# Page:Popular Science Monthly Volume 19.djvu/142

ment by a committee. In it all the units of measurement are referred to three fundamental units, the centimetre, the gramme, and the secund, whence it is called the centimetre-gramme-secund system of units (expressed by the symbol C. G. S.). The units practically employed—multiples or sub-multiples of the fundamental units—are the ohm, or unit of resistance (symbol R.), the volt, or unit of electro-motive force (symbol E.), and the weber, or unit of intensity (symbol I.). Their relation to each other is expressed by the equation, I ${\displaystyle =}$ ER' whence, the value of two of the elements being known, that of the other can be determined. The unit of resistance, or ohm, is determined by a long and complicated formula, so that it is easier to get it at once by comparison with the material standard which is kept at London. Graduated resistance-boxes containing electric coils carefully adjusted to the resistance-force they are intended to represent, are sold by the instrument-makers. Some idea of what the ohm is may be given by saying that a wire of pure copper a metre (or 3913 inches) long and a milimeter in diameter (or about 125 of an inch) represents a resistance of one fiftieth of an ohm; consequently, fifty metres, or one hundred and fifty and a half feet of such wire, will represent an ohm. Common copper wire offers a stronger resistance, so that only thirty or forty metres of it are required to represent an ohm. The volt, or unit of electro-motive force, is not represented by any actual exact standard, but several constant piles exist, the force of which has been exactly measured, which may be referred to. A Daniell battery, having its copper immersed in a saturated solution of sulphate of copper, and its zinc in a saturated solution of sulphate of zinc, has a force of 1·079 volt. The electro-motive force may be measured in practice by using galvanometers which are graduated in volts, the exactness of which is proportioned to the amount of the resistance they offer. One weber represents the intensity of a current having a force of a volt and passing over a circuit which offers an ohm of resistance. The intensities of currents in ordinary industrial use are represented by fractional units of the weber, the milliweber, or thousandth of a weber, for telegraphic, domestic, and medical currents, the microweber, or millionth of a weber, for telephonic currents. Telegraphic currents vary in intensity from five to twenty milliwebers; the currents of the Gramme machines that feed the Serrin regulators, of from twenty to thirty webers. Some machines used in electrotyping afford still more intense currents, often exceeding eighty webers, although their electro-motive force is very feeble. In France they sometimes measure by the kilometre of resistance, meaning by that the resistance which is offered by a telegraphic wire four millimetres or about one sixth of an inch in diameter, and a thousand metres or five furlongs long, which is equivalent to about ten ohms. The unit of Siemens (U. S.), employed in Germany, represents the elastic resistance of a column of mercury having the length of a metre and a section of a square millimetre, and is equivalent to 0·9536 of an ohm. Several units of intensity founded on the chemical action of electric currents are in use—such, for example, as may be founded on the quantity of gases disengaged in a minute by a voltameter placed in a circuit, or the amount of copper that may be deposited in an hour in an electrolytic bath which is traversed by the current to be measured. Standard apparatuses have also been made, so graduated as to furnish on a simple reading the intensities in webers and microwebers.