a circuit to the ocean. Fresh water will not conduct as well. In fact, distilled water is a very poor conductor. It all relies on the mineral content, and to some extent, temperature. Warm water conducts slightly better than cold water.
TEST* I use an identical piece of metal as an extension of my positive probe (reader) as that which has the voltage applied i.e. if I test downrigger wire, I will have several short sections (1 ft?) of the same reader wire to test with. One is all that is required, but if I do multiple tests, I change the used one for a new one for each test. Just in case the test wire itself has residual voltage from the previous test. Prepare one or more short sections of trolling (reader) wire by having it/them contact your trolling wire on the spool, or taped to a strand (dry or wet for 15 minutes). What this does is equalize most any voltage differences between the trolling wire and the test wires and gives you a "zero base line" for testing. Using rubber gloves, remove reader wire(s) and connect one to your positive meter probe (do not allow reader wire to contact any other metal or fingers and remain preferably dry). As in the diagram above, drop the cannon ball into the water a short ways (6 ft), connect the negative probe to the downrigger wire above the water. With your meter set on 2 volts DC, touch the tip of the reader wire (on positive probe) into the ocean or lake close to the trolling wire (1ft). Note the reading. This will be the true voltage that you are fishing with and what a fish sees. If your gurdies or downrigger are isolated from the bonding system, chances are that you will see very little voltage (pay attention to decimal points). You can try dropping the trolling wire deeper, but you should see little change if you have strictly stainless. If you have brass markers, test it up and down to see if more or less submerged brass markers affects the reading. Do not dip the meter probes or alligator clips into the water. This will give a false reading because probes are often copper, brass, and/or nickel plated.
To take it to the next level, have a jumper wire of any type and contact (short) the downrigger wire (above water) to your bonding system, engine, negative battery post or aluminum hull, etc, and read the voltage again in the same manner. In most cases you will now see a true trolling wire voltage.
Another confirmation test that I have done is to place a variety of 2 galvanically different metals, in contact or separated by varying distances and in plastic totes that have a couple inches of sea water in them. I have done this for up to 2 week periods each test in order to observe any galvanic reactions between the 2 metals. My conclusion was very simply, "no contact = no voltage". I confirmed my findings with my residual voltage test. Now apply the last paragraph to the jumper wire in the previous paragraph. No Contact = No Voltage!
If you do have a black box, re-connect, turn it on, apply voltage and test in the same manner as above. The test voltage of "what a fish sees", should closely equal what the voltage setting is on the black box.
For those that disagree with my method, If you can't see an adequate measure of voltage of "what a fish sees", (and I can't be more adamant) THERE IS NO VOLTAGE PRESENT, a fish will see nothing!!!. My method is exactly how I test and display my lure voltages at trade shows. Think of your zincs. Do you think they will protect underwater metals just because they are close together? Not going to happen!
I think people are confusing a corrosion cell with a simple voltaic cell (without a salt bridge). Sure, I can make a voltaic cell, like a battery, and run my portable alarm clock. I've done it. But there the circuit flows between separated anode and cathode through the water on one side and through the load (clock wired to both anode and cathode)on the other. Compare that to downrigger wires (cathode) and hull (anodes). There is the circuit through the water on one side, but your stainless wires are isolated from everything. Resulting in no completed circuit.
I originally set up a 30ft test trough to see how far the voltage would travel. In fresh water, lure voltage would decrease 30% for every 10ft of separation. In salt water, the voltage did not drop 5% over the entire 30ft. So if you do this test, the meter reading off of your reader wire will not vary if you are 1ft or 10ft away from the main trolling wire.
Good fishin.
Al Dampier
http://www.lurecharge.com
allan@lurecharge.com