An ammeter is said to be sensitive if it takes minimum current to produce large deflection. This is a generalized definition of sensitivity of ammeter.
The sensitivity of ammeter may be expressed in three ways as follows
(a) Current Sensitivity:
It is defined as the ratio of the deflection of the ammeter to the current producing the deflection. The current is expressed in micro amperes and the deflection in millimeters. When the scale is not marked in millimeters, the deflection can be taken in scale divisions. The current sensitivity of ammeter formula is
S_{1} = (d/I) (mm/μA) /(scale divisions / micro amperes)
Where d = the deflection of the ammeter in scale divisions.
I = the current through the ammeter in microamperes.
(b) Voltage Sensitivity:
It is defined as the ratio of the ammeter deflection to the voltage producing this deflection. Hence
S_{v} = (d/V) (mm/mV) /(scale divisions / micro amperes)
Where d = the deflection of the ammeter in scale divisions
V = the voltage applied to the ammeter in milli V.
(c) Megohm Sensitivity :
It is defined as the number of megohms required in series with the ammeter to produce one scale division deflection when 1 V is applied to the circuit. The applied current equals practically to 1/R micro amperes and produces one division deflection. Numerically, the current sensitivity and megohm sensitivity are equal. Therefore,
S_{R} =d /I = S_{r} (mm/μA) / (scale divisions / micro amperes)
Where d = deflection of the ammeter in scale divisions.
I = Ammeter current in micro amperes.
(d) Factors to be Considered in Improving the Sensitivity of Current Meter:
1. The mass of the moving parts must be kept minimum.
2. Friction losses must be minimised.
3. Sensitivity of an ammeter can be increased by increasing the as available flux density.. This can be done by increasing the number of turns of the coil.
If the area of winding is fixed, increase of number of turns can be done using thinner gauges of wire. However this increases the resistance of the winding.
It is to be noted that increase of the resistance of coil puts in a limit to, the sensitivity of the ammeter.
4. Sensitivity can be increased by having small control constant.
5. Sensitivity can also be increased by reducing the leakage flux produced by the coil.
Commercially Available Moving Coil Ammeters:
The following table shows the standard ranges of the commercially available moving coil ammeters. They are called basic meters or basic movements, The voltage across the meter, the resistance of the meter including swamping resistance and ohms per volt of the basic meters is given in this table.
Table: Sensitivity of Commercially Available Moving Coil Ammeters
Full scale deflection current

Voltage across the meter

Meter resistance

Ohms per volts

25.0 μA

100 mV

4,000 ohm

40,000

50.0 μA

100 mV

2,000 ohm

20,000

100.0 μA

100 mV

1,000 ohm

10,000

200.0 μA

100 mV

500 ohm

5,000

1.0 mA

100 mV

100 ohm

1,000

5.0 mA

50 mV

10 ohm

200

10.0 mA

50 mV

5 ohm

100

20.0 mA

50 mV

2.5 ohm

50

The above table is useful in calculating the values of shunt resistances to extend the range of the ammeters. It is also useful in determining the value of series resistance required in case of voltmeters.
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