Does a radio receiver add a load to the transmitter? If I have a dynamo connected to a switch and a load (such as a light bulb), the load on the dynamo is increased when the switch is closed and the light comes on. The dynamo gets harder to turn.
Is the same true for a radio transmitter and receiver? If the receiving aerial is initially not connected to a circuit and then is connected, can you detect at the transmitting end that the load was added at the receiving end?
If not, what is the difference between the two examples?
 A: While I agree with Alfred Centauri's answer, I am not sure it gives a direct answer to your specific question for the following reason. If there is a receiving antenna somewhere, there is always some reflection, however minute. If the antenna is connected to a circuit, the conditions of reflection will change, and the transmitter can "notice" that. In practice, this can be significant if the receiver is in the near-field region of the transmitter (http://en.wikipedia.org/wiki/Near_and_far_field ). For DC current, the near-field region is very large (as the wavelength is very large (infinite)). 
A: As an intermediate step, consider a sinusoidal source driving an infinite transmission line with some characteristic impedance $Z_0 = 50\Omega$.
The source "sees" a real impedance of $50\Omega$ and so, power is delivered to the line and, since the TL is infinitely long, the power is transported down the line, via an electromagnetic wave, without reflection.
Now, replace the infinite line with a finite length line terminated with a $50\Omega$ resistor.  From the perspective of the source, nothing has changed.
The source continues to deliver power to the line and the power is transported down the line via electromagnetic wave but, instead of propagating forever, the power is delivered to the resistor where it is converted to heat.
Now, how does this differ from a circuit?  Well, consider what would happen in the above if, instead of terminating the finite TL with the characteristic impedance, it were left open.
In that case, there would be reflections from the end resulting in standing waves and the source would see a vastly different load than the characteristic impedance.  Similarly for a short circuit termination and in-between.  If the termination doesn't equal the characteristic impedance, the source will "know" the difference.
Now, imagine shrinking the length of the TL down to zero and you essentially have circuit theory and the source "sees" the load directly.
Finally, make the step to radio.  A transmitter delivers power to electromagnetic waves that propagate through space* and, assuming there is no reflection, that's all the transmitter "sees" regardless of whether the waves propagate to infinity or are total or partially absorbed by receiving antennae.

*See akhmeteli's answer.  Essentially, my answer assumes the receiving antenna and receiver are in the far-field where there are effectively only the propagating EM waves from the transmitting antenna.  An antenna and receiver in the near-field couple to the transmitting antenna, i.e., become electrically part of the transmitting apparatus and, thus, affect the transmitter.
