The Standard Electrical Dictionary Part 46

Current Reverser.

A switch or other contrivance for reversing the direction of a current in a conductor.

Currents, Amp?rian.

The currents of electricity a.s.sumed by Ampere's theory to circulate around a magnet. As they represent the maintenance of a current or of currents without the expenditure of energy they are often a.s.sumed to be of molecular dimensions. As they all go in the same sense of rotation and are parallel to each other the result is the same as if a single set of currents circulated around the body of the magnet. More will be found on this subject under Magnetism. The Amp?rian currents are purely hypothetical and are predicated on the existence of a field of force about a permanent magnet. (See Magnetism, Amp?re's Theory of.)

If the observer faces the north pole of a magnet the Amp?rian currents are a.s.sumed to go in the direction opposite to that of a watch, and the reverse for the south pole.

Figs. 118-119 DIRECTION OF AMP?RIAN CURRENTS.

Currents, Angular.

Currents pa.s.sing through conductors which form an angle with each other.

Currents, Angular, Laws of.

1. Two rectilinear currents, the directions of which form an angle with each other, attract one another when both approach to or recede from the apex of the angle.

2. They repel one another, if one approaches and the other recedes from the apex of the angle.

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Currents, Earth.

In long telegraph lines having terminal grounds or connected to earth only at their ends, potential differences are sometimes observed that are sufficient to interfere with their working and which, of course, can produce currents. These are termed earth-currents. It will be noted that they exist in the wire, not in the earth. They may be of 40 milliamperes strength, quite enough to work a telegraph line without any battery.

Lines running N. E. and S. W. are most affected; those running N.W. and S. E. very much less so. These currents only exist in lines grounded at both ends, and appear in underground wires. Hence they are not attributable to atmospheric electricity. According to Wilde they are the primary cause of magnetic storms, q. v., but not of the periodical changes in the magnetic elements. (See Magnetic Elements.)

Synonym--Natural Currents.

Current, Secondary.

(a) A current induced in one conductor by a variation in the current in a neighboring one; the current produced in the secondary circuit of an induction coil or alternating current converter.

(b) The current given by a secondary battery. This terminology is not to be recommended.

Current, Secretion.

In electro-therapeutics, a current due to stimulation of the secretory nerves.

Current Sheet.

(a) If two terminals of an active circuit are connected to two points of a thin metallic plate the current spreads over or occupies practically a considerable area of such plate, and this portion of the current is a current sheet.

The general contour of the current sheet can be laid out in lines of flux. Such lines resemble lines of force. Like the latter, they are purely an a.s.sumption, as the current is not in any sense composed of lines.

(b) A condition of current theoretically brought about by the Amp?rian currents in a magnet. Each molecule having its own current, the contiguous portions of the molecules counteract each other and give a resultant zero current. All that remains is the outer sheet of electric current that surrounds the whole.

Current, Sinuous.

A current pa.s.sing through a sinuous conductor.

Currents, Multiphase.

A term applied to groups of currents of alternating type which constantly differ from each other by a constant proportion of periods of alternation. They are produced on a single dynamo, the winding being so contrived that two, three or more currents differing a constant amount in phase are collected from corresponding contact rings. There are virtually as many windings on the armature as there are currents to be produced. Separate conductors for the currents must be used throughout.

Synonyms--Polyphase Currents--Rotatory Currents.

167 STANDARD ELECTRICAL DICTIONARY.

Currents of Motion.

In electro-therapeutics, the currents produced in living muscle or nerves after sudden contraction or relaxation.

Currents of Rest.

In electro-therapeutics, the currents traversing muscular or nervous tissue when at rest. Their existence is disputed.

Currents, Orders of.

An intermittent current pa.s.sing through a conductor will induce secondary alternating currents in a closed circuit near it. This secondary current will induce a tertiary current in a third closed circuit near it, and so on. The induced currents are termed as of the first, second, third and other orders. The experiment is carried out by Henry's coils. (See Coils, Henry's.)

Currents, Thermo-electric.

These currents, as produced from existing thermo-electric batteries, are generated by low potential, and are of great constancy. The opposite junctions of the plates can be kept at constant temperatures, as by melting ice and condensing steam, so that an identical current can be reproduced at will from a thermopile.

Thermo-electric currents were used by Ohm in establis.h.i.+ng his law. (See Ohm's Law.)

Current, Swelling.

In electro-therapeutics, a current gradually increasing in strength.

Current, Undulatory.

A current varying in strength without any abrupt transition from action to inaction, as in the make and break current. The current may be continually changing in direction (see Current, Alternating), and hence, of necessity, may pa.s.s through stages of zero intensity, but such transition must be by a graduation, not by an abrupt transition. Such current may be represented by a curve, such as the curve of sines. It is evident that the current may pa.s.s through the zero point as it crosses the line or changes direction without being a make and break current.

When such a current does alternate in direction it is sometimes called a "shuttle current." The ordinary commercial telephone current and the alternating current is of this type. (See Current, Make and Break.)

Current, Unit.

Unit current is one which in a wire of unit length, bent so as to form an arc of a circle of unit length of radius, would act upon a unit pole (see Magnetic Pole, Unit,) at the center of the circle with unit force.

Unit length is the centimeter; unit force is the dyne.

[Transcriber's note: The SI definition of an ampere: A current in two straight parallel conductors of infinite length and negligible cross-section, 1 metre apart in vacuum, would produce a force equal to 2E-7 newton per metre of length.]

168 STANDARD ELECTRICAL DICTIONARY.