I came across a scenario where two people are spinning a wire with no specific magnetic field: enter image description here

In such a scenario, can any current be induced due to the change in magnetic flux (where the magnetic field is that of Earth)?

  • $\begingroup$ Steering of moving charges by Earth's magnetic field (which causes the effect you're asking about) is what converts the gamma rays from a nuclear explosion into an "electromagnetic pulse." There's a nice explanation in arxiv.org/abs/1611.03390 $\endgroup$
    – rob
    Commented Jan 21, 2020 at 13:59
  • $\begingroup$ Not exactly what you are asking, but somehow related: en.wikipedia.org/wiki/Electrodynamic_tether $\endgroup$
    – EigenDavid
    Commented Jan 21, 2020 at 15:03

2 Answers 2


Yes, while the Earth's geomagnetic field that the wire is moving through is very weak (0.25 to 0.65 gauss at Earth's surface) compared to commonly used generators, it does exist, and will produce some current in a moving coil.


With the apparatus as drawn it is unlikely that a reading will be noticed on the moving coil galvanometer.

There is a circuit which has a magnetic flux linked with it due to the horizontal component of the Earth’s magnetic field.
With a change of circuit area there will be a change in the magnetic flux linked with the circuit which will induce an emf in the circuit - Faraday.
As there is a complete electrical circuit an induced current will flow in the circuit.
However the induced emf and hence the induced current will be very small because the horizontal component of the Earth’s magnetic field is small, $\approx 30\,\rm \mu T$.
So if the area changes by $0.01\,\rm m^2$ in $0.3$ second the induced emf will by of the order of $1\,\rm \mu V$.

There is problem with detecting this small changing voltage.
Effectively the galvanometer has a short circuit across its terminal which will mean that the motion of its coil will be heavily damped. This is because the coil in the galvanometer will be moving in the magnetic field of its internal magnet, an emf will be induced and an induced current which will oppose the motion of the coil. Note that the induced current is made larger by reducing the resistance of the circuit connected to its terminals.


A variation of the method shown in the diagram using an Earth inductor and a ballistic galvanometer to measure the Earth’s magnetic field.


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