Principle of Cell:

To study electrical principles further we require a source of emf. Although an emf can be produced by any of the six methods discussed above, large amounts of useable power can only be produced chemically or by generation. Generation requires a more in depth study of magnetism and therefore cells and batteries will be studied first.
On an aircraft, the battery may be used for engine starting, but far more importantly, the battery is the source of emergency power when the generator fails. Although aircraft battery systems and servicing will be studied at a later date, battery principles and battery construction will be studied now and will not be repeated.

PRINCIPLES

A Cell is a portable device which converts chemical energy into electrical energy. A group of interconnected cells is known as a battery. Cells operate on a principle of the exchange of charges between dissimilar metals.
CELL & BATTERY SYMBOLS

The circuit symbols for cells and batteries are shown below. To identify the polarity of the terminals, a long thin line is used to represent the positive terminal and a short thick line the negative terminal. Sometimes the terminal voltage is indicated.