of voltage, the cgs system at the time being the customary system of units in science. They chose such a ratio because the cgs unit of voltage is inconveniently small and one volt in this definition is approximately the emf of a Daniell cell, the standard source of voltage in the telegraph systems of the day. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power. The "international volt" was defined in 1893 as 1/1.434 of the emf of a Clark cell. This definition was abandoned in 1908 in favor of a definition based on the international ohm and international ampere until the entire set of "reproducible units" was abandoned in 1948. A redefinition of SI base units, including defining the value of the elementary charge, took effect on 20 May 2019. See also Orders of magnitude (voltage) Rail traction voltage SI electromagnetism units SI prefix for unit prefixes Standardised railway voltages Voltmeter References External links History of the electrical units. SI derived units Units of electrical potential Alessandro Volta