Electric current is defined as electron flow. Coulomb is a unit of measurement of electron flow. Now suppose that the observer can make a short observation of the electron flow. If the observer starts the stopwatch and counts within 1 second that 6.25 x 1018 electrons flow through a certain point, the number of electrons is called 1 coulomb (C), and the flow of electrons per coulomb per second is called 1 ampere. (A).

Ampere (A) is the unit of measurement of electric current, abbreviated as “Ampere”, named after the French scientist Andre Ampere in the early 19th century. 1 ampere is defined as the cross section of a conductor passing through 1 coulomb of charge in 1 second, that is: 1 ampere = 1 coulomb/sec. (Note: This is not the definition of the International System of Units). In the written formula, the letter I (intensity) represents current, and its unit of measurement is ampere (A). For example, a certain current can be recorded as I = 10A, or the current flowing through a circuit is 10 amperes, or 10 amperes.

Figure 1 compares a hydraulic system with a circuit. Please note that in order to measure water flow and current, we connect the meter to the path of the “fluid”. The flow meter in the water measures the water flow, and the ammeter in the circuit can measure the current. The unit of measurement of water flow is gallons per second or gallons per minute, and the unit of measurement of current is coulombs per second, and 1 coulomb per second is equal to 1 ampere. In Figure 1, the water flow is limited by the valve, and the current is limited by the motor circuit resistance. A simple calculation can be used to determine the magnitude of the current: amperage is equal to the number of volts divided by the value of resistance, that is, amperage = number of volts/resistance value.

① Ammeter
An ammeter is also called an ammeter and is used to measure the current in a circuit. There are two types of ammeters, namely: series ammeter and clamp ammeter (current clamp). The series ammeter (see Figure 2) is directly connected to the current path when in use and becomes a part of the circuit, as shown in Figure 1. For this reason, the circuit must be disconnected first, and then the circuit must be connected after connecting the ammeter. In order to reduce the impact on the current, the internal resistance of the series ammeter is very low, usually less than 0.1 ohm. If the resistance of the ammeter itself is large, the current in the circuit will decrease, and it may even fail to work normally. The series ammeter can be an analog meter with a scale and pointer (see Figure 3a), or a digital meter with a digital display (see Figure 3b). The meter shown in Figure 3 is called a multimeter because it can measure a variety of electrical parameters such as current (amperes), voltage (volts), and resistance (ohms).

The clamp-on ammeter (see Figure 4) does not need to be connected to the current path when in use. This kind of ammeter can detect the magnetic field around the wire. It has an openable jaw to clamp a wire through which an electric current flows. If the jaws clamp multiple wires with current, the magnetic field around the wires will be weakened or even disappear, and the reading of the ammeter will be inaccurate or even zero. For some types of clamp ammeters, the magnetic field generated by the wire current will induce voltage to the coil in the clamp. The principle is similar to that of a transformer, and then it is converted into a current reading. This meter can only measure alternating current (AC).

In other designs, a semiconductor device called a Hall generator is used to detect the magnetic field. In the Hall circuit, a constant current generator provides current to a semiconductor module, and the terminal of this module is connected to the measuring mechanism (head) of the ammeter. If the magnetic field passing through the semiconductor module is zero, the voltage at the output terminal of the Hall device is zero, and the reading of the ammeter is zero. When the module senses a nearby magnetic field, it will generate a voltage across the module, the value of which is proportional to the current in the circuit under test. After the meter head is calibrated, the voltage can be converted into a current measurement value and displayed. The ammeters in Figure 4a and Figure 4b both have digital readings. The ammeter using the Hall generator principle can measure alternating current (AC) and direct current (DC).

②Conventional current
The early theory used to describe current is called conventional current theory. The theory holds that current flows from the most positive point to the most negative point in the circuit. This conventional current theory is still used in some occasions, for example, most cars ground the negative pole of the battery. When the negative electrode is used as the ground terminal, the voltage of the positive electrode is considered to be higher than that of the earth, which is the live wire (high voltage) end, and the car technicians believe that the current flows from the live wire end. Many engineering schools also use conventional current theory when teaching semiconductor courses. Circuit schematics and some component symbols use arrows to indicate the direction of current, and the direction of these arrows is the direction of conventional current. Components such as diodes and triodes all use this arrow symbol. (When using the electron flow theory, as discussed in the next section, the direction of the current in the schematic diagram is opposite to the direction of the arrow sign.) (Note: The conventional current theory defines the direction of the current as the direction of the flow of “positive charge”, so it differs from The electrons move in the opposite direction.
Photovoltaic (PV) cells are made of semiconductor materials, and the current in a photovoltaic cell is similar to that in a semiconductor diode. Some schematic diagrams that use photovoltaic cells as a power source show the normal current direction, so don’t be confused when you see that the current is flowing from the positive to the negative.

③Electronic current
The new current theory believes that the direction of current should be the direction of movement of electrons, from the negative potential end to the positive potential end. Electrons with negative charges are attracted by the positive electrode of the voltage source and flow from the most “negative” point in the external circuit to the most “positive” point in the circuit. Thus, current flows from the negative electrode of the battery to the positive electrode of the battery through the conductor. This theory of electron flow has been widely recognized.
Figure 5 shows the symbol and current direction of the diode. A diode is an element that only allows current to flow in one direction. The arrow end is called the anode, and the straight end is called the cathode. In the figure, the direction of the electron current is opposite to the direction of the arrow, while the direction of the conventional current is the same as the direction of the arrow. But in both circuit diagrams, the anode is connected to the positive (+) of the circuit, and the cathode is connected to the negative (-). If you connect the cathode to the positive terminal of the circuit, it is like the switch is turned off, and current will not flow through the circuit. Please note that electrons actually flow from the negative electrode to the positive electrode.

④Oriented electron flow
The electrons in conductors (such as copper wires) are always active due to the influence of heat and other factors. However, random electron movement does not create an electric current. If the electrons are to form an electric current, the movement of the electrons must be guided. Connecting a device such as a battery or generator to the circuit can generate a flow of oriented electrons. For example, a battery has a negative electrode and a positive electrode. The negative electrode repels electrons into the wire or load, and the positive electrode attracts these electrons. So the battery guides electrons through the circuit. In the first chapter, this principle is called the law of charge. Figure 6 shows the flow of guided electrons. The electrons are negatively charged and are therefore repelled by the negative electrode of the battery and attracted by the positive electrode.