SYMBOLS, ABBREVIATIONS AND DEFINITIONS.

QUES.

What is an ohm?

ANS. An ohm is the practical unit of electric resistance and many researches have been made to determine its working value. For a wire of a certain quality the resistance is in proportion to the length and also in proportion to the cross section or size.

QUES. What is Ohm's law?

ANS.

The question of relation between volts, amperes and resistance is expressed under one universal law, which is known as Ohm's law, c= in which "C" stands for current, "E" for electro-motive force or volts, and "R" for resistance.

E

R'

Current in amperes equals pressure in volts divided by resistance in ohms, or again, electro-motive force equals resistance multiplied by current; and again, resistance equals electro-motive force divided by the current; thus it will be seen that these terms are dependent upon each other, and that their relation to each other is expressed by this law.

These are written in three ways:

If one volt will force one ampere of current through a circuit having one ohm resistance it will take five volts to force five amperes through the same circuit. If this resistance should be increased to five ohms it would take five times five

SYMBOLS, ABBREVIATIONS AND DEFINITIONS.

amperes for the proper number of volts to force the amperes through, which would be 25 volts. From this it can be seen that it is very easy to obtain any one of these quantities when we have the other two.

QUES. ANS.

What is a volt?

The volt represents an electric pressure nearly equal to that of a Daniel battery cell described page 55; it is the practical unit of E. M. F. such as may be induced in a conductor which cuts lines of magnetic force at the rate of one hundred millions per second.

QUES. What is an ampere?

ANS. The ampere is the practical unit of electric current and represents a volume of a current produced by a pressure of one volt flowing through a conductor having a resistance of one ohm.

QUES. What symbol is used to indicate battery cells joined up in one row, or as they are said to be "in series."

ANS.

FIG. 53.

By the use of a symbol (see cut) in which a short thick line stands for the zinc and a longer thin line stands

for the copper (or carbon). Thus Fig. 53 represents six cells joined in series.

Similar symbols are used for "condensers" in diagrams of electric circuits.

SYMBOLS, ABBREVIATIONS AND DEFINITIONS.

QUES.

What is meant by reluctance?

ANS. The reluctance of a magnetic circuit corresponds with the resistance of an electric circuit.

QUES. What is the meaning of “coil"?

ANS. That which is gathered or wound into a ring or circle.

QUES. What is the definition of “laminated”?

ANS. It means plated-consisting of plates, layers, or scales laid one over the other.

QUES.

What is the definition of “periphery"?

ANS. It means the circumference or outside of a circle, ellipsis, or other regular curvilinear figure.

QUES. What is the definition of the word "potential"?

ANS. This oft-used word means in its primary sensepower to do work-in this connection it denotes power to do electric work.

QUES. What is the definition of “shunt"?

ANS. This is a contraction of "shun it." In railways a turning off to a siding that the principal line of rails may be left free-this idea is transferred to electricity literature.

QUES. What is the definition of calibration?

ANS. This word comes from one originally meaning "the diameter of a body" and is kindred to the word "calipers."

SYMBOLS, ABBREVIATIONS AND DEFINITIONS.

In electrical literature it means to determine the absolute or relative value of the scale divisions or of the indications of any electrical instrument, such as a galvanometer, watt-meter, etc. QUES. What is meant by periodicity?

ANS. The rate of succession of alternations, or the rate of change in the alternations or pulsations of an electric current.

QUES. What is staggering?

ANS. This term is used when one brush is placed slightly in advance of the other brush so as to bridge over a break in the circuit of the armature wires.

QUES. What is polarity?

ANS. Polarity is that quality possessed by a mineral when it attracts one pole of a magnetic needle and repels the other. QUES. What is meant by the term "inversely"?

ANS. This means to turn into a contrary direction—a change of order so that the last becomes first.

QUES. What is meant by permeability?

ANS. Permeability is the conductivity for magnetic lines of force. In other words, it is a measure of the ease with which magnetism passes through any substance. The permeability of good soft wrought iron is sometimes 3000 times that of air, varying with the quality of the iron.

NOTE.-The magnetic permeability decreases as the magnetization increases. When a piece of iron has been magnetized up to a certain intensity its substance shows a tendency to reach magnetic saturation. In good iron this is reached at about 125,000 lines of force to the square inch of area of cross section.

SYMBOLS, ABBREVIATIONS AND DEFINITIONS,

QUES.

What is the air gap?

ANS. A name given to the part of the magnetic circuit composed of air. Usually the air gap is between the pole pieces and the iron core of the armature. More or less air gap is necessary to provide room for the armature wires and also for mechanical clearance between armature and pole pieces.

QUES.

What is the output of a dynamo?

ANS. By the output of a dynamo is meant the electrical activity of the machine in watts, as measured at its terminals; or, in other words, the output is all the available electrical energy.

QUES.

What is the intake of a dynamo?

ANS. The intake of a dynamo is the mechanical activity it absorbs, measured in watts.

QUES. What is "torque"?

ANS. This is the force which produces the motion around a shaft or axis. It is often expressed as the pounds of "pull" excited at the end of a lever arm one foot long. An example is the "pulling" or turning force of an armature of an electric motor, on its shaft.

QUES. What is the definition of "electrical efficiency"?

ANS. In a dynamo or generator the relation of total electric energy produced, to the useful, or available electrical energy, thus: if a machine produced but half the work represented by the energy it absorbed, the rest disappearing in