question regarding the boundary behavior of electric current


I have a cable and I send electric current through it. I want to know the moment when the current has arrived at the other end of the cable.

Can I visualize this moment by using an oscilloscope ?

If the answer is yes, I have another question for a more complex experiment.

I have a very simple network with 2 nodes (A and B). There are 2 cables which are connecting these nodes. The cables have different lengths L1 and L2. Assume that L1 is shorter than L2.

I have draw a small picture here:

--A*-------L1---------*B-- | | | | |______L2_____|

I apply electric power to this device. Because the cables have lengths greater than zero I assume that it will take a while until the current traverse the path from A to B.

More than that, in node A, the current is split in 2, because there are 2 cables linking A with B. Because one of the cables is shorter I assume that "a part" of the current arrives earlier (denote this by moment M1) than the other "part" because it has to traverse a shorter path. Am I correct ?

The other "part" of the current which has traversed cable L2 will arrive later in B. Lets denote this by moment M2.

Can I measure these 2 moments by using an oscilloscope? Or what other options for measurement I have ?

Are there some fluctuations of the electric current at moments M1 and M2 ?

Are there high precision oscilloscopes for this experiment? Or should I use very very long cables?

I'm not interested what happens after those moments.

Thanks, Laura

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a measurement device called a time domain reflectometer (TDR) can be used to measure the length and character of a cable, usually a coaxial cable.

a more elaborate device is called a network analyzer (nothing to do with computer networks).

a rudimentary TDR can be created with a fast O'scope and a pulse generator.

the propigation velocity of a cable varies depending on dielectric type. see chart for examples

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types of cable called "radar delay lines" are designed to resolve timing issues in microwave circuits.

the following is a referance to a pioneer in measuring the speed of electricity

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It seems obvious that you are in over your head. You really need to learn something about transmission lines.

It is true that if you have two cables of different lengths excited from a common point, you can measure the time delays using a suitable oscilloscope. A wave travels on a line at approximately 1ft/ns. Thus, if you coil up one of the cables to get there outputs close to a dual trace oscilloscope, there should be no problem. But to carry out such experiments meaningfully, you have to know about how waves travel on transmission line. You have to know about terminations, mismatch, and reflections.


-- Fermez le Bush--about two years to go.

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Salmon Egg

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