The Standard Electrical Dictionary Part 31

Fig. 81. CHARACTERISTIC CURVE OF A DYNAMO.

FIG. 82. DROOPING CHARACTERISTIC.

Characteristic Curve.

A curve indicating the variations in electro-motive force developed during the rotations of the armature of a dynamo or other generator of E. M. F. The term as used in the electrical sense is thus applied, although the indicator diagram of a steam engine may be termed its characteristic curve, and so in many other cases. As the amperes taken from a series generator are increased in number, the E. M. F. rises, it may be very rapidly up to a certain point, and thereafter more slowly.

To construct the curve coordinates, q. v., are employed. The resistance of the dynamo and of the outer circuit being known, the current intensity is measured. To obtain variations in electro-motive force the external resistance is changed. Thus a number of ampere readings with varying known resistance are obtained, and for each one an electro-motive force is calculated by Ohm's law. From these data a curve is plotted, usually with volts laid off on the ordinate and amperes on the abscissa.

By other methods other characteristic curves may be obtained, for which the t.i.tles under Curve may be consulted.

114 STANDARD ELECTRICAL DICTIONARY.

Characteristic, Drooping.

A characteristic curve of a dynamo which indicates a fall in voltage when an excessive current is taken from the dynamo in question. It is shown strongly in some Brush machines, and is partly due to the arrangements for cutting out two of the coils as they approach the neutral line. It is an advantage, as it protects from overheating on short circuit.

Characteristic, External.

In a dynamo the characteristic curve in which the relations of volts between terminals to amperes in the outer circuit are plotted. (See Curve, External Characteristic.)

Characteristic, Internal.

A characteristic curve of a shunt dynamo, in which the relations of volts to amperes in the shunt circuit is plotted.

Characteristics of Sound.

Of interest, electrically, as affecting the telephone, they comprise:

(1) Pitch, due to frequency of vibrations.

(2) Intensity or loudness, due to amplitude of waves of sound.

(3) Quality or timbre, the distinguis.h.i.+ng characteristics of any specific sound due to overtones, discords, etc., by which the sound is recognizable from others. The telephone is held by the U. S. courts to be capable of reproducing the voice by means of the undulatory current.

(See Current, Undulatory.)

Charge.

The quant.i.ty of electricity that is present on the surface of a body or conductor. If no electricity is supplied, and the conductor is connected to the earth, it is quickly discharged. A charge is measured by the units of quant.i.ty, such as the coulomb. The charge that a conductor can retain at a given rise of potential gives its capacity, expressible in units of capacity, such as the farad. A charge implies the stretching or straining between the surface of the charged body, and some complimentary charged surface or surfaces, near or far, of large or small area, of even or uneven distribution.

Charge. v.

(a) To introduce an electrostatic charge, as to charge a condenser.

(b) To decompose the elements of a secondary battery, q. v., so as to render it capable of producing a current. Thus, a spent battery is charged or recharged to enable it to do more work.

Synonyms--Renovate--Revivify--Recharge.

115 STANDARD ELECTRICAL DICTIONARY.

Charge, Bound.

A charge of electricity borne by the surface of a body so situated with reference to another oppositely charged body, that the charge is imperceptible to ordinary test, will not affect an electroscope nor leave the surface if the latter is connected to the earth. To discharge such a body it must be connected to its complimentarily charged body.

The bound charge was formerly called dissimulated or latent electricity.

(See Charge, Free.)

The charge or portion of a charge of a surface which is neutralized inductively by a neighboring charge of opposite kind. The degree of neutralization or of binding will depend on the distance of the two charged surfaces from one another and on the electro-static nature of the medium intervening, which must of necessity be a dielectric. A charge not so held or neutralized is termed a free charge. Thus a surface may be charged and by the approach of a surface less highly charged may have part of its charge bound. Then if connected to earth.

it will part with its unbound or free charge, but will retain the other until the binding surface is removed, or until the electricity of such surface is itself bound, or discharged, or until connection is made between the two surfaces. Thus a body may have both a bound and a free charge at the same time.

Charge, Density of.

The relative quant.i.ty of electricity upon a given surface. Thus a charged surface may have an evenly distributed charge or one of even density, or an unevenly distributed charge or one of uneven density. In a thunderstorm the earth has a denser charge under the clouds than elsewhere.

Synonym--Electrical Density.

Charge, Dissipation of.

As every body known conducts electricity, it is impossible so to insulate a surface that it will not lose its charge by leakage. An absolute vacuum might answer, and Crookes in a high vacuum has retained a charge against dissipation for years. The gradual loss is termed as above.

Charge, Distribution of.

The relation of densities of charge on different parts of a charged body. On a spherical conductor the charge is normally of even distribution; on other conductors it is unevenly distributed, being of greatest density at points, edges, and parts of smallest radius of curvature. Even distribution can also be disturbed by local induction, due to the presence of oppositely charged bodies.

116 STANDARD ELECTRICAL DICTIONARY.

Charge, Free.

The charge borne by an insulated body, independent of surrounding objects. Theoretically it is an impossibility. A charge always has its compliment somewhere in surrounding objects. As a matter of convenience and convention, where the complimentary charge is so distributed that its influence is not perceptible the charge is called a free charge. If connected to earth the free charge will leave the body. If the body is connected with an electroscope the free charge will affect the same.

(See Charge, Bound.)

Charge, Residual.

When a Leyden jar or other condenser is discharged by the ordinary method, after a few minutes standing a second discharge of less amount can be obtained from it. This is due to what is known as the residual charge. It seems to be connected in some way with the mechanical or molecular distortion of the dielectric. The jarring of the dielectric after discharge favors the rapidity of the action, diminis.h.i.+ng the time required for the appearance of the residual charge. The phenomenon, it will be seen, is a.n.a.logous to residual magnetism. This charge is the reciprocal of electric absorption and depends for its amount upon the nature of the dielectric. (See Absorption, Electric, and Capacity, Residual.)

Synonym--Electric Residue.

Chatterton's Compound.

A cement used for cementing together layers or sheets of gutta percha, and for similar purposes in splicing telegraph cables. Its formula is: Stockholm Tar, 1 part.

Resin, 1 part.

Gutta Percha, 3 parts.

All parts by weight.

Chemical Change.

When bodies unite in the ratio of their chemical equivalents, so as to represent the satisfying of affinity or the setting free of thermal or other energy, which uniting is generally accompanied by sensible heat and often by light, as in the ignition of a match, burning of a candle, and, when the new compound exhibits new properties distinct from those of its components, a chemical combination is indicated. More definitely it is a change of relation of the atoms. Another form of chemical change is decomposition, the reverse of combination, and requiring or absorbing energy and producing several bodies of properties distinct from those of the original compound. Thus in a voltaic battery chemical combination and decomposition take place, with evolution of electric instead of thermal energy.