At this point in this documentary about the history of electricity: https://www.youtube.com/watch?v=oPnS2WO2_0k&t=4m40s the guy says the Maxwell calculations predicted the generation of certain types of waves, when there is a change in the direction of the current.

I did not have time to study the derivation of the Maxwell equations, so I don't know exactly how they work, but could somebody show me what calculations predicts this phonomena?

Further, in this video, the guy shows a wireles telegraph, simply made with this theory. What is the minimum knowledge I need to build one? (i'm gonna build it inside my mind)

I need to know if there's a paper or book that talks about this subject: how to generate this waves with an experiment, and how to construct a simple wireless telegraph.

  • 1
    $\begingroup$ How well versed are you in the field of multi-variable calculus? do you know these terms: gradient, divergence, curl? $\endgroup$ Jul 8, 2014 at 7:17
  • $\begingroup$ There is that old TV show, The Mechanical Universe that explained these subjects wonderfully, if you can find it. $\endgroup$
    – rodrigo
    Jul 8, 2014 at 8:00
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    $\begingroup$ The calculation to derive the EM wave equation is described in detail in the Wikipedia article Electromagnetic wave equation. $\endgroup$ Jul 8, 2014 at 10:05
  • $\begingroup$ Basically, you want a radio :) $\endgroup$
    – sailx
    Jul 8, 2014 at 11:06

1 Answer 1


First of all, I'm not an expert, but that can be an advantage in trying to explain the equations in lay terms...

Maxwell's equations are these, in differential form:

  1. $$ \nabla \cdot \mathbf{E} = \frac {\rho} {\varepsilon_0}$$
  2. $$ \nabla \cdot \mathbf{B} = 0 $$
  3. $$ \nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}} {\partial t} $$
  4. $$ \nabla \times \mathbf{B} = \mu_0\left(\mathbf{J} + \varepsilon_0 \frac{\partial \mathbf{E}} {\partial t} \right) $$

Eq. 1 means that an electric charge creates a proportional electric field .

Eq. 2 means that there is no magnetic charges (monopoles).

Eq. 3 means that a variation in the magnetic field creates an electric field.

Eq. 4 means that a variation in the electric field plus currents (moving chages) creates a magnetic field.

Now, with that in mind think of what all this means: an electric charge creates an electric field, the charge moves, the electric field changes, that change creates a mangnetic field... All the known electric and magnetic phenomena.

But, hidden in these equations there is another interesting possibility. Without the need of any electric charge, a sinusoidal electric field could create a (cosinusoidal?) magnetic field, that in turn will create another electric field, and so on. That is, a standing electro-magnetic wave, without the need of any charge at all!

Those are what they call solutions to the Maxwell's equations.

Now, you want to create a radio transmitter. The simplest form is just an antenna, that is a long piece of wire, in which you inject an electric current that oscillates at the same frequency you want to transmit (from a few kHz to hundreds of MHz). The variable electric field in the wire will create an electro-magnetic wave.

The funny part is to build the receiver. For that you will need another antenna. There, the coming electro-magnetic wave will induce a current (fractions of a mA), that you will amplify with an electronic circuit (a valve or transistor will do) and send to whatever device you use to generate the final output.


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