The relationship between voltage and current (amperes) is Ohm's law V=IR. In words voltage is equal to current multiplied by resistance. Welding machines are classified as constant voltage or constant current. Constant current machines, stick and tig mostly, try to keep the current more or less constant. The voltage will vary mostly based on how short an arc you hold. A short arc reducing the resistance of the arc and hence the voltage. The controls of a constant current machine will effect current (amps). Constant voltage machines, mig and flux core mostly, try to keep the voltage constant. In "short arc" transfer the wire is allowed to touch the puddle, since the machine tries to keep constant voltage the current (since the resistance of the short is very small) will be very large. This heats the wire until it melts (like a fuse) an arc forms across the gap which melts the work side stub into the puddle and the wire side into a ball. The gap burns wider until the arc can no longer maintain itself. Then after some period the wire which is being fed at a constant rate will touch again and the process repeats. Since each repetition uses a more or less constant amount of energy the effect of an increase in wire speed is an increase in amperage. There is a limit to the amount of wire speed that a particular voltage can support. On this machine the controls adjust voltage and wire speed.
Bill

Amperage/Current controls penetration. Voltage controls the height and width of the weld deposit. This is irregardless of the process. Constant Current/Amperage power source's main control, controls the amperage and voltage is adjusted with the arc length. Constant voltage power sources requires wirefeed speed adjustment to control amperage which controls penetration. The cv power source's voltage is controlled by its main control. Weld procedures call for heat input which is volts X Amps divided by the travel speed.

Hi Sidekick

This is a very BIG question, but I will try to give you some of my ideas. (You already have a couple of good answers.)

Some power sources will only allow you to set the Amperage. These are the so called "constant current" power sources. (Also called variable voltage or drooper power sources.) They are typically used for the manual welding processes. (GTAW, SMAW) With this power source, the voltage is a function of the arc gap. The longer the arc, the higher the voltage. As such, you can not "set" the voltage. It is a function of the welder technique. Bill mentioned the V=IR relationship. (Ohm's law) This relationship holds across the arc, but does not hold for the power source itself. With this power source, when the voltage is changed (by the welder varying the arc length.) the amperage does not change linearly as is given by Ohm's law. Rather, the amperage will only change slightly with a change in the voltage. (Hence the name constant current.)

Some power sources allow you to set the Voltage. These are the so called "constant voltage" power sources. They are generally used for the automatic and semi-automatic welding processes. (GMAW, FCAW) With this power source, the voltage is set with a knob on the machine and the amperage is regulated by the wire feed speed of the wire feeder. This is so because with a small change in voltage, there is a very large increase in amperage. By speeding up the wire feed, the arc length is shortened, leading to a slightly lower voltage which leads to a much higher current. This higher current then burns off more wire. We can see that this is a "self regulating" system, preventing the wire from "stubbing" into the work piece. There are obviously limits to this. What complicates the voltage and amperage (wire feed speed) settings on this power source is that you can move between different "transfer modes" depending on your voltage, amperage and gas used as shielding. (In the case of GMAW.) This is however another day's topic.

For any given situation, an increase in either voltage or amperage will increase the amount of energy that is introduced into the work piece. A higher energy usually means better penetration, although this too is a rather complicated subject.

Hope this helps. If you have a more specific question, then by all means ask it.

Regards
Niekie Jooste
Fabristruct Solutions