NIEASUREIIENT ELECTEICAl. QUANTITIES. .V.; already explained, the intensity of an electric cur rent may lie measured in terms of the earth's magnetic force by a galvanometer, and the value of this fierce may he determined by suitable meas urements, (See Thus the numeri cal value of any current on the G. S. electro magnetic system may be obtained. (There are other and better methods, depending upon the attraction of two parallel coils of Wire carrying currents.) The intensities of two currents may be compared by making them pass through a. voltanwter (q.v.), and then applying, Faraday's first law of electrolysis. To measure the leetrieal resistance of a conductor the simplest method—at least theoretically—is to measure by means of a calorinuo..ric experiment the heating (-fleet in it produced by passing through it a cur rent whose intensity is measured. The heating effect is ands as both i and f are easily olithined. It can be calculated. This will lee its immerieal value on the •('. C. S. cleetro-magnctie To compare the electrical resistances of different conductors, the simplest and most accu rate method is the use of `NVIleatsione's bridge' (q•v.). If the of a given conductor is known, and if the current produced through it by any E. Al. F. can lee ineaseired. the numerical value of this E. Al. F. is given by c thin's law. Alethoels for the comparison of different electro motive forces described in all text-books On physics and electricity. OWill to the incon venient magnitude of limit: of resistance and of ele•t•oonotive force on the C. G. S. clectro magnetic system, other units have been adopted for practical use. t Ett:urnicAt. UNITS.) The 'ampere• (q.v.), or practical Unit of cur rent, is one-tenth the C. S. electro-magnetic the 'ohm.' or practical unit of resistance, is substantially DV clectr-unagnetic units; the 'volt,' or practical unit of electroano liye substantially lie G. S. electro magnetic units.
EtEcrtne Cl'intENrs. if the magnetic field inclosed by any conductor. e.g. a wire bent into a closed circle. is varied in any way. there will be produced in the conductor an electric cur rent, which continues only while the change goes on. This is called an Induced current.' and it is said to lie due to an induced Ell. F. if there was a current in the conductor orielnally it will be either increased or decreased, depending upon the direction of the induced current. This last is in such a direction that by its electromagnetic action it will oppose that change in the magnetic field \ vhieh produces it. Thus. if the change is
clue to the approach of a magnet having its north pole foremost. the induced current will be such as to cause the repulsion of the north pole while. if the change is due to the removal of the north pole of the magnet the induced current will be in the opposite direction. The change in the mag netic field may he due, also, to the approach or removal of electric currents, to the change in the intensity of neighboring currents. or to changes in the original current in the conductor itself. Special cases of this last kind are afforded when a eonductor carrying a current is broken. and also when the broken parts are reunited. thus again starting the current. In the former case the magnetic field starts at once to decrease; and owing to this there will be an induced current tending to neutralize the change in the magnetic field, i.e. in the same direction as the original current: this is shown by the spark which passes between the ends of the conductor where the cir cuit is broken. (This 'extra current on breaking is used often to produce a shock in the muscles of the arms or body: for. by increasing the magnetic field of the original current by winding the wire in a helix, a powerful shock may be felt.) Mum the circuit is again made, if the enrrent regained its original value at once there would be a sudden increase in the magnetic field; as the field increases. however. there are induced currents tending to oppose the change. and hence the current regains its former value at a rate de on the self-induction and the electrical resistance of the conductor—a very short time, however. in general is required. In all eases the induced E. 11. F. land therefore the intensity of the induced current) varies directly as the change in the tubes of magnetic induction, and inversely as the time taken in which to this change. The induced E. AI. F. equals the change in the number of tubes of induct inn in one second. In fact. the simplest way of defining a unit of self or mutual induetion—or 'unit induction'—is to say that it is the induction in a circuit when the induced E. 31. F. in it has the value one, while the inducing current varies at the rate of one unit per second. For. if Al is the coefficient of mutual induction between two coils, and if a eurrent i passes through tine. their \i tubes through the other ; therefore, if the eurrt nt i varies at the rate of one per second. N' ill V'11'y at the rate of 3I per second; but the induced E. M. F.