Polarity (electricity)
In electrical engineering, polarity refers to the quality that makes it possible to distinguish each of the terminals of a cell, battery or other direct current electrical machines. Each of these terminals, called poles, can be positive or negative.
Before it was discovered that electric current is a flow of electric charge carriers, which in metals are electrons and circulate from the negative pole or cathode to the positive pole or anode, said current was defined as a flow of positive charges and the conventional direction of circulation of the current was established as a flow of charges from the positive to the negative pole.
The term "polarity" to refer to a component whose terminals (typically there are two) are interchangeable in a circuit, without altering its operation. Examples of components without polarity are resistors, capacitors (excluding electrolytic ones), coils, switches... Components with polarity can be transistors, electrolytic capacities, diodes, etc.
Importance
Knowing the polarity of the power supply is extremely important since, in most cases, small direct current devices do not usually incorporate protections against a possible change of polarity in their power supply, which can cause damage irreversible in the device.
To avoid this, and since the cause of the problem is usually the improper placement of the batteries, it is common for these devices to incorporate a diagram showing how they should be placed. Likewise, the contacts are distinguished by using, conventionally, a metallic spring for the negative pole and a plate for the positive pole or else contacts in the form of brackets that only allow them to be fastened in the correct direction. Similarly, in appliances with rechargeable batteries, the transformer-rectifier has such an output that the connection to the powered appliance can only be made one way, thus preventing polarity reversal.
When it comes to large-scale installations, for example telephone exchanges or other telecommunication equipment, where there is a centralized distribution of direct current for the entire equipment room, adequate connection and protection elements are used to avoid an erroneous change of polarity