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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 voltage 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 serve as the electrolyte in the Edison storage battery. 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 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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armature coils are in parallel with the field coils. Only a portion of the current produced in the armature passes through the field coil.

Spacers.--Wood strips used in some types to separate the cells in the case and divided to provide a space for the tie bolts.

Specific Gravity.—The density of the electrolyte compared to water

a standard. It indicates the strength and is measured by the hydrometer.

Starvation.—The result of giving insufficient charge in 'relation to the amount of discharge, resulting in poor service and injury to the battery.

Steel.-An alloy composed of iron and carbon in its simplest forms. It contains from .05 to 1.80 per cent. of carbon. It is a variety of iron that can be hardened and softened by heat treatment, as well as having all the properties of iron as regards malleability, etc. It forms an important part of Edison Storage Battery elements and container.

Strap.-The leaden casting or small busbar to which the plates of a group are joined.

Sulphated.—The condition of plates having an abnormal amount of lead sulphate caused by “starvation,” or by allowing battery to remain discharged for lengthy periods.

Switch.—A switch interposed in an electrical conductor will, when opened, leave an air-gap in this conductor that offers so much resistance to the flow of current that the electricity cannot pass. Closing the switch so that the continuity of the conductor is re-established will enable the current to flow.

Tie Bolts.—Bolts which, in some types, extend through the battery case between the cells and clamp the jars in position.

Time Cutout.—Cutout devices which automatically break the charging circuit of storage batteries when the current has passed through a sufficient time to insure proper charging. A time cutout is merely a switch operated by clock work.

Transformer.-A form of induction coil to “step up” or increase voltage or to “step down” or decrease voltage. As a rule, when the potential is increased the amperage or current is reduced and vice versa. An ignition coil transforms a current of low voltage and strong amperage to one of high potential and very low amperage. A transformer can also change high-voltage current to one of low potential and secure an increase in amperage.

Top Nut.—The hexagon nut which, in batteries with bolted connections, screws on the post and holds the connectors and sealing nut in place.

Vaseline.—One of the residues left after distilling off the lighter constituents of crude petroleum oil. Used as a coating for brass or copper terminal screws on storage batteries to prevent corrosion or formation of verdigris by the chemical action of the electrolyte on the metal.

Vent.-Special fittings placed in the cover of sealed battery cells to allow passage of gas and prevent electrolyte from splashing out.

Voltage.--Electrical potential or pressure, of which the volt is the unit.

Watt.—A watt is a unit of quantity, or amount of electric energy, and corresponds to a current of one ampere at a pressure of one volt. Thus a watt is a volt-ampere-second, and 746 watts indicate an amount of electrical energy equal to one mechanical horse-power. A kilowatt is 1,000 watts.

Zinc.-A silvery white metal having a crystalline fracture and somewhat similar to lead in many respects, though not nearly so heavy. This material is widely used in primary batteries as the active plate, but is seldom made into storage-battery plates.

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