A voltage reference is an electronic component or circuit that produces a constant DC(direct-current) output voltage regardless of variations in external conditions such as temperature, barometric pressure, humidity, current demand, or the passage of time. The earliest voltage references were electrochemical components, notably the Weston standard cell and the lead-acid cell. The Weston cell contained two electrodes immersed in a solution of cadmium sulfate, and produced a predictable 1.018 volts DC at room temperature. A lead-acid cell, still used in automotive and emergency-power applications today, contains a solution of sulfuric acid and produces 2.1 volts DC at room temperature. The main problem with all electrochemical cells and batteries, when forced to serve as voltage references, lies in the fact that the output voltage varies depending on the current demand. In general, when the cell or battery must deliver more current, its output voltage goes down because of its own internal resistance. Simple contemporary voltage references rely on semiconductor components known as Zener diodes. When a certain minimum amount of current passes through a Zener diode, a predictable voltage develops across it, and this voltage (called the avalanche voltage or Zener voltage) remains the same as the current continues to increase. The exact voltage depends on the design of the device, and can range from around 1 volt to more than 100 volts. More sophisticated voltage references take advantage of the properties of junctions between specific semiconductor materials, comparing multiples of the voltages between those materials to the voltage developed across a conventional semiconductor diode. These devices are called bandgap voltage references. Voltage references are used in the power supplies of precision electronic equipment of all kinds, particularly systems of the sort whose performance is greatly affected by small changes in the voltage applied to them. Personal computers and peripherals can tolerate a fair amount of voltage fluctuation (up to several percent above or below the nominal value), as can most consumer communications devices. Scientific and medical laboratory equipment can tolerate voltage variations of only a tiny fraction of one percent above or below the nominal value