The Standard Electrical Dictionary Part 9

Anvil.

An intermittent contact, or "make and break" of the current is sometimes produced by directly pressing a key down upon a metallic surface, the two being terminals of the circuit. The surface or stud on which such pressure is produced is called the anvil. The ordinary telegraph key, which makes a contact by the pressure of the operator's fingers does it by making a contact between a contact piece upon the front end of the key and the anvil. In the induction coil the anvil is also found. Thus in the cut representing the end of an induction coil and its circuit breaker in which O and O' and P and P' represent the secondary circuit terminal connections A is the core of soft iron wires, h is the anvil; the hammer when resting upon it so as to be in contact closes the circuit. When the current coming from the primary to the post i, pa.s.ses through the hammer and anvil h, and emerges by m, it magnetizes the core; this attracts the hammer, which is made of or is armed with a ma.s.s of iron. This breaks the circuit. The hammer falls at once on the anvil, again making the circuit, and the action is repeated with great rapidity. Hammer and anvil or key and anvil connections should be made of platinum.

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 gla.s.s 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 princ.i.p.ally 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 gla.s.s, 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 gla.s.s 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.

Fig. 22. DIAGRAM OF WHEATSTONE'S BRIDGE.

Armature.

(a.) A ma.s.s or piece of iron or steel, or a collection of pieces of iron designed to be acted on by a magnet. While nickel or cobalt might be used, they rarely or never are except in experimental apparatus. The armature of a permanent horse shoe magnet is simply a little bar of soft iron. When the magnet is not in use it is kept in contact with the poles with the idea of retaining its magnetism. It is then said to be used as a keeper. A bar magnet does not generally have an armature. The armature is also used to exhibit the attraction of the magnet.