>>the knobs . . .
this is true, but one thing not mentioned,,,,,
except for 'southern fried chicken' - which is around here about 'half-deep frying' in cast iron - I use a deep copper/stainless sauce pot for deep frying. we don't do a lot of it. I have a fry thermometer with a clip.
even with nothing in the pot, the temperature of the oil from top to bottom will vary by 40F degrees, no problem! I use a big metal slotted spoon to 'agitate' the oil - working on getting the temperature about in the middle depth of the pan where I want it.
when the food goes in, it chill shocks the oil; I have to up the knob. then I have down the knob to prevent overheating - but if it will over heat depends on what and how much of what goes in the pot of oil and how long it needs to cook through.
beer-battered butterflied shrimp is the most 'common' deep fry around here. in a four quart sauce pan, on a gas stove, once it's up to temp and the shrimp go in 6-8 at a time, I normally only have to increase the heat and leave it there. shrimp cooks pretty quick, so the temp does go 'down' then 'up' but within an acceptable 'it's crisp but not burnt' range.
the thermometer on a clip is installed 'resident' so I can see the temps falling then rising - and I do stir the stuff around to distribute the heat.
thermostatic control is a nice touch - note that in most cases the thermostat and the heating elements are near each other on the bottom, and the food bits are floating on the top - where, lacking some stirring - it's cooler (making for greasy / soggy / etc.)
from my great aunt I had a purpose built deep fryer - with a plug-in controller,,, eventually the controller got too hot to touch, the plug too hot to touch, the cord - which was black&white cloth wrapped,,,(yeah, that old) - got too hot. could not find a new plug/wire/controller with the matching geometry so I opted to ditch it before it made fire-in-the-kitchen.....