I vary the volume control

Does my radio get louder if I vary the volume control  

We want to turn up the radio because it s noisy outside, and we want to hear what is broadcast. We therefore turn the volume knob toward loud . At its most basic, the volume control is a variable resistor, across which we pass a current from the battery, acting much like a kettle element. If we turn up the volume control then a larger current is allowed to flow, causing more energy to be produced by the resistor. As a listener, we hear a response because the sound from the speakers becomes louder. The speakers work harder. 

While the magnitude of the noise and the position of the volume knob are both variables, they represent different types, with one depending on the other. The volume control is a controlled variable because the listener dictates its position. The amount of noise is the observed variable because it only changes in response to variations in the controlled variable, and not before. 

We consciously, carefully, vary the magnitude of the controlled variable and look at the response of the observed variable. 

Physical chemists often depict relationships between variables by drawing graphs. The controlled variable is always drawn along the x-axis, and the observed variable is drawn up the y-axis. 

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