History of the Multimeter

The first pointer meter for measuring current was called a galvonometer, invented in 1820. It was used with a Wheatstone Bridge to compare the unknown resistance and voltage to be measured with the known voltage and resistance, and then measure the relevant voltage, current, resistance, etc. This method of measurement is laborious and inconvenient in the laboratory. The device is cumbersome and complex and not easy to carry.

▲ Galvanometer

The galvanometer can only roughly reflect whether there is current, but cannot give the exact value of the current. However, the ammeter using the moving coil mechanism (D'Arsonval/Weston transmission mechanism) can display the current.

The hollow coil wound with fine enameled wire is suspended in the magnetic pole of the permanent magnet. After the DC current passes through it, it can generate a rotational torque to drive the pointer to rotate. The magnetic field designed in a circular ring makes the Ampere force on the coil with current independent of the angle. A thin metal spring wire is used to generate a restoring torque, which makes the angle of rotation of the pointer proportional to the current passing through the coil. This device is called the D'Arsonval transmission mechanism, and it is still widely used in various types of analog electronic meters.

▲ D'Arsonal transmission mechanism

The ammeter based on the active coil mechanism can easily and conveniently measure the current without the need for a Wheatstone bridge. On this basis, by adding a shunt resistor, a series resistor and a stable DC power supply, the voltage, current and resistance of different ranges can be measured.

In the 1920s, as electron tube equipment became more and more widely used, the multimeter was born. It is said that the first modern multimeter was invented by Donald Macaie, an engineer at the British Post and Telegraph Office in 1920. In his work, in order to repair communication facilities, he needed to constantly measure the voltage, current, resistance, etc. in the circuit. He couldn't stand the trouble of carrying multiple meters at the same time, so he developed a multimeter that could measure voltage, current and resistance at the same time, which was called the Avometer at the time.

▲ Donald Macadie's multimeter

The ampere-ohm multimeter uses a movable coil mechanism pointer ammeter, with external precision voltage divider resistors and shunt resistors, and uses a gear switch and socket to select the measurement category and process range.

Macadie transferred his Avometer design to the Automatic Winding and Electrical Equipment Company (ACWEEC, founded in 1923), and it was commercialized and sold that year. Before the improved model 8, the Avometer could only measure DC voltage and current signals.

At that time, a pocket watch-style voltmeter was also popular. The shell was metal and much cheaper than the Avometer. Its shell was usually connected to the negative pole of the meter head. Although this simplification made operation easier, it also caused many careless electronic engineers to suffer from electric shocks.

This type of meter is usually relatively simple. For example, the instruction manual only indicates 33Ω/V, the dial is often not uniform, and there is no pointer zero adjustment screw.

▲ Pocket watch voltmeter

Pointer multimeters usually need to absorb a certain amount of current from the measured circuit to drive the rotating coil, such as a 50 microampere full-scale meter head, which is a commonly used high-sensitivity meter head. During measurement, if the pointer is fully deflected, it will need to continue to receive 50 microamperes of current from the measured circuit, which will affect the measurement results of some high-impedance circuits, making the read value lower than the normal value.

使用真空电子管来提高万用表的输入阻抗是非常必要的，它们被称为真空管万用表(VTVM ,VVM）。这种电子真空管万用表通常具有1MΩ以上的输入阻抗，它利用了真空管阴极跟随输出（电压串联负反馈）电路来提高输入阻抗，这样便使得万用表在测量时对被测电路不会产生显著的 影响。

▲ Vacuum tube multimeter

Before the invention of digital (integrated) multimeters, high impedance analog transistor circuits, or field effect transistors (FETs), were used to replace vacuum tubes in multimeter equipment. Modern digital multimeters use high impedance integrated circuits, and the input impedance can match or exceed the original vacuum tube multimeter.

▲ Modern digital multimeter

Today's multimeters have added many additional functions, such as decibel meters for measuring power, capacitance, transistor gain, frequency, duty cycle, display hold, etc. The buzzer on the multimeter can make a sound when measuring the on and off of the circuit, giving quick measurement feedback. With some special sensors, the multimeter can also measure temperature, luminosity, etc., and has a computer interface. Some can also read the measured value in language.