The Standard Electrical Dictionary Part 71

253 STANDARD ELECTRICAL DICTIONARY.

The distorted lines must be regarded as resultants of the two induced polarities of the armature, one polarity due to the induction of the field, the other to the induction from its own windings. The positions of the brushes have much to do with determining the amount and degree of distortion. In the case of the ring armature it will be seen that some of the lines of force within the armature persist in their polarity and direction, almost as induced by the armature windings alone, and leak across without contributing their quota to the field. Two such lines are shown in dotted lines.

In motors there is a similar but a reversed distortion.

Fig. 169. DISTORTION OF FIELD IN A RING ARMATURE OF AN ACTIVE DYNAMO.

Fig. 170. DISTORTION OF FIELD IN A RING ARMATURE OF AN ACTIVE MOTOR.

254 STANDARD ELECTRICAL DICTIONARY.

Field, Drag of.

When a conductor is moved through a field so that a current is generated in it, the field due to that current blends with the other field and with its lines of force, distorting the field, thereby producing a drag upon its own motion, because lines of force always tend to straighten themselves, and the straightening would represent cessation of motion in the conductor. This tendency to straightening therefore resists the motion of the conductor and acts a drag upon it.

Field of Force.

The s.p.a.ce in the neighborhood of an attracting or repelling ma.s.s or system. Of electric fields of force there are two kinds, the Electrostatic and the Magnetic Fields of Force, both of which may be referred to. A field of force may be laid out as a collection of elements termed Lines of Force, and this nomenclature is universally adopted in electricity. The system of lines may be so constructed that (a) the work done in pa.s.sing from one equipotential surface to the next is always the same; or (b) the lines of force are so laid out and distributed that at a place in which unit force is exercised there is a single line of force pa.s.sing through the corresponding equipotential surface in each unit of area of that surface. The latter is the universal method in describing electric fields. It secures the following advantages:--First: The potential at any point in the field of s.p.a.ce surrounding the attracting or repelling ma.s.s or ma.s.ses is found by determining on which imaginary equipotential surface that point lies.

Second: If unit length of a line of force cross n equipotential surfaces, the mean force along that line along the course of that part of it is equal to n units; for the difference of potential of the two ends of that part of the line of force = n; it is also equal to F s (F = force), because it represents numerically a certain amount of work; but s = I, whence n = F. Third: The force at any part of the field corresponds to the extent to which the lines of force are crowded together; and thence it may be determined by the number of lines of force which pa.s.s through a unit of area of the corresponding equipotential surface, that area being so chosen as to comprise the point in question. (Daniell.)

Field of Force, Electrostatic.

The field established by the attracting, repelling and stressing influence of an electrostatically charged body. It is often termed an Electrostatic Field. (See Field of Force.)

255 STANDARD ELECTRICAL DICTIONARY.

Field of Force of a Current.

A current establishes a field of force around itself, whose lines of force form circles with their centres on the axis of the current. The cut, Fig. 172, shows the relation of lines of force to current.

Fig. 171. EXPERIMENT SHOWING LINES OF FORCE SURROUNDING AN ACTIVE CONDUCTOR.

Fig. 172. DIAGRAM OF FIELD OF FORCE SURROUNDING AN ACTIVE CONDUCTOR.

Fig. 173. LINK OF FORCE INDUCED BY A CURRENT SHOWING THE MAGNETIC WHIRLS.

The existence of the field is easily shown by pa.s.sing a conductor vertically through a horizontal card. On causing a current to go through the wire the field is formed, and iron filings dropped upon the card, tend, when the latter is gently tapped, to take the form of circles. The experiment gives a version of the well-known magnetic figures, q. v. See Fig. 171.

The cut shows by the arrows the relation of directions of current to the direction of the lines of force, both being a.s.sumptions, and merely indicating certain fixed relations, corresponding exactly to the relations expressed by the directions of electro-magnetic or magnetic lines of force

256 STANDARD ELECTRICAL DICTIONARY.

Field, Pulsatory.

A field produced by pulsatory currents. By induction such field can produce an alternating current.

Field, Rotating.

In a dynamo the field magnets are sometimes rotated instead of the armature, the latter being stationary. In Mordey's alternator the armature, nearly cylindrical, surrounds the field, and the latter rotates within it, the arrangement being nearly the exact reverse of the ordinary one. This produces a rotating field.

Field, Rotatory.

A magnetic field whose virtual poles keep rotating around its centre of figure. If two alternating currents differing one quarter period in phase are carried around four magnetizing coils placed and connected in sets of two on the same diameter and at right angles to each other, the polarity of the system will be a resultant of the combination of their polarity, and the resultant poles will travel round and round in a circle. In such a field, owing to eddy currents, ma.s.ses of metal, journaled like an armature, will rotate, with the speed of rotation of the field.

Field, Stray.

The portion of a field of force outside of the regular circuit; especially applied to the magnetic field of force of dynamos expressing the portion which contributes nothing to the current generation.

Synonym--Waste Field.

Field, Uniform.

A field of force of uniform density. (See Field Density.)

Figure of Merit.

In the case of a galvanometer, a coefficient expressing its delicacy. It is the reciprocal of the current required to deflect the needle through one degree. By using the reciprocal the smaller the current required the larger is the figure of merit. The same term may be applied to other instruments.

It is often defined as the resistance of a circuit through which one Daniell's element will produce a deflection of one degree on the scale of the instrument. The circuit includes a Daniell's cell of resistance r, a rheostat R, galvanometer G and shunt S. a.s.sume that with the shunt in parallel a deflection of a divisions is obtained. The resistance of the shunted galvanometer is (GS/G+S ; the multiplying power m of the shunt is S+G/S; the formula or figure of merit is m d (r+R +G S/G+S).

The figure of merit is larger as the instrument is more sensitive.

Synonym--Formula of Merit.

257 STANDARD ELECTRICAL DICTIONARY.

Filament.

A thin long piece of a solid substance. In general it is so thin as to act almost like a thread, to be capable of standing considerable flexure. The distinction between filament and rod has been of much importance in some patent cases concerning incandescent lamps. As used by electricians the term generally applies to the carbon filament of incandescent lamps. This as now made has not necessarily any fibres, but is ent.i.tled to the name of filament, partly by convention, partly by its relative thinness and want of stiffness. (See Incandescent Lamps--Magnetic Filament.)

Fire Alarm, Electric, Automatic.

A system of telegraph circuits, at intervals supplied with thermostats or other apparatus affected by a change of temperature, which on being heated closes the circuit and causes a bell to ring. (See Thermostat.)

Fire Alarm Telegraph System.

A system of telegraphic lines for communicating the approximate location of a fire to a central station and thence to the separate fire-engine houses in a city or district. It includes alarm boxes, distributed at frequent intervals, locked, with the place where the key is kept designated, or in some systems left unlocked. On opening the door of the box and pulling the handle or otherwise operating the alarm, a designated signal is sent to the central station. From this it is telegraphed by apparatus worked by the central station operator to the engine houses. The engines respond according to the discipline of the service.