That's not reversing the polarity! That's cancelling the polarity. That's switching it off!
Here's a clue: get two magnets and stick the ends together so you have North-South touching. They stick, right? Now, reverse the polarity so that instead of North-South you now have South-North. Guess what? They're still sticking together! If you reverse the polarity on a 240v AC motor it still turns in the same direction until the earth leakage breaker trips. (3 phase motors and washing machine motors are different.)
STOP REVERSING THE POLARITY! it's bad SF! Just because Doctor Who reversed the polarity doesn't make it a good solution. It's BULLSHIT.
SF Problems that should be caused by reversing the polarity:
-The engines run backwards thrusting you to a stop, then into reverse at high speed.
-The shields let shots in but you can't shoot out
-The magnetically sealed door stays magnetically sealed (only in reverse)
-All ship controls would operate in reverse (turns right when you turn the wheel left)
-Fault Lights turn on when there is no problem, and go off to indicate a problem
Just once I'd like to see an engineer reverse the polarity and fry the circuits.
This is a real peeve I have. It got to me so badly I had to add it into The Novel What I Am Writing.:
They worked on the drive for an hour before they felt the problem had been repaired.
"How is that, Effie?" asked Mitch.
"The chance of failure is now one percent."
"I told you that wasn't the problem," said Peib.
"Yes, well. Now we know what it isn't," said Mitch. "Effie has been leading us astray."
After another hour of repairs Effie announced "Chance of failure is now two percent." Mitch kicked the drive in anger. "Chance of failure is now one hundred percent," said Effie.
"Oh dear," said Bren.
"Never mind," said Peib, patting Mitch on the shoulder. "It was going to fail anyway."
"Shut up you bastards," grumbled Mitch.
"The primary power coupling has failed," said Effie.
"There!" said Mitch waving a finger at Peib. "There is no way a little kick would do that."
Five minutes later they had removed the damaged unit. Peib examined it closely. "There is a string of burned contacts in there," he said eventually as he handed it to Mitch. "You may gloat now."
"I told you a little kick wouldn't do that," said Mitch as he peered at the unit. "I'd better get a spare."
"There are no useable primary power couplings on board," said Effie.
Bren closely examined the failed coupling. "I think I can fix this," he said. The hairs around his nose bristled and quivered. "Is there a microscope on board?"
"No," said Peib.
"Can't you do something?"
"Such as what?"
"Oh, I don't know." Bren waved his hands vaguely. "You could reverse the polarity or something." Mitch and Peib stared at him for long seconds. They burst out laughing and he lowered his head in embarrassment.