Moving Coil Galvanometer
The main parts of a moving-coil galvanometer are shown in figure.
The current to be measured is passed through the galvanometer. As the coil is in the magnetic field `vecB` of the permanent magnet, a torque `vecGamma=nivecAxxvecB` acts on the coil. Here `n=` number of turns, `i=` current in the coil `vecA=` area-vector of the coil and `vecB=` magnetic field at the site of the coil. This torque deflects the coil from its equilibrium position.
The pole pieces are made cylindrical. As a result, the magnetic field at the anns of the coil remains parallel to the plane of the coil evei)' Where even as the coil rotates. The deflecting torque is then `Gamma = niAB`. As the upper end of the suspension strip W is fixed, the strip gets twisted when the coil rotates. This produces a restoring torque acting on the coil. If the deflection of the coil is `theta` and the torsional constant of the suspension strip is `k`, the restoring torque is `ktheta`. The coil will stay at a deflection `theta` where
`niAB=ktheta`
`i=k/(nAB) theta`
Hence, the current is proportional to the deflection. The constant `k/(nAB)` is called the galvanometer constant.
We define the current sensitivity of the galvanometer as the deflection per unit current. From Eq. this current sensitivity is.
`phi/I=(NAB)/k`
A convenient way for the manufacturer to increase the sensitivity is to increase the number of turns N.
We choose galvanometers having sensitivities of value, required by our experiment.
We define the voltage sensitivity as the deflection per unit volt of applied potential difference
`phi/I=((NAB)/k)I/V=((NAB)/k)1/R`
The current to be measured is passed through the galvanometer. As the coil is in the magnetic field `vecB` of the permanent magnet, a torque `vecGamma=nivecAxxvecB` acts on the coil. Here `n=` number of turns, `i=` current in the coil `vecA=` area-vector of the coil and `vecB=` magnetic field at the site of the coil. This torque deflects the coil from its equilibrium position.
The pole pieces are made cylindrical. As a result, the magnetic field at the anns of the coil remains parallel to the plane of the coil evei)' Where even as the coil rotates. The deflecting torque is then `Gamma = niAB`. As the upper end of the suspension strip W is fixed, the strip gets twisted when the coil rotates. This produces a restoring torque acting on the coil. If the deflection of the coil is `theta` and the torsional constant of the suspension strip is `k`, the restoring torque is `ktheta`. The coil will stay at a deflection `theta` where
`niAB=ktheta`
`i=k/(nAB) theta`
Hence, the current is proportional to the deflection. The constant `k/(nAB)` is called the galvanometer constant.
We define the current sensitivity of the galvanometer as the deflection per unit current. From Eq. this current sensitivity is.
`phi/I=(NAB)/k`
A convenient way for the manufacturer to increase the sensitivity is to increase the number of turns N.
We choose galvanometers having sensitivities of value, required by our experiment.
We define the voltage sensitivity as the deflection per unit volt of applied potential difference
`phi/I=((NAB)/k)I/V=((NAB)/k)1/R`