A dynamo in which the iron of the field magnet is of such shape as to enclose the field magnet coils as well as the armature.

Dynamometer.

A device or apparatus for measuring force applied, or rate of expenditure of energy by, or work done in a given time by a machine. A common spring balance can be used as a force dynamometer, viz: to determine how hard a man is pulling and the like. The steam engine indicator represents an energy-dynamometer of the graphic type, the instrument marking an area whence, with the aid of the fixed factors of the engine, the work done may be determined. p.r.o.ny's Brake, q. v., is a type of the friction dynamometer, also of the energy type. In the latter type during the experiment the whole power must be turned on or be expended on the dynamometer.

Dynamo, Motor.

A motor dynamo is a machine for (a) converting a continuous current at any voltage to a continuous current of different strength at a different voltage or for (b) transforming a continuous current into an alternating one, and vice versa.

For the first type see Transformer, Continuous Current; for the second type see Transformer, Alternating Current.

Dynamo, Multipolar.

A dynamo having a number of field magnet poles, not merely a single north and a single south pole. The field magnet is sometimes of a generally circular shape with the poles arranged radially within it, the armature revolving between the ends.

Dynamo, Non-polar.

A name given by Prof. George Forbes to a dynamo invented by him. In it a cylinder of iron rotates within a perfectly self-contained iron-clad field magnet. The current is taken off by brushes bearing near the periphery, at two extremities of a diameter. A machine with a disc 18 inches in diameter was said to give 3,117 amperes, with 5.8 volts E. M.

F. running at 1,500 revolutions per second. The E. M. F. of such machines varies with the square of the diameter of the disc or cylinder.

Dynamo, Open Coil.

A dynamo the windings of whose armatures may be grouped in coils, which are not connected in series, but which have independent terminals. These terminals are separate divisions of the commutator and so s.p.a.ced that the collecting brushes touch each pair belonging to the same coil simultaneously. As the brushes come in contact with the sections forming the terminals they take current from the coil in question. This coil is next succeeded by another one, and so on according to the number of coils employed.

Dynamo, Ring.

A dynamo the base of whose field magnets is a ring in general shape, or perhaps an octagon, and with poles projecting inwardly therefrom.

201 STANDARD ELECTRICAL DICTIONARY.

Dynamo, Coupling of.

Dynamos can be coupled exactly like batteries and with about the same general results. An instance of series coupling would be given by the dynamos in the three wire system when no current is pa.s.sing through the neutral wire, and when the lamps on each side of it are lighted in equal number.

Dynamo, Self-exciting.

A dynamo which excites its own field. The majority of dynamos are of this construction. Others, especially alternating current machines, are separately excited, the field magnets being supplied with current from a separate dynamo or current generator.

Dynamo, Separate Circuit.

A dynamo in which the field magnet coils are entirely disconnected from the main circuit, and in which current for the field is supplied by special coils carried for the purpose by the same armature, or by a special one, in either case a special commutator being provided to collect the current.

Dynamo, Separately Excited.

A dynamo whose field magnets are excited by a separate current generator, such as a dynamo or even a battery. Alternating current dynamos are often of this construction. Direct current dynamos are not generally so. The term is the opposite of self-exciting.

Fig. 140. SERIES DYNAMO.

Dynamo, Series.

A dynamo whose armature, field winding, and external circuit are all in series.

In such a dynamo short circuiting or lowering the resistance of the external circuit strengthens the field, increases the electro-motive force and current strength and may injure the winding by heating the wire, and melting the insulation.

202 STANDARD ELECTRICAL DICTIONARY.

Dynamo, Shunt.

A dynamo whose field is wound in shunt with the external circuit. Two leads are taken from the brushes; one goes around the field magnets to excite them; the other is the external circuit.

In such a dynamo the lowering of resistance on the outer circuit takes current from the field and lowers the electro-motive force of the machine. Short circuiting has no heating effect.

Fig. 141. SHUNT DYNAMO.

Dynamo, Single Coil.

A dynamo whose field magnet is excited by a single coil. Several such have been constructed, with different shapes of field magnet cores, in order to obtain a proper distribution of poles.

Dynamo, Tuning Fork.

A dynamo in which the inductive or armature coils were carried at the ends of the p.r.o.ngs of a gigantic tuning fork, and were there maintained in vibration opposite the field magnets. It was invented by T. A.

Edison, but never was used.

Dynamo, Uni-polar.

A dynamo in which the rotation of a conductor effects a continuous increase in the number of lines cut, by the device of arranging one part of the conductor to slide on or around the magnet. (S. P. Thomson.) Faraday's disc is the earliest machine of this type.

203 STANDARD ELECTRICAL DICTIONARY.

Dyne.

The C. G. S. or fundamental unit of force. It is the force which can impart an acceleration of one centimeter per second to a ma.s.s of one gram in one second. It is equal to about 1/981 the weight of a gram, this weight varying with the lat.i.tude.

Earth.

(a) The earth is arbitrarily taken as of zero electrostatic potential.

Surfaces in such condition that their potential is unchanged when connected to the earth are said to be of zero potential. All other surfaces are discharged when connected to the earth, whose potential, for the purposes of man at least, never changes.

(b) As a magnetic field of force the intensity of the earth's field is about one-half a line of force per square centimeter.

(c) The accidental grounding of a telegraph line is termed an earth, as a dead, total, partial, or intermittent earth, describing the extent and character of the trouble.

[Transcriber's note: Fallen power lines can produce voltage gradients on the earth's surface that make walking in the area dangerous, as in hundreds of volts per foot. Lightning may be a.s.sociated with substantial changes in the static ground potential.]