The experiment is as follows

A solenoid is connected to a galvanometer and a magnetised iron rod is passed through it. (the apparatus is not ideal, and there is no zero error in the galvanometer)

I pulled the rod very rapidly out of the solenoid and stopped, i observed that the needle of the galvanometer moved to the right instantaneously, but while coming back to zero again, it didn't move instantaneously and its angular velocity was quite less than during it moves to right...

A similar observation is made when the rod is pushed and stopped.

The question that arose in my mind is that as the induced current is proportional to the time rate of the magnetic field then why didn't the needle moved to zero instantaneously when the rod id stopped?

here is a link showing this experiment check from 6:00 to 11:00


Assuming that the galvanometer has a coil in it then the effect is due to the relatively low total circuit resistance.

What this means is that we are dealing with a spring-mass system which is massively over-damped.

The mass is the coil and former and the spring is the return spring(s) which moves the coil to the equilibrium position when there is no current passing through it.

If the galvanometer is of a conventional type then the coil is probably wound on an aluminium former and the system is normally critically damped to enable the pointer to reach its final position in the shortest possible time.
The damping being achieved by an emf induced (Faraday) in the former because it is moving through a magnetic field which in turn induced currents in the former which oppose the motion of the former (Lenz).
If it is a ballistic galvanometer then the coil former will either be wooden or plastic and in normal use underdamped.

So what happens when a solenoid is connected to a galvanometer?
In effect there is coil and a complete circuit with a very low resistance.
As the coil moves through the magnetic field an emf is induced (Faraday) and because the resistance of the circuit of the circuit is so low the induced current is larger which produces a large opposition to the motion of the coil (Lenz) thus the system is heavily over-damped and the return to zero is slower than expected.

I suggest as an experiment adding a resistance box into the galvanometer and solenoid series circuit and noting the effect of changing the resistance which has been added. I would expect that depending on the value of the added resistance the maximum deflection might actually increase compared with having no extra resistance in the circuit and the return to zero might also be observed to be quicker.

  • $\begingroup$ please check out the video I have mentioned, I don't know if it is a conventional galvanometer.... and by the way, will this have, no effect(on the needle) when the needle moves to the right? $\endgroup$ – Abhinandan Angra May 20 '20 at 13:34
  • $\begingroup$ Going away from the zero position might involve a larger force than returning back to the zero position. In the video it is a centre zero moving coil galvanometer which in general use is critically damped. $\endgroup$ – Farcher May 20 '20 at 14:06
  • $\begingroup$ Thanks, got it. $\endgroup$ – Abhinandan Angra May 20 '20 at 15:28

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