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Hard Rubber.-A rubber compound that has been hardened by heat treatment so it has greater stiffness than rubber in its natural form, and will keep its shape indefinitely after forming. This material is very brittle and not very strong. It is an excellent insulator of electricity, and as it is not affected by sulphuric acid it is widely used for cell jars.

Hold-Down Clips.-Brackets for the attachment of bolts for holding the battery securely in position on the car.

Horse-Power.—The accepted unit of mechanical work. The ability to move 550 pounds one foot in one second or 33,000 pounds one foot in one minute. An electrical horse-power is 746 watts.

H.P.-Abbreviation for horse-power.

Hydrogen.—One of the basic elements existing as a gas under natural conditions. It inay be liquefied by the simultaneous application of great pressure and abstraction of heat. It is the lightest known substance. The chemical symbol is H.

Hydrogen Flame.--A very hot and clean flame of hydrogen gas and compressed air used for making burned connections.

Hydrogen Generator.-An apparatus for generating hydrogen gas for lead-burning

Hydrometer.-An instrument for measuring the specific gravity of the electrolyte.

Hydrometer Syringe.-A glass barrel enclosing an hydrometer and provided with a rubber bulb for drawing up electrolyte.

Induction.—The creation of a current in a conductor not connected to a source of electricity by the juxtaposition of one that is carrying the current.

Induction-Magnetic.—The magnetization of any magnetic substance, such as iron or steel, by placing it in a magnetic field but not in actual contact with tie energizing magnet.

Insulating Tupe.-A textile fabric impregnated with insulating compound of an adhesive nature. Used to cover bare spots in insulated wires, re-enforce insulation, and for protecting joints where wires are joined together.

Insulating Varnish.-Shellac or sealing wax dissolved in alcohol, or gum copal dissolved in ether, may be used as a varnish for insulating purposes.

Insulator.—Materials such as wood, glass, rubber, etc., and air, conduct electricity so badly as to be termed insulators. What would normally be an insulator to a current of low potential may be ruptured by a current of higher potential or pressure which can break down the resistance.

Iron Oxide.—Commonly known as "rust.” It is packed in steel pockets, which are assembled into negative plates of Edison Storage Battery. Expressed chemically as Feo.

Jar.-The hard rubber container holding the element and electrolyte. KOH.—Chemical symbol for caustic potash or potassium hydrate.

Lead.-An abundant and widely used metal of bluish-white color, and one of the softest and heaviest of metals. It is not acted upon by sulphuric acid unless an electric current is passed through it. It forms the main part of most storage-battery plates, either as a metallic lead or as a lead oxide.

Lead-Burning.-Making a joint by melting together the metal of the parts to be joined.

Lead Oxide.—Material on plates when a cell is discharged according to some theories of storage-battery action. This is expressed chemically as PbO, differing from peroxide only because there is less oxygen combined with the lead.

Lead Peroxide.—The active material on positive plates of lead batteries after charging. Expressed chemically as PbO2.

Lead Sulphate.—Material on storage-battery plates when cells are discharged, caused by absorption of sulphate from the electrolyte. Expressed chemically as PbSO4.

Lime, Slaked.-First quicklime is obtained by burning limestone, chalk or marble in kilns and afterward removing its caustic properties by watering it and allowing it to remain in the air for a time. This is used in battery compartments of electric vehicles to neutralize spilled acid, as it is of an alkaline nature.

Litharge.—A yellow or reddish oxide of lead that is partially fused.

Local Action.-Wasteful oxidization of zinc in a primary battery when it is not in use, or abnormal sulphation of storage-battery plates due to impurities in the electrolyte.

Lug.-The extension from the top frame of each plate, connecting the plate to the strap or busbar.

Magnetism.—This is a property possessed by certain substances, and is manifested by the ability to attract and repel other materials susceptible to its effects. When this phenomena is manifested by a conductor or wire through which a current of electricity is flowing, it is termed "electro-magnetism.” Magnetism and electricity are closely related, each being capable of producing the other.

Magnetic Substances.—Only certain substances show magnetic properties, these being iron, nickel, cobalt and their alloys. The earliest known substance possessing magnetic properties was a stone or iron ore first found in Asia Minor. It was called the “lodestone,” or leading stone, because of its tendency, if arranged so it could move freely, of pointing one particular portion toward the north.

Magnetic Attraction. If the north pole of one magnet is brought near the south pole of another, a strong attraction will exist between

them, this depending upon the size of the magnets used and the air-gap separating the poles. Magnets will attract all magnetic substances.

Magnetic Repulsion.—1f the south pole of one magnet is brought close to the end of the same polarity of the other there will be a pronounced repulsion of the forces. The like poles of magnets will repel each other because of the obvious impossibility of uniting two influences or forces of practically equal strength but flowing in opposite directions.

The unlike poles of magnets attract each other because the force is flowing in the same direction.

Magnetic Flow.—The flow of magnetism is through the magnet from south to north, and the circuit is completed by the flow of magnetic influence through the air-gap or metal armature bridging it from the north to the south pole.

Maximum Gravity-The highest specific gravity which the electrolyte will reach by continued charging, indicating that no acid remains with the plates.

Meters.—Most of the electrical measuring instruments depend upon the principle of electro-magnetism or induction. These measuring instruments are made in portable and switchboard types. The windings in an instrument designed to measure current quantity or amperage are usually of coarse wires, while the windings of an instrument to measure electro-motive force or voltage will be of finer wire. The gauge used to measure current quantity is called an ampere meter or ammeter, while that used to measure current pressure is a volt meter.

Mica.-An insulator of natural mineral derivation that will stand considerable heat. Not suited for use with high-potential currents, because it is apt to contain impurities of a metallic nature. Commonly known as “isinglass.

Motor.—A machine that is capable of delivering current in one direction when driven by mechanical power and which will produce mechanical energy if electric current is passed through the winding in a reverse direction.

Motor-Generator.–An electrical machine that may be used either as a current producer or for generating electricity if driven by mechanical means, or as a power producer if driven by electrical means.

Negative Pole.—The terminal of a current-generator to which the current flows after leaving the outer circuit.

Nickel.--A silver white malleable and ductile metal, that can be applied to others by thin surface coating through an electro-deposition or plating process.

Nickel-Hydrate.-A green powder used as the active material in the positive plates of the Edison storage battery.

Ohm.-The ohm is the unit by which resistance is judged. Everything has electrical resistance. Some elements have very little, such as

a short length of good conductor; others have so much as to form a most effectual barrier to the passage of the current, these being commonly known as insulators.

Ohms Law.—The fundamental rules expressing the relation between voltage, amperage and resistance. It may be expressed thus: the current strength (C) is equal to the voltage or electro-motive force (E) divided by the resistance (R), or C=E R. Naturally, the age is equal to the current strength multiplied by the resistance, or E=C X R. The resistance is equal to the voltage divided by the amperage, or R= E = C. .

Oil of Vitriol.-Commercial name for concentrated sulphuric acid (1.835 specific gravity). This is never used in a battery, and would quickly ruin it.

Oxidization.—The chemical combination of oxygen with any substance. Iron rust is ferrous oxide, and has been caused by oxidization.

Oxygen.-One of the most active of the elements that naturally exist as a gas, though it may be liquefied. Owing to its great affinity for various substances, it is not found free or uncombined. The chemical symbol is 0.

Pb.—Chemical symbol for lead.

Paraffine Wax.-A white substance produced by distillation of crude petroleum, and one of the best insulators known.

Pickling.–The process of cleaning metal by dipping in an acid solution. This solution is known as a “pickle.”

Plates.—Metallic grids supporting active material. They are alternately positive (brown) and negative (gray).

Polarity.-A difference in electrical condition. The positive terminal of a cell or battery, or the positive wire of a circuit, is said to have positive polarity; the negative, negative polarity.

Post. The portion of the strap extending through the cell cover, by means of which connection is made to the adjoining cell or to the car circuit.

Positive Pole.—The terminal of a current-generator from which the current is intended to flow to the outer circuit. Potassium Hydrate.—An alkaline substance combined with water to

as the electrolyte in the Edison storage ba ry. Commonly known as caustic potash. Expressed chemically as KOH.

Potential or E.M.F. (Electro-Motive Force).—The greater the difference in the quantities of the electrical charge the greater the tendency to reach the state of equilibrium. This difference in electrical conditions, or amount of electrical charge, is termed “difference of potential,” and high or low potential, or “electro-motive force,” in any electrical system indicates a large or small difference of charge or electrical condition at different parts. This is measured in volts.

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Rectifier.-Any device capable of changing alternating current to one having the properties of direct current.

Resistance.--Material (usually lamps or wire) of low conductivity inserted in a circuit to retard the flow of current. By varying the resistance, the amount of current can be regulated.

Resistance, External.—The resistance of those parts of the circuit outside of the dynamo or battery producing the current.

Resistance, Internal.-The resistance of the windings of a generator or that of the electrolyte and separators of a storage battery as distinguished from that of the parts comprising the outer circuit.

Resistance, Ohmic.—Resistance measured in ohms is a true resistance.

Return, or Ground.—The conductor which is supposed to carry the current to its starting point after it has passed through parts of the outer circuit. In an automobile lighting, starting and ignition system the metallic chassis frame is often used as a “ground return” to the battery. In large power installations the earth is actually used as a return conductor.

Rheostat.-A device having coils of different resistance that can be brought into action progressively to control electric-current flow, as when charging batteries.

Rubber Sheets.--Thin, perforated hard-rubber sheets used in combination with the wood separators in some types of batteries. They are placed between the grooved side of the wooden separators and the positive plate.

Sealing Compound.—The acid-proof compound used to seal the cover to the jar.

Sealing Nut.—The notched round nut which screws on the post and clamps the cell cover in place in Exide batteries.

Sediment.-Active material which gradually falls from the plates and accumulates in the space below the plates provided for that purpose.

Series Winding.–A method of winding electric machines where the armature winding is in series with the field winding. All current produced in the armature coils must pass through the field coils as well before reaching the external circuit.

Separators.—Sheets of grooved wood, specially treated, inserted between the positive and negative plates to keep them out of contact.

Shellac.-A resinous, vegetable substance, soluble in alcohol, and having good insulating qualities.

Short Circuit.-A metallic connection between the positive and negative plates within a cell. The plates may be in actual contact or material may lodge and bridge across. If the separators are in good condition, a short circuit is unlikely to occur.

Shunt Winding.-A method of winding electric machines where the

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