About system disturbance due to measurement
Disturbance of the measured system by the act of measurement is one source of systematic error. If we were to start with a beaker of hot water and wished to measure its temperature with a mercury-in-glass thermometer, then we would take the thermometer, which would be initially at room temperature, and plunge it into the water. In so doing, we would be introducing a relatively cold mass (the thermometer) into the hot water and a heat transfer would take place between the water and the thermometer. This heat transfer would lower the temperature of the water. Whilst in this case the reduction in temperature would be so small as to be undetectable by the limited measurement resolution of such a thermometer, the effect is finite and clearly establishes the principle that, in nearly all measurement situations, the process of measurement disturbs the system and alters the values of the physical quantities being measured.
Another example is that of measuring car tire pressures with the type of pressure gauge commonly obtainable from car accessory shops. Measurement is made by pushing one end of the pressure gauge on to the valve of the tire and reading the displacement of the other end of the gauge against a scale. As the gauge is used, a quantity of air flows from the tire into the gauge. This air does not subsequently flow back into the tire after measurement, and so the tire has been disturbed and the air pressure inside it has been permanently reduced.