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# Current

Current. 1. In Electricity, current is the term used to denote motion of electric quantity, the magnitude of current at any instant being measured by the rate at which displacement of electricity takes place. It is generally understood that the motion does not take place in one direction only, there being apparently displacements in opposite directions at the same time, in the same sort of way that waves may meet and pass each other on the surface of a liquid. Any such fluid analogies must, however, be employed with caution in the case of electricity. For a current to flow from one point to another a conductor must exist continuously between the two points, and a difference of pressure of the electricity, or, as it is termed, a difference of potential, must likewise exist between them. So, in hydraulics, a conducting channel must exist for a current of water to pass from one point to another, between which in the same way a difference of pressure must exist. The magnitude of the electric current will necessarily depend on the conducting power or conductance of the medium between the points, and also on the difference of potential between them. The ordinary unit of current is the ampere, which is the intensity of the current when the unit quantity of one coulomb is displaced across any section of the conductor per second. It is also the current which flows through one ohm resistance, when its two ends are at one volt difference of potential. With a steady current the quantity displaced across any section of an electric circuit must be the same. Instruments for measuring current intensity are called amperemeters, or ammeters. Of these there are many types in commercial use. They should be sensitive, that is to say, they should give obvious readings for small currents; they should be dead-beat, that is, recording the steady reading at once without wasting time in oscillations of the pointer or needle; they should be unaffected by neighbouring magnets or electric circuits and should be portable, direct-reading, and cheap. All these qualifications are not possessed by any single instrument, but circumstances may permit one or more of the conditions to be neglected. Currents are either continuous or alternating. In the former the displacements are always in the same direction, whereas in the latter the displacements periodically change their direction. Thus, in an alternating-current circuit the magnitude of displacement in any one direction in a given time is zero. Nevertheless; alternating currents have many industrial applications.

2. Current, in Hydraulics, signifies the flow of water or other liquid from place to place. The current in any channel may be measured by the quantity of water flowing past any section per second. This quantity will depend upon the area of this section, and upon the average speed of flow at right angles to the section. To measure the current we may in the case of small channels erect a triangular or rectangular or gauge-notch, through which the water shall flow. Observations of the upper level of the water and of the dimensions of the notch afford a means of calculating the quantity flowing through per second. If the stream be large, the exact shape and dimensions of a suitable section of the stream are obtained by soundings, and the speed or flow at various points in the section determined by specially designed current meters. These data, if sufficiently numerous, enable us to calculate the current.