first contact, a chemical reaction called a 'half-cell reaction' occurs on the metal surface. As metal ions are transferred to or from the electrolyte, and as the metal and electrolyte become oppositely charged, the increasing voltage at the thin insulating layer between metal and electrolyte will oppose the motion of the flowing ions, causing the chemical reaction to come to a stop. If a second piece of a different type of metal is placed in the same electrolyte bath, it will charge up and rise to a different voltage. If the first metal piece is touched against the second, the voltage on the two metal pieces will be forced closer together, and the chemical reactions will run constantly. In this way the 'contact electrification' becomes continuous. At the same time, an electric current will appear, with the path forming a closed loop which leads from one metal part to the other, through the chemical reactions on the first metal surface, through the electrolyte, then back through the chemical reactions on the second metal surface. In this way, contact electrification leads to the invention of the Galvanic cell or battery. Metallic contact If two metals having differing work functions are touched together, one steals electrons from the other, and the opposite net charges grow larger and larger; this is the Volta effect. The process is halted when the difference in electric potential (electrostatic potential) between the two metals reaches a particular value, namely the difference in work function values - usually less than one volt. At this point, the Fermi levels for the two metals are equal, and there is no voltage difference between them. [If there were a voltage difference between them, then a current would flow between them: so "zero current" implies "zero voltage difference".] Semiconductor contact If a metal touches a semiconductor material, or if two different semiconductors are placed into contact, one becomes charged slightly positive and the other slightly negative. It is found that if this junction between semiconductors is connected to a power supply, and if the power supply is set to a voltage slightly higher than the natural voltage appearing because of contact electrification, then for one polarity of voltage there will be a current between the two semiconductor parts, but if the polarity is reversed, the current stops. Thus contact between materials lead to the invention of the semiconductor diode or rectifier and