Fig. 17. INDUCTION COIL CIRCUIT BREAKER.
38 STANDARD ELECTRICAL DICTIONARY.
A. O. C.
Abbreviation for Anodic Opening Contraction, q. v.
Aperiodic. adj.
In an oscillating apparatus, or in the oscillating member of apparatus,
the fact of having no reference to time of vibration; dead-beat.
Synonym. Dead-beat.
39 STANDARD ELECTRICAL DICTIONARY.
Fig. 18. ARAGO'S DISC.
Arago's Disc.
An apparatus consisting of a disc of copper mounted horizontally, or on
a vertical spindle, and so arranged as to be susceptible of rapid
rotation. Immediately over it, and best with a pane of glass
intervening, a magnetic needle is mounted on a pivot directly over the
axis of the disc. If the disc is rotated the lines of force of the
magnet are cut by it, and consequently currents are produced in the
copper. These currents act upon the needle and cause it to rotate,
although quite disconnected. It is advisable for the needle to be strong
and close to the disc, which should rotate rapidly.
Arc v.
To form a voltaic arc.
Arc, Compound.
A voltaic arc springing across between more than two electrodes.
Arc, Metallic.
The voltaic arc produced between terminals or electrodes of metal. The
characteristics of such arc as contrasted with the more usual arc
between carbon electrodes are its greater length for the same
expenditure of energy, its flaming character and characteristic colors
due to the metals employed. It is sometimes, for the latter reason, used
in spectroscopic investigations.
Arc Micrometer.
A micrometer for measuring the distance between the electrodes of a
voltaic arc.
Arc, Simple.
A voltaic arc produced, as usual, between only two electrodes.
40 STANDARD ELECTRICAL DICTIONARY.
Arc, Voltaic.
The voltaic arc is the arc between two carbon electrodes slightly
separated, which is produced by a current of sufficient strength and
involving sufficient potential difference. The pencils of carbon are
made terminals in a circuit. They are first placed in contact and after
the current is established they are separated a little. The current now
seems to jump across the interval in what sometimes appears an arch of
light. At the same time the carbon ends become incandescent. As regards
the distance of separation with a strong current and high electro-motive
force, the arc may be several inches long.
The voltaic arc is the source of the most intense heat and brightest
light producible by man. The light is due principally to the
incandescence of the ends of the carbon pencils. These are differently
affected. The positive carbon wears away and becomes roughly cupped or
hollowed; the negative also wears away, but in some cases seems to have
additions made to it by carbon from the positive pole. All this is best
seen when the rods are slender compared to the length of the arc.
It is undoubtedly the transferred carbon dust which has much to do with
its formation. The conductivity of the intervening air is due partly,
perhaps, to this, but undoubtedly in great measure to the intense
heating to which it is subject. But the coefficient of resistance of the
intervening air is so much higher than that of any other part of the
circuit that an intense localization of resistance occurs with
corresponding localization of heating effect. This is the cause of the
intense light. Thus if the carbons are but 1/32 of an inch apart as in a
commercial lamp the resistance may be 1.5 ohms. The poor thermal
conductivity of the carbon favors the concentration of heat also. The
apparent resistance is too great to be accounted for by the ohmic
resistance of the interposed air. A kind of thermoelectric effect is
produced. The positive carbon has a temperature of about 4,000° C.
(7,232° F.), the negative from 3,000° C. (5,432° F.) to 3,500° C.
(6,322° F.). This difference of temperature produces a
counter-electro-motive force which acts to virtually increase the
resistance of the arc. The carbon ends of an arc can be projected with
the lantern. Globules are seen upon them due to melted silica from the
arc of the carbon.
Fig. 19. EXPERIMENTAL APPARATUS FOR PRODUCING THE VOLTAIC ARC.
41 STANDARD ELECTRICAL DICTIONARY.
Areometer.
An instrument for determining the specific gravity of a fluid. It
consists of an elongated body ballasted so as to float vertically and
provided with a mark or a scale. It floats deeper in a light than in a
heavy liquid. If it carries but one mark weights are added until that
mark is reached, when the weights required give the specific gravity. Or
the scale may give the reading directly based upon the depth to which it
sinks. Areometers are often made of glass, ballasted with shot or
mercury enclosed in their bottom bulb as shown. They are used in
regulating battery solutions, and in watching the charging and
discharging of storage batteries.
Fig. 20. AREOMETER
Fig. 21. BEAD AREOMETER
Areometer, Bead.
A tube of glass containing beads of different specific gravities. It has
apertures at top and bottom. When immersed in a liquid, the same fills
it, and the specific gravity within certain limits, depending on the
factors of the beads, is shown by the beads which float and those which
sink. It is used for storage batteries and other purposes where acids
and solutions have to be tested.
Argyrometry.
The method of ascertaining the weight and inferentially the thickness of
an electroplater's deposit of silver. It is done by weighing the article
before and after plating.
Arm.
The four members of a Wheatstone bridge, q. v., are termed its arms.
Referring to the diagram of a bridge, P, Q, R, S, are the arms.