Resistive Networks

F. Poyo (yoshi.tamori@gmail.com)


Let us think about the following resistive network.

               (A)
        +---R---o---R---+
        |       |       |
        R       R       R
        |       |       |
        +---R---o---R---+
               (B)

The equivalent resistance of the above resistive circuit (thus, the resistance observed between terminals (A) and (B)) is


              3
        Re = --- R
              5

The following one

        +---R---+---R---+---R---+---R---+
        |       |       |       |       |
        R       R       R       R       R
        |       |       |       |       |
        +---R---+---R--(A)--R---+---R---+
        |       |       |       |       |
        R       R       R       R       R
        |       |       |       |       |
        +---R---+---R--(B)--R---+---R---+
        |       |       |       |       |
        R       R       R       R       R
        |       |       |       |       |
        +---R---+---R---+---R---+---R---+

has


              49
        Re = ---- R.
              93

It may have mistake. (Sorry about that in the case.)

Question

The question is to solve the equivalent resistance between (A) and (B) for the following infinitely extended circuit.


                       ... 

        |       |       |       |       |
  ...---+---R---+---R---+---R---+---R---+---...
        |       |       |       |       |
        R       R       R       R       R
        |       |       |       |       |
  ...---+---R---+---R--(A)--R---+---R---+---...
        |       |       |       |       |
        R       R       R       R       R
        |       |       |       |       |
  ...---+---R---+---R--(B)--R---+---R---+---...
        |       |       |       |       |
        R       R       R       R       R
        |       |       |       |       |
  ...---+---R---+---R---+---R---+---R---+---...
        |       |       |       |       |

                       ... 

It is good idea to think about the following resistive ladder.


     (A)o---R---+---R---+---R--- ...        
                |       |
                R       R      ..... (repeat infinitely)
                |       |
     (B)o-------+-------+------- ...

Someone may have a hunch that the equivalent resistance of the above resistive ladder be an irrational number. Do you believe that? Then what about the first network in question?


Solutions

Mathemusement

yoshi.tamori@gmail.com