📚 Measuring Current and Potential Difference | 测量电流与电势差
Understanding how electric charge flows and how energy is transferred in a circuit begins with two fundamental quantities: current and potential difference. Mastering the techniques to measure them accurately is a core practical skill in electromagnetism. This guide explains the concepts behind these measurements, the correct use of ammeters and voltmeters, and the essential circuit rules that govern their behaviour.
理解电荷如何流动以及能量如何在电路中转移，要从两个基本物理量入手：电流和电势差。掌握准确测量它们的技巧是电磁学中的核心实验技能。本指南解析这些测量背后的概念，讲解电流表和电压表的正确使用方法，以及支配其行为的基本电路规则。
1. What is Electric Current? | 什么是电流？
Electric current is the rate of flow of electric charge past a point in a circuit. It is measured in amperes (A), where 1 ampere equals 1 coulomb of charge flowing per second. In metallic conductors, current consists of a drift of free electrons moving from the negative terminal towards the positive terminal of a power supply. Conventional current, however, is defined as flowing from positive to negative.
电流是电荷流过电路中某一点的速率。它以安培（A）为单位，1安培等于每秒流过1库仑的电荷。在金属导体中，电流由自由电子从电源负极向正极漂移形成。然而，传统电流方向被定义为从正极流向负极。
Mathematically, current I is expressed as:
I = Q / t
where Q is the charge in coulombs and t is the time in seconds. For example, if 12 coulombs pass a point in 4 seconds, the current is 3 A.
数学上，电流 I 表示为：
I = Q / t
其中 Q 是电荷量（库仑），t 是时间（秒）。例如，若 12 库仑电荷在 4 秒内通过某点，则电流为 3 安培。
2. What is Potential Difference (Voltage)? | 什么是电势差（电压）？
Potential difference, often called voltage, measures the energy transferred per unit charge between two points in a circuit. It is the ‘push’ that drives current through a component. The unit of potential difference is the volt (V), where 1 volt means 1 joule of energy is transferred for every coulomb of charge that passes.
电势差，常称作电压，衡量电路中两点间每单位电荷转移的能量。它是推动电流通过元件的“推力”。电势差的单位是伏特（V），1伏特表示每通过1库仑电荷就有1焦耳的能量被转移。
In equation form: V = W / Q, where V is the potential difference in volts, W is the work done (energy transferred) in joules, and Q is the charge in coulombs. A 6 V battery does 6 joules of work on each coulomb of charge passing through it.
公式为：V = W / Q，其中 V 是电势差（伏特），W 是做功或转移的能量（焦耳），Q 是电荷（库仑）。一节 6 伏电池对每个通过它的库仑电荷做 6 焦耳的功。
3. The Ammeter: Principle of Operation | 电流表：工作原理
An ammeter is designed to measure the current flowing through a branch of a circuit. To do this without altering the current significantly, an ideal ammeter has an extremely low internal resistance. Most analogue ammeters work on the magnetic effect of current: a coil placed in a magnetic field experiences a torque proportional to the current, moving a pointer across a scale.
电流表用于测量流过电路某支路的电流。为了在测量时不显著改变电流，理想的电流表具有极低的内阻。大多数模拟电流表利用电流的磁效应工作：置于磁场中的线圈会受到与电流成正比的转矩，从而带动指针在刻度盘上偏转。
Digital ammeters use a known shunt resistor and amplify the tiny voltage drop across it to obtain current. They are typically more precise and easier to read, but understanding their analogue counterpart helps comprehend the underlying physics.
数字电流表利用已知的分流电阻，放大其上微小的电压降来获得电流。它们通常更精确且易于读数，但理解模拟电流表有助于领会其背后的物理原理。
4. Connecting an Ammeter Correctly | 正确连接电流表
The ammeter must always be connected in series with the component whose current you wish to measure. This ensures that the same current flows through the ammeter as through the component. Break the circuit at the point of interest and insert the ammeter so it becomes part of the single conducting loop.
电流表必须始终与被测元件串联连接。这样才能确保流过电流表与被测元件的电流相同。在待测点断开电路，将电流表接入，使其成为单一导电回路的一部分。
Never connect an ammeter directly across a power supply or in parallel with a component. Its extremely low resistance would create a short circuit, causing dangerously large currents that can damage the meter, blow fuses, or overheat wires.
绝对不要将电流表直接跨接在电源两端或与元件并联。其极低的电阻会形成短路，产生危险的大电流，可能损坏电表、烧断保险丝或导致导线过热。
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Incorrect: Ammeter placed in parallel – draws huge current, risk of damage.
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错误：电流表并联接入——会流过巨大电流，有损坏风险。
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Correct: Ammeter in series – current is limited by circuit resistance.
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正确：电流表串联——电流受电路电阻限制。
5. The Voltmeter: Principle of Operation | 电压表：工作原理
A voltmeter measures the potential difference between two points. For accurate readings, an ideal voltmeter should have infinite resistance so that it draws no current from the circuit. In practice, voltmeters have very high internal resistance (often millions of ohms), minimising their impact on the circuit.
电压表测量两点之间的电势差。为获得准确读数，理想电压表应具有无穷大电阻，从而不从电路中抽取电流。实际电压表的内阻非常高（通常以兆欧计），以减小对电路的影响。
Analogue voltmeters are essentially sensitive ammeters with a large series resistor (multiplier) built in. The amount of deflection is proportional to the voltage across the terminals. Digital voltmeters convert the input voltage into a digital readout via an analogue‑to‑digital converter.
模拟电压表本质上是一个灵敏电流表，内部串联了一个大电阻（倍压器）。指针偏转量与端子间的电压成正比。数字电压表通过模数转换器将输入电压转化为数字显示。
6. Connecting a Voltmeter Correctly | 正确连接电压表
A voltmeter must be connected in parallel with the component across which the potential difference is to be measured. Connect the voltmeter’s positive (red) terminal to the more positive side of the component and the negative (black) terminal to the more negative side, respecting the polarity of the circuit.
电压表必须与待测电势差的元件并联连接。将电压表的正（红）极连接到元件电位较高的一侧，负（黑）极连接到电位较低的一侧，注意电路的极性。
Because the voltmeter has a very high resistance, it takes a negligible current, leaving the circuit essentially undisturbed. If wrongly connected in series, the high resistance would drastically reduce the current in the loop, and the voltmeter would read close to the supply voltage while the intended components would not function correctly.
由于电压表电阻极高，它从电路抽取的电流可以忽略不计，电路基本不受干扰。如果错误地串联连接，高电阻会大幅减小回路电流，电压表读数将接近电源电压，而预期的元件将无法正常工作。
7. Range Selection and Scale Reading | 量程选择与刻度读数
Before taking a measurement, always select a suitable range on the meter. Choose a range that is higher than the maximum expected value to prevent the meter from overloading or banging the pointer against the end stop. Once you have an approximate reading, you may shift to a lower range for greater precision.
测量前，务必在电表上选择合适的量程。选择高于预期最大值的量程，以防止电表过载或指针猛烈撞击止挡。得到大致读数后，可切换至更低的量程以获得更高的精度。
Analogue scale reading requires careful estimation. Note the full‑scale deflection value and the number of divisions to determine the value per division. Always view the pointer perpendicular to the scale to avoid parallax error. Many analogue meters include a mirror strip behind the scale to help align the eye.
模拟表盘的读数需要仔细估读。注意满偏值以及分度数量，以确定每分度的值。始终垂直表盘观察指针，以避免视差。许多模拟电表在刻度背后装有镜条，帮助视线对准。
Digital meters display the value directly, but attention must be paid to the decimal point and unit prefix (e.g., mA, μA, mV). An ‘overload’ or ‘OL’ warning indicates the range is too low.
数字电表直接显示数值，但需要注意小数点和单位前缀（如 mA、μA、mV）。显示“超量程”或“OL”表示量程太低。
8. The Role of Shunts and Multipliers | 分流器与倍压器的作用
To extend the range of an ammeter, a low‑resistance shunt is connected in parallel with the meter movement. Most of the current bypasses the meter coil through the shunt, allowing a larger total current to be measured while only a known fraction passes through the sensitive movement.
要扩展电流表的量程，需将一个低电阻的分流器与表头并联。大部分电流会通过分流器绕过表头线圈，从而能够测量较大的总电流，而只有已知比例的小电流流过灵敏表头。
Similarly, to extend the range of a voltmeter, a high‑resistance multiplier is connected in series with the meter coil. This multiplier drops the majority of the voltage, ensuring that only a small safe voltage appears across the movement itself. By selecting different multipliers, multirange voltmeters can be constructed.
类似地，要扩展电压表的量程，需将高电阻的倍压器与表头线圈串联。倍压器承担了大部分电压，确保表头本身只承受很小的安全电压。通过选择不同的倍压器，可以制成多量程电压表。
9. Digital vs Analogue Meters | 数字仪表与模拟仪表
Digital meters are widely used in today’s laboratories because they offer high input impedance, reduce human reading errors, and can auto‑range. Their accuracy is usually specified as a percentage of the reading plus a certain number of least‑significant digits. They are also less fragile than analogue movements.
数字电表在当今实验室中广泛使用，因为它们具有高输入阻抗，可减少人为读数错误，并能自动选择量程。其精度通常表示为读数的百分比加若干个最低有效数字。它们也比模拟表头更坚固。
Analogue meters, while less common, still provide a visual indication of trends and fluctuations that might be missed on a digital display. They do not require a battery for the voltmeter/ammeter function (except for the ohmmeter). Understanding both types is vital for physics education, as analogue meters illustrate the magnetic principles of current measurement beautifully.
模拟电表虽然不常见，但仍能提供数字显示可能忽略的趋势和波动视觉指示。它们在进行电压/电流测量时无需电池（欧姆表除外）。理解两种类型对物理教育至关重要，因为模拟电表精美地展示了电流测量的磁学原理。
10. Practical Investigation: Measuring I and V in a Simple Circuit | 实践探究：简单电路中的电流与电压测量
A typical experiment to investigate the relationship between current and potential difference for a fixed resistor involves setting up a circuit with a power supply, a variable resistor (rheostat), an ammeter in series, and a voltmeter in parallel across the resistor. By adjusting the rheostat, you can vary the current and record corresponding pairs of voltage and current readings.
探究固定电阻的电流与电势差关系的典型实验，需要搭建一个由电源、可变电阻器（变阻器）、串联的电流表以及并联在电阻两端的电压表组成的电路。通过调节变阻器，可以改变电流并记录对应的电压和电流读数组。
Data collected often produces a straight line through the origin, confirming Ohm’s law V = IR. During the practical, it is important to take readings swiftly to avoid heating the resistor, which can change its resistance and lead to a curved graph. Safety components such as a fuse and switch should be included.
收集到的数据通常会形成一条过原点的直线，验证欧姆定律 V = IR。实验过程中，务必快速读取数据，避免电阻发热，因为温度变化会改变电阻值，导致图像弯曲。电路中应包含保险丝和开关等安全元件。
11. Common Mistakes and Safety Precautions | 常见错误与安全注意事项
Common errors include reversing the meter connections, which can cause analogue needles to deflect backward and damage delicate movements, using the wrong range, and forgetting to reconnect the circuit properly after inserting an ammeter. Always practice correct polarity: red to positive, black to negative.
常见错误包括电表接线反接，这会导致模拟指针反向偏转、损坏精密表头；使用错误的量程；以及插入电流表后忘记正确重接电路。务必遵守正确的极性：红接正极，黑接负极。
Safety precautions: always start with the highest range before turning on the circuit, never let bare wires touch, use insulated crocodile clips, and ensure the power supply is switched off while modifying the circuit. For high‑voltage experiments, use meters with shrouded sockets and never touch exposed conductors. Even low‑voltage circuits can produce high currents; thus a fuse is a necessary safeguard.
安全注意事项：接通电路前始终从最高量程开始；不要让裸线触碰；使用带绝缘套的鳄鱼夹；修改电路时确保电源已关闭。对于高压实验，使用带护套插孔的仪表，切勿触碰裸露导线。即使低压电路也可能产生大电流，因此保险丝是必要的防护措施。
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