Reversing the Polarity

Factual Questions
41

Thanks, Longhair75. You just reminded me of a good example of reversing the polarity related to fire alarm systems.

In many fire alarm systems, the audible appliances have internal diodes installed in series with the sound-producing component (vibrating horn or whatever). The fire alarm control keeps a 24 VDC current on the audible appliance circuit to supervise the integrity of the circuit through and end-of-line device (usually a resistor). The diode prevents the audible appliance from sounding in this state. To sound the appliance, the polarity of the circuit is reversed at the circuit terminals. Current can then flow through the diode and produce the sound required.

TL;DR: Polarity one way to supervise, reverse it to sound.

42

The U2 guitar part mentioned in the OP reminded me of something I once heard regarding Pink Floyd’s “Echoes”. That part in the middle that sounds like seagulls squawking was supposedly due to David Gilmour plugging in one of his guitar effects pedals backwards. Not sure if that is “reversing the polarity” or just “playing guitar while stoned.”