Prof. Walter Lewin Explains "That's that rho of s

[anhnha]

Please help me. What does the Prof. Walter Lewin mean by this "that is that rho of s"?
Here is the video: http://www.youtube.com/watch?v=rp1orH7OC1c&list=SP854AA255B15C574C&index=17#t=31m20

If I take this wire and I make a cross-section of that wire-- this is wire 1 and this is wire 2-- then at any moment in time, as the voltage wave passes by, this may be positive, that is that rho of s, and this will be negative, that's that rho of s .

 

[Pythagorean]

I think he starts talking about it around 8:30 and eventually makes his way to the right end of the board around 9:30. He has an electric field dependent on rho_s I think he meant "rho sub s" (rho with subscript s).

 

[anhnha]

Thanks. Can you explain why the positive or negative of these wires are related to the rho of s?

 

[anhnha]

Any help?

 

[Pythagorean]

I don't remember the exact setup and currently on mobile, but I thought the setup was the two wires were each on opposite terminals of a battery, so you have a positive charge on one wire and a negative on the other.