What type of cell is known for maintaining a constant EMF?

A fuel cell is designed to maintain a constant electromotive force (EMF) as long as it is supplied with a continuous source of fuel and an oxidizer. This characteristic stems from the way fuel cells operate; they convert the chemical energy of fuels, like hydrogen, directly into electrical energy through electrochemical reactions.

Fuel cells produce electricity at a nearly constant voltage, provided the conditions remain stable. This is in contrast to other types of cells, which can experience voltage fluctuations based on factors like charge state, temperature, and internal resistance. For example, dry cells and nickel-cadmium cells will show varying EMF outputs depending on their state of charge and usage. Lead-acid cells also exhibit similar characteristics where the voltage decreases as the battery discharges.

The steady voltage output of fuel cells makes them particularly advantageous for applications that require a reliable and consistent power supply, such as in vehicles and stationary power generation.