WHAT A TRANSFORMER TAP CHANGER IS AND WHAT IT DOES.
A tap changer is a device fitted to power transformers for regulation of the output voltage to required levels. This is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropriate transformer/s. Supply authorities are under obligation to their customers to maintain the supply voltage between certain limits. Tap changers offer variable control to keep the supply voltage within these limits. About 96% of all power transformers today above 10MVA incorporate on load tap changers as a means of voltage regulation. Tap changers can be on load or off load.

On load tap changers generally consist of a diverter switch and a selector switch operating as a unit to effect transfer current from one voltage tap to the next. It was more than 60 years ago on load tap changers were introduced to power transformers as a means of on load voltage control.

Here the resistor color code for 4 band color and 5 band color.
Usually the 5 band color resistor also known as metalfilm resistor (blue body color).

The physicist Gustav Robert Kirchhoff (1824-1887) was a researcher and experimentalist in electricity back in the time before radio, before electric lighting, and before much was understood about how currents flow.

Kirchhoff reasoned that current must work something like water in a network of pipes, and that the current going into any point has to be the same as the current going out. This is true for any point in a circuit, no matter how many branches lead into or out of the point. Two examples are shown in Fig. 5-5.

In a network of water pipes that does not leak, and into which no water is added along the way, the total number of cubic feet going in has to be the same as the total volume going out.

Conductors
In some materials, electrons move easily from atom to atom. In others, the electrons move with difficulty. And in some materials, it is almost impossible to get them to move. An electrical conductor is a substance in which the electrons are mobile.

The best conductor at room temperature is pure elemental silver. Copper and aluminum are also excellent electrical conductors. Iron, steel, and various other metals are fair to good conductors of electricity.

In most electrical circuits and systems, copper or aluminum wire is used. Silver is impractical because of its high cost.

Some liquids are good electrical conductors. Mercury is one example. Salt water is a fair conductor.

Gases are, in general, poor conductors of electricity. This is because the atoms ormolecules are usually too far apart to allow a free exchange of electrons. But if a gas becomes ionized, it is a fair conductor of electricity.

Electrons in a conductor do not move in a steady stream, like molecules of water through a garden hose. Instead, they are passed from one atom to another right next to it (Fig. 1-5). This happens to countless atoms all the time. As a result, literally trillions of electrons pass a given point each second in a typical electrical circuit.

You might imagine a long line of people, each one constantly passing a ball to the neighbor on the right. If there are plenty of balls all along the line, and if everyone keeps passing balls along as they come, the result will be a steady stream of balls moving along the line. This represents a good conductor.