Building circuits
All electronic systems are made from just a few basic types of components. By themselves, these cannot do anything useful. It is the way they are connected to form circuits that creates the rich variety of electronic technology. Circuit design depends, among other things, on mathematics, much of it worked out in the 19th century. The rules governing the flow of electricity between components were formulated in 1845 by the German scientist Gustav Kirchhoff. Though simple, they are one of the keys to understanding how circuits work. For many basic functions, ready-made circuit designs exist in books or as integrated circuits (pp. 52-53) The designer has to know about these and decide how to link them to perform a given task. When the design is complete, the circuit must be built. Until the 1960s most circuits were put together by hand, with components linked by wires. The development of the printed circuit board -a piece of plastic with holes to accept the components and metal tracks to connect them-means that circuits can now be assembled by machine, greatly reducing the cost of electronic devices.