A few notes on using a rheostat. (a high wattage potentiometer)
-A rheostat's wattage is proportional to its setting. So a 100 watt, 100 ohm rheostat is only a 50 watt rheostat when set to 50 ohms.
-A trick that can be used in rating rheostats, (well, any resistor actually), is to rate them in amps, not watts.
For instance, a 100 ohm rheostat set to 100 ohms, with 100 volts across it will dissipate 100 watts and will have 1 amp flowing in it. If you set the rheostat to 50 ohms, in order not burn up your rheostat, you will have to lower the voltage to 50 volts, which will give you a dissipation of 50 watts. This will again be 1 amp. And so on.
So you need a rheostat that can take the field current. 2 amps? (I'm too lazy to look it up right now.) But now we run into a problem. When you first put the rheostat inline, it has to take just about the entire field current. (not much limiting is taking place yet.) A 50 ohm rheostat for 2 amps is 200 watts. Kind of big. But, if you simply switch in a 20 ohm resistor, the current (about) gets cut in half, and you only need a 20-ish watt resistor. Play with some numbers and you will see how this works. It's not totally obvious.