Electrical Instruments

Statement -1: The sensitivity of a moving coil galvanometer is increased by placing a suitable magnetic materials as a core inside the coil.

Statement -2: Soft iron has a high magnetic permeability and cannot be easily magnetized or demagnetized.

In a moving coil galvanometer, the torque on the coil can be expressed as $\mathrm{\tau }=\mathrm{ki}$, where $\mathrm{i}$ is the current through the wire and $\mathrm{k}$ is a constant. The rectangular coil of the galvanometer having number of turns $\mathrm{N}$, area $\mathrm{A}$, and moment of inertia $\mathrm{I}$, is placed in magnetic field $\mathrm{B}$. Find the torsional constant of the spring, if a current ${\mathrm{i}}_0$ produces a deflection of $\frac{\pi }{2}$ in the coil in reaching the equilibrium position.

A moving coil galvanometer on connecting with a high resistance in series is converted into:

The reading on a high resistance voltmeter, when a cell is connected across it, is 2.5 V. When the terminals of the cell are also connected to resistance of  $5 \mathrm{\Omega }$  as shown in the circuit, the voltmeter reading drops to 2.0 V. The internal resistance of the cell is:

The deflection in galvanometer falls to ${\left(\frac{1}{4}\right)}^{th}$, when it is shunted by $3 \Omega$ . If additional shunt of $\mathrm{2 \Omega }$ is connected in parallel to earlier shunt, the deflection in galvanometer falls to ${\left(\frac{1}{\alpha }\right)}^{th}$ value. Write the value of $2\alpha$.

A moving coil galvanometer has the following characteristics- Number of turns of coil$=50$, Area of coil$=70{\mathrm{mm}}^2$, Resistance of coil$=30 \Omega$, the flux density of radial field$=0.1\mathrm{T}$, Torsional constant of suspension wire$=7\times {10}^{-8}\mathrm{Nm}/\mathrm{rad}$. Calculate the voltage sensitivity. 

The voltage sensitivity is defined as the deflection per unit current.

What is the SI unit of the voltage sensitivity?

Write the expression of the voltage sensitivity.

A moving coil galvanometer produces deflection $20.5\mathrm{rad}$ for current $10\mathrm{\mu A}$. Calculate current sensitivity?

The SI unit of current sensitivity in galvanometer is

A convenient way for the manufacturer to increase the current sensitivity is to increase the number of turns in the galvanometer.

Write the expression for the sensitivity of the ammeter.

A galvanometer has current sensitivity of $10$ divisions per $\mathrm{mA}$ with $150$ equal divisions. The shunt resistance in $\mathrm{m\Omega }$ required to convert given galvanometer into an ammeter of range $6 \mathrm{A}$ is $x$. Write the value of $10x$ to the nearest integer. Here, galvanometer voltage is $75 \mathrm{mV}$.

A galvanometer has $36 \mathrm{\Omega }$ resistance. If a $4 \mathrm{\Omega }$ shunt is added to this, the fraction of current that passes through the shunt is:-

If an ammeter is to be used in place of a voltmeter, then with the ammeter we must connect a,

The ammeter has range of $1$ $\mathrm{A}$, without shunt. The range can be varied by using different shunt resistances. The graph between shunt resistance and range will have the nature as