The Standard Electrical Dictionary Part 12

Armature, Rolling.

(a) An armature for a permanent horseshoe magnet consisting of a straight cylinder of soft iron on which a heavy wheel is mounted. When the legs of the magnet are inclined downward and the bar is laid across them it rolls down to the poles, across their ends, and back up the under side. It is merely a magnetic toy or ill.u.s.trative experiment.

Synonym--Wheel Armature.

(b) Another form consists of little bars of iron with bra.s.s discs attached to the ends. On placing two of these together and bringing the poles of a magnet near them, as shown, they become magnetized with like polarity by induction and repel each other, rolling away in opposite directions.

Fig. 30. ROLLING OR WHEEL ARMATURE.

Fig. 31. ROLLING ARMATURES.

Armature, Shuttle.

The original Siemens' armature, now discarded. The core was long and narrow, and its cross section was nearly of the section of an H. The grooves were wound full of wire, so that the whole formed almost a perfect cylinder, long and narrow comparatively. (See Winding Shuttle.)

Synonym--Siemens' Old Armature--Girder Armature--H Armature.

Fig. 32. SHUTTLE OR H ARMATURE.

Armature, Spherical.

An armature of a dynamo which is wound on a spherical core, so as to be almost a sphere. It is employed in the Thomson-Houston dynamo, being enclosed in a cavity nearly fitting it, formed by the pole pieces.

Armature, Stranded Conductor.

A subst.i.tute for bar-armatures in which stranded copper wire conductors are subst.i.tuted for the solid bar conductors, to avoid Foucault currents. (See Armature, Bar.)

50 STANDARD ELECTRICAL DICTIONARY.

Armature, Unipolar.

An armature of a unipolar dynamo. (See Dynamo Unipolar.)

Armor of Cable.

The metal covering, often of heavy wire, surrounding a telegraph or electric cable subjected to severe usage, as in submarine cables.

Synonym--Armature of Cable.

Arm, Rocker.

An arm extending from a rocker of a dynamo or motor, to which arm one of the brushes is attached. (See Rocker.) Ordinarily there are two arms, one for each brush.

Articulate Speech.

Speech involving the sounds of words. It is a definition which has acquired importance in the Bell telephone litigations, one contention, concerning the Bell telephone patent, holding that the patentee did not intend his telephone to transmit articulations, but only sound and music.

Astatic. adj.

Having no magnetic directive tendency due to the earth's magnetism.

Examples are given under Astatic Needle; Circuit, Astatic; and Galvanometer Astatic.

Fig. 33. n.o.bILI'S PAIR.

FIG. 34. VERTICAL PAIR ASTATIC COMBINATION.

Astatic Needle.

A combination of two magnetic needles so adjusted as to have as slight directive tendency as possible. Such a pair of needles when poised or suspended will hardly tend to turn more to one point of the compa.s.s than another. The combination is generally made up of two needles arranged one above the other, with their poles in opposite directions. This combination is usually called n.o.bili's pair. If of equal strength and with parallel magnetic axes of equal length they would be astatic. In practice this is very rarely the case. A resultant axis is generally to be found which may even be at right angles to the long axis of the magnets, causing them to point east and west. Such a compound needle requires very little force to turn it one way or the other. If one of the needles is placed within a coil of insulated wire a feeble current will act almost as strongly to deflect the system as if the other was absent, and the deflection will only be resisted by the slight directive tendency of the pair of needles. This is the basis of construction of the astatic galvanometer. Sometimes coils wound in opposite directions and connected in series, or one following the other, surround both needles, thus producing a still greater effect of deflection.

Other astatic needles are shown in the cuts below. [Figures 33 to 35.]

51 STANDARD ELECTRICAL DICTIONARY.

FIG. 35. SIMPLE ASTATIC NEEDLE.

Asymptote.

A line continuously approached by a curve, but which the curve, owing to its construction or nature of curvature, can never touch, be tangent to, or intersect.

Atmosphere.

(a) A term applied to the atmospheric pressure as a practical unit of pressure equal to 15 lbs. to the square inch as generally taken. It is really about 14.7 lbs. per square inch, or 1,033 grams per square centimeter.

(b) Air, q. v.

Atmosphere Residual.

The atmosphere left in a vessel after exhaustion. The term may be applied to any gas. In an incandescent lamp after flashing the residual atmosphere consists of hydro-carbons.

Atmospheric Electricity.

The electricity of the atmosphere, rarely absent, but often changing in amount and sign. Benjamin Franklin, in a memoir published in 1749, indicated the method of drawing electricity from the clouds by pointed conductors. In June, 1752, he flew a kite and by its moistened cord drew an electric spark from the clouds, confirming his hypothesis that lightning was identical with the disruptive discharge of electricity. To observe electricity in fine weather a gold-leaf or other electroscope may be connected to the end of a long pointed insulated conductor. The electricity during thunderstorms can be shown by a similar arrangement, or burning alcohol or tinder gives an ascending current of warm air that acts as a conductor. Quite elaborate apparatus for observing and recording it have been devised. Atmospheric electricity is usually positive, but occasionally negative. When the sky is cloudless it is always positive, increasing with the elevation and isolation of the place. In houses, streets, and under trees no positive electricity can be found. In the Isle of Arran, Scotland, a rise of 24 to 48 volts per foot of increase in elevation was found by Sir William Thomson. At sunrise the electrification of the air is feeble, it increases towards noon and decreases again to reach a second maximum a few hours after sunset. It increases with the barometric pressure generally. In cloudy weather it is sometimes negative and the sign often changes several times in the same day. In a thunderstorm the changes in sign and potential are very rapid. The cause of atmospheric electricity is far from clear. Tait attributes it to a contact effect between air and water vapor, Solmeke to friction of water vesicles against ice particles in the upper atmosphere, he first showing that the two may coexist. The cause of the enormous increase of potential producing lightning is attributed to the decreased capacity due to the change of water from cloud vesicles to drops, thus diminishing the electrostatic capacity of the water in question. (See Lightning.)

52 STANDARD ELECTRICAL DICTIONARY.

Atom.

The ultimate particle or division of an elementary substance; the smallest part that can exist in combination, and one which cannot exist alone. An elementary substance is composed of molecules just as truly as a compound one, but the atoms in the molecule of an elementary substance are all precisely alike. Hence atoms are the units of chemistry, they have to do with combinations, but the physical unit, the smallest particle of matter that can have an independent existence, is the molecule. The two are often confounded, especially by writers of a few years ago, so that by "atom" the molecule is often meant. There is nothing to be said of their size or ma.s.s. All such calculations refer to the molecule, q. v., often spoken of and called the atom.