I have an ancient 125 amp AC arc welding machine on which the current is varied by moving the negative lead from to different terminals. However the terminals are in pairs and joined by slotted metal plates, next to each pair two amperages are written, eg. 50/80 or 60/125. There are 3 pairs of terminals. What do the numbers mean and how do i obtain the low current settings?(i can only get it down to about 80amps)

Your dialarc HF has a selector for polarity, DCEP (electrode positive), DCEN (electrode negative) and AC. DC TIG welding is done in DCEN only with the high frequency turned to start. When welding aluminum, you must be in AC with the high frequency turned to continuous. Other machines may be different, but that setup will be how you need to set that dialarc HF.

The old school tungsten recommendation has always been the green or pure tungsten when welding aluminum on AC with a transformer machine, which is what you have. However, pure tungsten will not be a good choice for inverter style machines or when welding other metals besides aluminum. So instead of having multiple types of tungsten running around in your tool box, I use the blue or 2% lanthanated tungsten for everything. I find that it has excellent arc characteristics on both AC and DC, you can also sharpen it to a point when welding on AC, which is very useful in directing the arc when tacking up your aluminum job or starting out at low amps.

The black lead is connected to the metal you are attempting to weld, it has the silver ground clamp connected to it

The remaining red lead with the electrode holder connected to it holds the electrode
These welders are also capable of using 125 & 230 vac, 1-phase current from your mains there is usually a switchable indicator on the machine allowing you to select the input voltage

Check the dial indicator. Sometimes if someone changes the amps while welding, there will be a break in the coil. It will work at a different setting as long as you are away from the break.

The Rossi 220bz mig welder comes set up to use gas welding, that is with the Mig electrode connected to the positive terminal, inside the machine.
To use gasless mig wleding, the mig wire must be disconnected from the positive terminal inside the machine and reconnected to the Negative terminal - inside the machine. Open both the side coveres of the welder using a phillips screwdriver. Use a 13mm spanner to undo the hex nut holding the thick copper Mig lead to the terminal named positive, then lead it through the machine to the terminal on the oppisite side of the machine, labelled negative and screw it in place there with a 13mm hex screw. Be carefull not to bump any of the many electric parts inside the machine.
If you run the Mig welder in gasless mode with positive earth and positive elecrode it will blow the household electric fuse.
If you run the Mig welder in gasless mode with negative earth and positive electrode it will work, but the end of the Mig gun tip will get very very hot and start smoking abnormally from excessive heat.
But if you take the side covers off and rewire it (takes about 10 minutes) to negative Mig Electrode, the Mig gun tip remains cool and does not overheat. You will have to cut just one white cable tie, to get an extra inch or so of free Mig cable, so the thick copper Mig lead will reach accross to the terminal on the other side of the machine named negative. Stick the earth lead at the front in the positive terminal.
I did this on my Rossi 220bz mig welder.
http://www.weldequip.com/gasless-mig-wire.htm

Buyer beware. Contact the manufacturer and replace the needed parts.

my welder doesn't weld but my electric meeter goes wild.

If you are using flux wire and not shielding gas, your clamp should be positive and the wire should be negative. Heat is provided by the electrical arc that happens between the wire and the material. Your welder by itself does not pre heat the material and with 3/16 material you should not need to preheat anyway.

The electrical arc is kept consistent by adjusting the wire feed rate and the current applied. You will need to experiment with these settings to find what works best with your welder. For this welder and 3/16 material, I would suggest you start at the upper end of the current scale and about a third of the way up on wire speed. The welder might have a chart for these settings in the manual or inside cover.

If the wire feeds too fast, it will push the probe around and you will feel pressure as the wire feeds out. If the speed is too slow you will get large spatters and intermittent arcs. When properly adjusted the arc will sound even and consistent. The arc gap should always be about 1/8 of an inch.

To maintain enough heat for good penatration, do not move the probe too fast, work in a pattern and watch the weld pool (melted metal) and not the arc. Watching the weld pool will clue you in if you are moving too fast or too slow.

I hope this helps.

-Scott