Electric current is a flow of electrons through an object. In a conductor, electric current can flow in either direction.Metals are the best conductors of electricity, and even some liquids and gases can conduct electrical current. The study of certain substances, such as gallium, silicon, and germanium, revealed that they too had the ability to conduct electricity, but they had special properties. These substances came to be called semiconductors.

In the 1870s, German-born physicist Karl Ferdinand Braun (1850-1918) observed that a crystal called galeria (an ore of lead sulfide) was able to conduct electricity in one direction only. This property soon made galeria very useful in electrical devices. Its one-directional conduction gave it the ability to convert alternating current into direct current. Similarly, it could also change radio waves, with their positive and negative alternation, into electrical pulses that could easily be made into audible sounds. Thus...