In everyday life we are familiar with electric currents in wires and other conductors. Indeed, most practical electrical devices depend on electric current: current through a lightbulb, current in the heating element of a stove or electric heater, and currents in electronic devices. Electric currents can exist in conductors such as wires, and also in other devices such as the CRT of a television or computer monitor whose charged electrons flow through space.
In the 1780s, Luigi Galvani, a professor at the University of Bologna, discovered that a frog's leg muscle would contract when touched by dissimilar metals, similar to how it contracted under static electricity. He believed the electric charge originated within the animal's own muscle or nerve tissue, with the metals simply acting as conductors.
When he published his findings in 1791, Galvani called this phenomenon "animal electricity," and both he and others speculated it might reveal the fundamental "life-force."
Alessandro Volta, working at the University of Pavia, doubted this interpretation and instead proposed that the electricity came from the contact between the two different metals, not the animal itself. He determined that a moist conductor—like the frog muscle or dampness at the metal junction—was essential to complete the circuit, and recognized that the frog's muscle was actually a more sensitive detector of electrical "tension" (what we'd now call voltage) than the electroscopes of the time.
Through further experimentation, Volta ranked different metal combinations by how much electrical effect they produced, creating what is now known as the electrochemical series, still used in chemistry today, and found that carbon could substitute for one of the metals.
Building on this work, he made his greatest contribution: stacking alternating discs of zinc and silver separated by cloth or paper soaked in salt solution or dilute acid, forming a "pile" that produced a much greater potential difference than a single pairing—strong enough that bringing metal strips from either end close together produced a spark. This was the first electric battery, which Volta published in 1800.
A battery converts chemical energy into electrical energy, and even today's simplest cells work on the same basic principle: two electrodes made of dissimilar metals (or metal and carbon) are immersed in an electrolyte solution, such as dilute acid. In a simple carbon-zinc cell, the acid dissolves zinc atoms off the zinc electrode, leaving electrons behind and making that electrode negatively charged, while the resulting positive ions in the electrolyte pull electrons off the carbon electrode, making it positively charged.
This charge separation creates a potential difference between the two terminals, which stabilizes once the electrode's growing negative charge starts attracting new zinc ions back before they escape; connecting the terminals through a wire or circuit lets the reaction—and current flow—continue until an electrode is used up and the cell dies.
The voltage a battery produces depends on the electrode materials and how readily they give up electrons. Connecting cells in series (positive terminal to negative terminal) adds their voltages together, which is why two $1.5$ V flashlight batteries in series yield $3.0$ V, and why a car's six $2$V cells combine for $12$V. This same current can power devices like an incandescent lightbulb, whose thin coiled filament heats to around $3000$K and glows as charge passes through it.
The purpose of a battery is to produce a potential difference, which can then make charges move.
When a continuous conducting path is connected between the terminals of a battery, we have an electric circuit. On any diagram of a circuit, we use this symbol to represent a battery.
Conductors contain many free electrons. Thus, if a continuous conducting wire is connected to the terminals of a battery, negatively charged electrons flow in the wire. When the wire is first connected, the potential difference between the terminals of the battery sets up an electric field inside the wire and parallel to it.
Free electrons at one end of the wire are attracted into the positive terminal, and at the same time other electrons leave the negative terminal of the battery and enter the wire at the other end.
In many real circuits, wires are connected to a common conductor that provides continuity. This common conductor is called ground and really is connected to the ground in a building or house.
