Adjusting appliance temps w/ no rheostat
Adding a temp control to the wire would require a temp sensor probe going
into the area that heats the yogurt. That would not be so practical --
however you could splice in a heavy duty dimmer switch
Don't use a dimmer
that is only for lights, as that would be too small current -- one for a
fan (motor) would be OK for something that only warms to 130°.
I definitely do not recommend using a fan speed control. They're
designed for inductive loads, and are at least overkill, and may not
work right at all. Almost any lamp dimmer is rated for more than the 100
Watts *max* a yogurt maker might run...