>>1004596
>Where I can find the the same system of a knife switch ( pic related ) but has a potentiometer ( for speed control )
You won't find any potentiometer that can handle 30 amps. Potentiometers are low-watt devices (usually <2 watts or so, most are less than half a watt) that you apply a low voltage to, and then you use that low voltage or current output to control a power-dividing circuit.

You could use a rheostat.
A rheostat works just like a potentiometer (it's a variable resistor) but it is made BIG and wire-wound on a ceramic base, so it can dissipate a lot of heat. The one in the picture is about 8 inches across and 2 inches thick.

A lot of them have an "off" position (the wiper is raised) at one end--since they are often used for direct-control of something. The green arrow is pointing to what is likely the "off" position. That little nub (probably) lifts the wiper up enough to break contact with the metal parts entirely. Since both ends of the wire resistor are provided, you can use it with the 'off' position at the low-end or the high-end.

Rheostats are rather "old-fashioned" in that before semiconductors became relatively inexpensive (before the 1960's) the cheapest and easiest way to add variable-electric-control to something was to use a rheostat. Old churches and movie theaters that had dimmable lights had these big handwheels on the wall--that was a big rheostat--that the power for the lights ran through.

Be aware that rheostats also waste quite a bit of power in use.... They are a poor idea for anything that is battery-powered.

>>1004596

it's called a chadburn

and you shouldn't switch engine power directly with a potentiometer. use the potentiometer to control the power circuit.

>>1004640
A Chadburn is a signalling device, not a control device, and not a potentiometer. The linked article constrasts an EOT with a remote throttle.

https://en.wikipedia.org/wiki/Engine_order_telegraph#Compared_to_remote_control_throttle

>>1004646
>>1004640

how hard could it be to stick one of these in one of those. seriously...

>>1004677
it would burn up.
A rheostat passes all current through it. It dissipates a fair amount of power. These will be able to set a control voltage, but not be wired in series with whatever it is you're controlling.

>>1004596
https://www.youtube.com/watch?v=P9UjxG8sN1c