Using Formula on Page 13: What Are the Rules?

[oneplusone]

When can you use this formula?
On page 13 it uses it here: http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/coursenotes/modules/guide06.pdf

however my instructor said that you can only use it when you are using a wire and the current is flowing parallel to the axis.

 

[Simon Bridge]

That sounds right - it only works for a nice geometry.

consider a rectangular plate LxWxT dimensions (length x width x thickness)
the resistance between opposite corners won't be $\rho L/WT$

Even if I attach the wires to the center of areas WxT, imagine W=T=1000m ... how does the current travel through the resistor then?