Galvanometer and Wheatstone Bridge

Wheatstone Bridge
Wheatstone Bridge

Today is Physics, Let's move from the past to the present with a new article. I completed my high school education in Diyarbakır vocational high school, Department of Machinery. In the first year of high school, the name of the department was given as Construction Machinery and the courses other than the workshop courses were followed by the Department of Electricity. The students of the engineering department and the students of the electrical department were taking lessons together in common lessons. In the middle of the first semester, when the winter season was experienced, Tacettin Authorized from the Electricity Department (I may have spelled his last name wrong, to be honest), his two classmates from the electrical department, and Bülent Yılmaz, a Mechanical-Civil Engineer with whom I still share his friendship, were sitting in the garden of the school, Tacettin and two friends next to him. Bülent and I came to me and asked that used and idle electrical units were sold around the Great Mosque and that we were with them for the purchase of materials and to be a bridge in the shopping. Anyway, we arrived at the Great Mosque area. They talk to a middle-aged man who sells electrical equipment. That man said that the friends studying in the Electrical department can bring the material they want after 45 minutes. We left there to meet after 45 minutes, while I and Bülent were going to meet the man at the appointed time, Tacettin and two friends were not there.

Man: I brought the materials, which one do you want?
Brother: We are the bridge…
Speaking of bridge, Galvanometer came to my mind. The galvanometer device is an electrical device that works with the principle that the change in the electric current creates a magnetic field. If the electric current passing through a wire wound in the form of a solenoid changes, the magnetic field created around it moves the needle (Solenoid is a coiled coil in the form of a compressed spiral curve. The name solenoid is known to have been discovered by the French physicist André-Marie Ampère to design a helical coil). At this point, all ammeters and voltmeters that you see and use in secondary education or in industry are galvanometers.

At high school level, the Ammeter is connected in series with the circuit, and the Voltmeter is connected in parallel, as the technical term, it can be said that a resistance with a smaller value than the wire used in the Ammeter mechanism is connected in series with the mechanism, and in the voltmeter, a higher value resistor is connected in parallel.

The galvanometer shows small electrical currents, at this point we can say that it measures like a micro voltmeter or a micro ammeter. The name of the galvanometer; It comes from the Italian Physicist Luigi Galvani, known for his successful work on electricity. According to sources, in 1820, Danish physicist Hans Christian Örsted discovered that a wire running an electric current deflected the magnetized needle of a nearby compass. At this point, we can say that Örsted's discovery and the production of the galvanometer device, which consists of a coil around a magnetized needle that can rotate on a pin, is based on the same principle. Figure 1 shows the Galvanometer display.


Wheatstone Bridge Figure
Figure 1: Wheatstone Bridge

We said that the tiny deviations in the circuit show the value of micro ammeter or micro voltmeter in the environment and Galvanometer is used as a measurement tool. If we examine the unit where the galvanometer is located in the middle in Figure 1, a method will be needed to find the value of the resistance with an unknown value, with the help of the resistors with known values ​​used in the circuit. At this stage, "wheatstone bridges" are commonly used for small resistors in the circuit and especially for cable resistance measurements.

When we examine the Wheatsone circuit diagram in this method, which is used for resistors of unknown value,


Wheatstone Bridge Figure
Figure 2: Wheatstone Circuit Diagram (with Voltmeter)

In Figure 2, Wheatstone circuit diagram two is shown. When the Wheatsone bridge is examined here, there are two known value resistors (R1 and R3), one regulated resistor (R2) and one unknown value resistor (Rx) in the circuit. Here, the set resistor R2 is changed until the galvanometer shows zero.
when the galvanometer will show zero value.
Last word, Galvanometer is a device that measures current. It indicates whether there is current in the circuit and if there is current, it indicates its direction. If there is a pointer and scale scale to show the amplitude of the current in the circuit, depending on the sensitivity; It is called micro ammeter, milli ammeter or ammeter. If the scale is calibrated for quantities such as volts or ohms, it is called a voltmeter or ohmmeter.
Let's end with a simple example question,

Wheatstone Bridge Example
Wheatstone Bridge Example

Since the value obtained from the product of the blue arrows and the red arrows will be of equal magnitude, no current will flow through the middle resistor.
Dear reader, stay tuned.

📩 12/02/2022 17:12

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