ELECTRICAL MEASURING INSTRUMENTS

For the detection or measurement of electric current, potential difference, and resistance certain instruments have been devised viz. The galvanometer for the detection of small currents measurements of small currents of the order of micro amperes or mili amperes the voltmeter or potentiometer for the measurement of potential difference (and voltage) between two points of a circuit are the EMF of a source the ammeter for the measurement of large currents the wheatstone bridge the meter Bridge the post office box and the ohmmeter for the measurement of resistance.

THE MOVING COIL GALVANOMETER

The moving coil galvanometer is a basic electrical instrument it is used for the detection (and Measurement) of small currents.

Its underlying principle is the fact that when a current flows in a rectangular coil placed in a magnetic field it experiences a magnetic torque. If it is free to rotate under a controlling torque, it rotates through an angle proportional to the current flowing through it. The rotation or deflection thus indicates a current through it.The rotation or deflection indicates a current through it although the Galvanometer detects full scale by a small current. Nevertheless it can measure such small currents if the deflection is property calibrated. The primitive form of the moving coil galvanometer was developed by the French scientist D'Arsonval, edward Weston improved upon D'Arsonval original version and gave us the modern moving coil galvanometer.

The essential parts of the the moving coil galvanometer are:

1. A u shaped permanent magnet with cylindrical concave pole pieces
2. A flat coil of thin enamel insulated wire (usually rectangular)
3. A soft iron cylinder
4. A pointer
5. Scale

The flat rectangular coil of thin enamel insulated wire of suitable number of turns wound on a light non metallic frame is suspected between the cylindrical concave pole pieces of the permanent u-shaped magnet by a phosphor bronze strip. One end off the wire of the coil is soldered to the strip. The other end of the strip is fixed to the frame of the galvanometer and connected to an external terminal. It serves as 1 current lead through the current enter or leaves the coil.The other end of the wire of the coil is s soldered to a loose and soft Spiral of wire connected to another external terminal. The soft Spiral of wire serves as the other current lead. A soft iron cylinder, coaxial with the pole pieces, is placed within the frame of the coil and fixed to the body of the galvanometer. In the space between it and the pole pieces, where the coil moves freely, the soft iron cylinder makes the magnetic field stronger and radial such that into whatever position the coil rotates, the magnetic field is always parallel to its plane.

When a current passes through the galvanometer it experiences a magnetic deflecting torque which tends to rotate it from its rest position. As the Coil rotates its produces a twist in the suspension strip. The twist in the strip produces an elastic restoring torque. The coil rotates until the elastic restoring torque due to the strip does not equal and cancel the deflecting magnetic torque and then it attains equilibrium and stops rotating any further.

In the previous article the deflecting magnetic torque was derived as;

Deflecting magnetic torque=BIAN cos alpha where B equals to strength of the magnetic field.

• I equals to current in the coil
• A equals to area of the coil
• N equals to number of turns in the coil
• Alpha equals to the angle of deflection of the coil

The restoring elastic torque is proportional to the angle of Twist of the suspension strip provided it obeys hooks law. Thus restoring elastic torque equals to C theta, where is the angle of Twist of the suspension strip (Theta is different from but proportional to Alpha) and C is the torque per unit Twist of the suspension strip for equilibrium.

Deflecting magnetic torque equals to you re storing elastic torque.

If the magnetic field were uniform. Alpha would continuously increase with theta and cos alpha and factor would not be constant. Then the current I would not proportional to theta and the scale of the Galvanometer not linear. However, due to the radial magnetic field the plane of the coil is always parallel to the field irrespective of the position the coil rotates. So,alpha, the angle between the plane of the coil and the direction of the field is always zero. Hence cos alpha equals to 1.

The current through the coil is directly proportional to the angle of Twist of the suspension.

The coil instead of being suspended by strip is pivoted between two jewelled bearings. The controlling torque is provided by 2 to hair Springs one on either side of the coil and curling in the opposite sense. The two ends of the coil are connected one to each spring. The hair spring also serve as current leads to the coil. Light aluminium pointer is fixed to the coil which moves over a calibrated circular scale with equal divisions which measures the deflection or current directly.

AMMETER AND VOLTMETER 

Ammeter and voltmeter are simple pivoted type moving coil galvanometer with suitable modifications. We have already seen that when a current flows in the galvanometer its coil is deflected. The current is proportional to the deflection. If the scale is calibrated for the current, it can be used as an ammeter. Again since the galvanometer coil has fixed resistance, the current in it is proportional to the potential difference across its terminals. Each value of the current in it correspondence to a certain potential difference across its terminals. If it's scale is calibrated for voltage, it can be used as a voltmeter. Most of the galvanometer give full scale deflection for a small current or for a small potential difference apply to them. They cannot measure large current or potential difference that we usually come across in our daily life. For measuring them Galvanometer has to be modified.

THE AMMETER 

An ordinary Galvanometer usually has some fixed resistance R of its coil and it gives full scale deflection when a certain Maximum current I I passes through it. The maximum current is called the range of the the the Unided galvanometer. A current which lies within the range can be measured directly with the galvanometer. It can serve as an ammeter.for measuring a current this ammeter must be connected in series with the circuit to allow throw it the full current which is to be measured if R is large, the Inception of this ammeter will increase the resistance in the circuit and decrease the current it is intended to measure and undesirable situation. So as not to affect the current to be measured an ammeter essentially must have very small resistance.

On the other hand if it is desired to measure a current much larger than I the galvanometer can not be used directly as an ammeter.If tried it will certainly be changes, to measure any current upto I larger than I, the galvanometer has got to be modified such that while the current in the main circuit is I, the current in the galvanometer coil never exceeds I.This objective is achieved  bu connecting a by pass resistance, called a shunt, of appropriate small value across in parallel with the galvanometer coil which allows the large excess current through itself while known fraction of I within the value Ig passes through the galvanometer coil.The main current in the galvanometer coil and the shunt together is always a simple multiple of the current in the coil which can be found and the scale calibrated accordingly to read the main current directly.

Consider a galvanometer G whose resistance is Rg and which gives full scale deflection when current is Ig flows through it.Suppose we want to modify it into an ammeter to measure a maximum current I (or to increase its range to I).A shunt Rg of appropriate small resistance should be connected in parallel with the galvanometer such that while the current in the main circuit is I, the current in the galvanometer coil is Ig producing full scale deflection and that in the shunt is :

Ig = I - Ig

As it clear, the galvanometer coil of resistance Rg and shunt resistance Rs are connected in parrallel between junctions a and b, the potential difference , Vg across the galvanometer and that across the shunt Vg are the same so Vg=Vs=V

MULTI RANGE AMMETERS:

Sometimes an ammeter has more than one range which means it has as many different shunts as the ranges.The desired range is selected by inserting the proper shunt in position,In one type, one end and each shunt is permanently connected to a common terminal while the other end of each is connected through a range switch to second common.

In the other type the shunts are arranged.In sub-type (a) separate range terminals are provided together with the common terminal marked (+) and in sub-type (b) the proper range terminal can be connected by a range switch to the second common terminal.