Ammeter Design

The maximum current that the iron-vane movement can read independently is equal to the current sensitivity of the movement. However, higher currents can be measured if additional circuitry is introduced. This additional circuitry, as shown in [Fig. 1], results in the basic construction of an ammeter.
Fig. 1: Basic ammeter.
Fig. 2: Multirange ammeter.
The resistance $R_{shunt}$ is chosen for the ammeter in [Fig. 1] to allow $1 mA$ to flow through the movement when a maximum current of $1 A$ enters the ammeter. If less than $1 A$ flows through the ammeter, the movement will have less than $1 mA$ flowing through it and will indicate less than full-scale deflection.
Since the voltage across parallel elements must be the same, the potential drop across a-b in [Fig. 2] must equal that across c-d; that is,
$$(1 mA)(43 Ω) = R_{shunt} Is$$
Also, Is must equal $1 A - 1 mA = 999 mA$ if the current is to be limited to $1 mA$ through the movement (Kirchhoff's current law). Therefore,
$$(1 mA)(43 Ω) = R_{shunt}(999 mA)$$
$$R_{shunt} = {(1 mA)(43Ω) \over 999 mA}$$
$$ = 43 mΩ \text{(a standard value)}$$
In general,
$$\bbox[5px,border:1px solid grey] { R_{shunt} = {R_m I_{CS} \over I_{max} - I_{CS}}} \tag{1}$$

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