Magnetic object affected by electromagnetic wave

Question

If a dipole antenna is strong enough, can it make a compass needle oscillate?

Answer 1

yes and no, with exceptions ;-) as follows.

A compass needle possesses rotary inertia and, if fluid-filled, viscous damping which means that by design, it responds sluggishly to fast changes in the direction of a magnetic field, and accurately to slow changes. Another way of expressing this is to say that to prevent the needle from jumping around upon getting perturbed, its RL time constant will be of order ~ several seconds.

(You can overcome this tendency by making the oscillating magnetic field hugely strong so as to overcome the inertia and damping of the needle.)

Now what this means is that if the frequency of the radio waves is of order ~10 hertz or anything greater, the needle has no time in which to even begin reorienting itself to the changing field and the needle will not wiggle in time with the radio waves.

How strong are these effects? They are significant. In my well-equipped minivan I have a large (5" diameter!) fluid-damped floor-mounted WWII-era master navigation compass as used in big cargo planes and passenger aircraft. All minivans should have at least one of these IMHO. And under the floor of the minivan are wires which carry current when the ignition is switched on.

With the compass card indicator motionless, I can turn on the ignition and at a rate of about one degree per second, the compass deflects to a maximum of about 20 degrees off true direction and holds there until I turn the ignition off. Then within 20 seconds, it settles back to true direction once again. With that time constant, flicking the ignition on and off at 10Hz will not get the needle to respond.

To make a compass respond to EM waves, then, you need frequencies of less than 1 hertz. These are extraordinarily difficult to generate artificially but occur all the time in thunderstorms, and are strong enough to perturb navigation compasses in aircraft.

Answer 2

It's extremely easy to do that! Just an ordinary ferrite rod antenna (a magnetic dipole) will do so, if you place the compass no more than a few inches away from it and feed the antenna with a current of 1 Hertz, as suggested in the other answer.

The easiest way to do the experiment is to take a ferrite antenna from an old radio, and intermittently connect it to a battery (i.e. half a second on, half a second off, repeated).

One should note that at these low frequencies the antenna actually does not transmit very efficiently (ferrite antennas are meant for frequencies in the order of 1 MHz) but that is not prohibitive here, because we are in the near-field region. And we should stick to the question: "can it make a compass needle oscillate?" Which does not at all require that this should involve waves that can be received very far away.