General facts.--The names of many of the original investigators of electrical phenomena are perpetuated in the familiar names of electrical measurements. For, notwithstanding its seeming subtlety, there is no force in use, or that has ever been used by men, capable of being so definitely calculated, measured, determined beforehand, as electricity is. As time passes new measurements are adopted and named, some of them being proposed as lately as 1893. An instance of the value of some of these old determinations of a time when all we now know of electrical science was unknown, may be given in what is known as Ohm's Law. Ohm was a native of Erlangen, in Bavaria, and was Professor of Physics at Munich, where he died in 1874. He formulated this Law in 1827, and it was translated into English in 1847. He was recognized at the time, and was given the Copley medal of the Royal Society of London. The Law--for by that distinctive name is it still called, though the name "Ohm," also expresses a unit of measurement--is that _the quantity of current that will pass through a conductor is proportional to the pressure and inversely proportional to the distance_. That is: Current = Pressure / Resistance. Transposing the terms of the equation we may get an expression for either of those elements, current, pressure, or resistance, in the terms of the other two. This relation holds true and is accurate in every possible case and condition of practical work. This remarkable precision and definiteness of action has made possible the creation of an extensive school of electrical testing, by which we are not only enabled to make accurate measurement of electrical apparatus and appliances, but also to make determinations in _other_ fields by the agency of electricity. When an ocean cable is injured or broken the precise location of the trouble is made _by measuring the electrical resistance of the parts on each side of the injury_. The magnitudes of measurements of electricity are expressed in the following convenient electrical units: The VOLT (named from Volta) equals a unit of _pressure_ that is equal to one cell of a gravity battery. The OHM, as a unit of measurement, equals a unit of _resistance_ that is equivalent to the resistance of a hundred feet of copper wire the size of a pin. The AMPÈRE (named from Ampère, 1775-1836, author of a "Collection of Observations on Electro-Dynamics" and other works, and a profound practical investigator) equals a unit of _current_ equivalent to the current which one Volt of pressure will produce through one Ohm of wire (or resistance). The Coulomb (1736--inventor of the means of measuring electricity called the "Torsion balance," and general early investigator) equals a unit of _quantity_ of one Ampere flowing for one second.