Observer effect
In physics, a more mundane observer effect can be the result of instruments that by necessity alter the state of what they measure in some manner. For instance, in electronics, ammeters and voltmeters usually need to be connected to the circuit, and so by their very presence affect the current or the voltage they are measuring. Likewise, a standard mercury-in-glass thermometer must absorb some thermal energy to record a temperature, and therefore changes the temperature of the body which it is measuring.