Physically, current strength is defined as the amount of charge that passes through the cross-section of an electrical conductor per unit of time. Mathematically, the current is described by the derivative of the charge $Q$ with respect to time $t$:
$$
I = \frac{\mathrm{d} Q}{\mathrm{d} t}
$$
The unit of electric current is the ampere (A), which corresponds to exactly one coulomb per second. The device used to measure electric current is called an ammeter and is denoted by the symbol A in circuit diagrams. Similarly, the device used to measure voltage is called a voltmeter and is denoted by the symbol V.
Even at currents of only a few milliamperes — values commonly found in technical applications — a large number of charge carriers move through the wire. For now, we restrict ourselves to direct current (DC), meaning the electrons in the conductor flow at every moment with the same current in the same direction. In this case, the current corresponds to the number $n$ of elementary charges $e$ passing through the conductor’s cross-section per second:
$$
I = \frac{ne}{T}
$$
Although the electric field in the conductor propagates at nearly the speed of light, the electrons themselves move with only a very small drift velocity, usually less than 1 mm/s. The fact that a light bulb still lights up immediately when switched on can be compared to a water hose already filled with water: as soon as the tap is opened, water flows out almost instantly at the other end. Similarly, the electric field propagates as a wave through the conductor, interacting with all electrons almost simultaneously.
Depending on the conductivity of a material, one distinguishes between conductors, insulators, and semiconductors. Metals such as gold, copper, and aluminum are good conductors with very low electrical resistance. In general, materials that conduct heat well also have a low electrical resistance. Examples of insulators include glass, ceramics, and plastics. Semiconductors have conductivities between those of conductors and insulators; common semiconductors used in technology include silicon (Si) and germanium (Ge).
The human body, although it has a relatively high electrical resistance, is not a perfect insulator. Its resistance varies significantly with moisture,