From the telegraph equation, it is found that electrical signals travel at fraction of light speed over transmission lines. For copper cables, the speed is about 0.5 c.

Is such speed experimentally verified. Can someone point to some experiments verifying the telegraph equations.

[edit] Is this thought experiment valid: 1) I have a switch S that triggers a step voltage for only a small time interval. 2) S is connected to a free non-powered transmission line pair with a resistor load at other end. 3) When S is triggered (on then off), a step signal will travel along wire and will be detected at the load.

The wires need not be initially powered for the signal to be transmitted.

  • $\begingroup$ The speed depends on the material between the conductors. Standard TDR (time domain reflectometry) equipment en.wikipedia.org/wiki/Time-domain_reflectometry can measure directly that speed. $\endgroup$
    – hyportnex
    Sep 29 '17 at 15:11
  • $\begingroup$ A standard experiment is to find the speed of a pulse down a coaxial cable. Terminating the end of the cable with a variable resistor also allows investigation of how the reflection of pulses changes with the resistance of the termination wiki.brown.edu/confluence/download/attachments/1163042/… $\endgroup$
    – Farcher
    Sep 29 '17 at 15:32
  • $\begingroup$ related. $\endgroup$
    – The Photon
    Sep 29 '17 at 15:51
  • $\begingroup$ Your thought experiment is essentially what a TDR does (although usually it waits for the pulse to be reflected back to the source end and detects it there). $\endgroup$
    – The Photon
    Sep 29 '17 at 16:06

This is the Very Large Array telescope in New Mexico: enter image description here (Image source: nasa.gov)

This telescope works by receiving radio signals from distant objects from multiple small antennas, then combining these signals to image the objects emitting those signals.

Since the antenna elements are quite far apart, to combine the signals means they must be carried from the elements to some common location. (Alternately, they might be digitized at each antenna element, but then a common timing reference must be distributed to all the antennas) If the phase delay due to the interconnecting cables between the antennas was not predictable, the entire scheme would not work.

So the experimental evidence for the correctness of the telegraphers equations is that this telescope (and numerous other radio devices built over the last 150 years) can be made to work.


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