Question

Rheostat -> Changes current without changing voltage

But if current inverses, voltage should also because V proportional to I. Then how does rheostat do it?

Answer 1

According to Ohm's law, the voltage across and current through a resistor are proportional.

If one places a variable resistance (rheostat) across a voltage source, the voltage across the rheostat is fixed.

By fixing the voltage across the rheostat, the current through becomes inversely proportional to the resistance:

i(R)=V /R

So we can change current by changing resistance as voltage is constant.