What instrument should I use to vary the power given to a heating element? I have a nickel-chrome wire as a heating element and I want to vary the power given to it to control the amount of heating the wire produces. Which instrument should I use? Please don't give theoretical explanations here, tell me how I can do it practically.
 A: Power is voltage times current, either of which is relatively easy to control.  If your accuracy requirement is not so tight that the change in resistance of the heating element matters, you can simply assume power is either proportional to the square of the voltage or the square of the current.
For tighter control, you need to do the math.  Generally you'd have a microcontroller controlling a power supply.  The micro measures both the voltage and current, multiplies them, then controls the power supply up or down to regulate the voltage x current product to the desired value.
It's no stretch at all for a micro to do a loop iteration every 10 µs (100 kHz rate), for example.  That will be much faster than the thermal time constants, so the micro is effectively controlling power in "real time", apparently continuously from the outside point of view.
In summary, this is a fairly basic control problem, with the speed requirements well within the range of what many microcontrollers can handle.  Attack it accordingly.
A: In simpler times, a rheostat would do the job. It diminishes and increases the current according to need:

A rheostat is a variable resistor which is used to control current. They are able to vary the resistance in a circuit without interruption. The construction is very similar to the construction of a potentiometers

Microelectronics is much more efficient and so the use has fallen off

Rheostats were often used as power control devices, for example to control light intensity (dimmer), speed of motors, heaters and ovens. Nowadays they are not used for this function anymore. This is because of their relatively low efficiency. In power control applications they are replaced by switching electronics

A: Multiple options available. The most simple one is a potentiometer that changes it's resistance when you turn it around.
