First ask why the potential difference between the metal rod in the guide rail 0
Because of the smooth metal rail, the MPN resistance is ignored.
In the title, the metal rail resistance does not count.So CD is short circuited. That is, U (CD) =0 V
The metal rod in rail between the part of the equivalent power framework is equivalent to using electrical appliances, the framework is not equivalent to the power of resistance, positive and negative pole is short circuited, external voltage is the voltage between the two poles is outside the 0. circuit.
