I know of two ways to control the current going through the coin. One way is to vary the voltage of the power supply and the other way is to change the resistance of the electrical path that the current flows. The problem is, most AC to DC adapters don't have a way to vary the voltage. This means we need to vary the resistance.
Lets have a look at Ohm's law and see how all this works.
Ohm's law states that E=I*R E= Volts I= Amps R= Resistance
Since we are trying to control the current lets transpose the formula so we have the I on the left side of the equal sign.
Transposing we arrive at I=E/R (current in Amps) is equal to the (voltage in Volts) divided by the (resistance in Ohms).
We can see from this that changing either the voltage or the resistance the current will change. If the voltage increases or the resistance decreases, the current will increase. If the voltage decreases or the resistance increases the current will go down.
Since we don't have a variable voltage supply, we are going to have to vary the resistance if we want to control current. For this experiment you will need an ammeter that can measure DC mili-Amps. You can purchase an inexpensive one anywhere that sells automotive supplies (Wal-Mart ect.). If you have an analog meter you have to make sure it is connected properly, if it is hooked up backwards the needle will move down the scale, simply reverse the leads if this happens and it will read correctly. If you have a digital meter it will not matter, you might see a negative sign in front of the number but don't worry about that.
Ammeters must be connected in series with the load (the coin).
Attach the negative lead to the coin. Attach the positive lead to one lead of
the ammeter and the other lead of the ammeter to the anode (the spoon).
Remember, if the analog meter reads backwards just reverse the two leads.
Using only tap water as your electrolyte, put the coin and the anode in the
water and plug the adapter into the wall. Depending on the sensitivity of
your meter, you should see zero, or very little current flow. Next unplug
the power supply and add a pinch of salt to the water and stir until the salt is
dissolved. Keep unplugging the power supply and adding small pinches of
salt until further addition does not increase the flow of current. You
controlled the current by varying the resistance of the electrolyte. The
following images show the above procedure. This also demonstrates just
1. Tap water only 29 mA
|2. With a pinch of salt 86 mA||3. A tiny bit more salt. 231 mA.|
|4. With Just a touch more
salt we are at 339 mA
this is already over the adapters current rating.
When the solution was saturated, the current
went to 750 mA, this is one of the main reasons
these adapters burn out.
|The next step involves touching the spoon (anode) while the power supply is plugged in. While it is virtually impossible to get a shock from a low voltage DC supply, even with wet salty hands, things can happen that are out of my control. You can where a dry rubber kitchen glove for safety if you wish, or you can unplug and support the spoon somehow. This is up to you. Keep the same electrolyte that you made by saturating the water with salt in the above experiment. Plug in your power supply and slowly lift the spoon out of the electrolyte, you should see that the current is dropping. Because the anodes area is decreasing the resistance of the current path is increasing and the current flow decreases. This can also be accomplished by increasing the distance between the anode and the cathode (coin). The following images show the current dropping as the spoon is removed.|
| 1. The spoon
is just starting to be pulled
out and the current has dropped to 230
| 2. The spoon is
half out and the current
has dropped to 184 mA.
| 3. The spoon is
just barely touching the
water and the current is only 31 mA.