The first time ever I kissed your mouth
I felt the earth move in my hand
Like a trembling heart of a captive bird
That was there at my command.
– From ‘The First Time Ever I Saw Your Face’, Ewan MacColl
Voltage and current are basic elements of electrical measurement. These two have an inseparable relationship, which signifies interdependence of control between two elements, so if you change one element then the other changes automatically and behaves responsive. How are they changing? It depends on materials and devices, and the relationship becomes characteristics.
It is now very common in electrical measurement that one applies voltage (or current) stimulus and measure current (or voltage) response. There are instances, of course, where electrical responses are measured at other physical stimulation such as light, heat, vibration, and so on. In such cases both voltage and current becomes objects of the measurement, but the interdependence between them still exists.
When you apply a constant voltage bias to an unknown material, the amount of current flowing throughout can be expressed as an electrical resistance. It is determined by the amount of transported electrical charges at given bias, and by the mobility of the charge carriers. The electrical resistance of a metal is low enough due to high density of highly mobile free electrons, while the resistance of insulator is extremely high due to the lack of charge carriers. The semiconductor shows specific nonlinear behaviors along the nature of electronic structure of the material.
When you apply a time-varying voltage to a device, the time-varying current response depends upon capacitance and inductance of the material. The dielectric property of the material generates electrical current in the course of charging and discharging at device interfaces upon time-varying voltage, without actual migration of charge carriers inside the material, and electromagnetic induction also causes induced current. The electrical properties of the device can be more specifically analyzed by measurement of electrical impedance differentiated as relationship between time-varying voltage and current, which is useful to describe a device model in terms of linear parameters such as electrical resistance, capacitance, and inductance, or to extract various nonlinear parameters from specific device model.
In technical aspects of electrical measurement, it is of great practical importance to design electrical and electronic circuits for precise control and sensing of voltage and current at DC and AC conditions with proper calibrations. Trigger functions and high-speed digital samplings are necessary for effective measurement simultaneity of time-varying voltage and current. The electrical modulation techniques using synthesized DC and AC signal are is frequently used for various device characterizations. Bias canceling and phase lock-in techniques are useful for measurement of fine perturbation signals running on DC bias with accuracy.
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