How Current Affects a Magnetic Field
Current flowing through a coil produces the magnetic field that surrounds the coil. The strength of the magnetic field is proportional to the amount of current flowing through the coil. As the current increases, the strength of the magnetic field increases, and as the current decreases, the strength of the magnetic field decreases. For example, if the current through a certain coil is increased from 1 to 1.6 amperes, the magnetic field around the coil will be stronger for 1.6 amperes than it was for 1 ampere.
Now suppose a spring is attached to the iron bar so that it holds back the bar. The magnetic field, therefore, will have to overcome the spring tension. The stronger the field, the more the spring tension will be overcome. Therefore, the greater the current flowing through the coil, the greater will be the magnetic field and the further the iron bar will be drawn into the coil. The greater the current flowing through a coil enclosing two iron bars, the further each iron bar will be repelled from the other. Similarly, the moving coil will rotate further as the current through the coil increases. All electromagnetic current meters operate on the principle that the strength of the magnetic field about a coil is proportional to the amount of current flowing through it.
Types of Electromagnetic Current Meters
Magnetic fields can be used to cause motion between magnetized objects and the amount of motion is proportional to the strength of the magnetic field, which, in turn, is proportional to the current that produces the field.
There are two basic types of electromagnetic current meter movements in use today: the moving-coil type and the moving-iron type. Both types operate on electromagnetism, but each type uses magnetic fields in a slightly different way to indicate the amount of current flowing in a circuit. Also, each type has certain advantages and disadvantages.
It is not easy to tell the difference between the different types of meters just by looking at them or by using them. From the outside, they appear the same, and they are generally used in the same manner to take current measurements. But, when you know how each type works, you can easily identify them when you examine their movements.
The Moving-Coil Meter Movement
In 1882, Arsene d’Arson val, a Frenchman, invented the galvanometer, named in honor of Italian scientist Galvani. The meter was basically a device that used a stationary permanent magnet and a moving coil. Although the early galvanometer was very accurate, it could only measure very small currents and was very delicate. Over the years, many improvements were made that extended the range of the meter and made it very rugged.
Because it is extremely accurate and rugged, the moving-coil movement is by far the most common meter movement used today. It is the basic meter movement used to measure current, voltage, resistance, and a wide variety of other electrical quantities. Therefore, a thorough understanding of the moving coil meter is a “must” for anyone studying electricity.
In its simplest form, the moving-coil meter uses a coil of fine wire wound on a light aluminum frame. A permanent magnet surrounds the coil. The aluminum frame is mounted on pivots to allow it and the coil to rotate freely between the poles of the permanent magnet. When current flows through the coil, it becomes magnetized, and the polarity of the coil is repelled by the field of the permanent magnet. This causes the coil frame to rotate on its pivots, and the distance it rotates depends on how much current flows through the coil. Therefore, by attaching a pointer to the coil frame and adding a scale calibrated in units of current, the amount of current flowing through the meter can be measured.
References
http://www.explainthatstuff.com/movingcoilmeters.html
https://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_movement.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/movcoil.html