| No. 40 | No. 40 | No. 38 | No. 36 ------------------------------------------------------------------------

TABLE OF SPARKING DISTANCES IN AIR.[13]

| Distance.

Volts. | (Inches.) ------------------------- 5000 | .225 10000 | .47 20000 | 1.00 30000 | 1.625 35000 | 2.00 45000 | 2.95 60000 | 4.65 70000 | 4.85 80000 | 7.1 100000 | 9.6 130000 | 12.95 150000 | 15.00

=Points Relating to Ignition Coils.=--1. Most ignition induction coils or "spark coils" as they are called, have terminals marked "battery,"

"ground," etc., and to short circuit the timer for the purpose of testing the vibrator, it is only necessary to bridge with a screw driver from the "battery" binding post to the "ground" binding post.

2. In adjusting the vibrator of an ignition coil, the latter should not require over one-half ampere of current.

[Ill.u.s.tration: FIG. 147 to 161.--Wiring diagrams showing connections of some standard spark coils. Key: B, to battery; C, to commutator or timer; G, to ground (engine frame); P, to plugs; S, to switch. 1, 6 terminal standard non-vibrator coil; 2, 3 terminal standard vibrator coil; 3 and 4, terminal standard vibrator coil; 5, standard double vibrator coil; 6, standard triple vibrator coil; 7, standard quadruple vibrator coil; 8, single dash coil; 9, single dash coil with switch; 10, double dash coil; 11, double dash coil with switch; 12, triple dash coil; 13, triple dash coil with switch; 14, quadruple dash coil; 15, s.e.xtuple dash coil.]

3. A half turn of the adjusting screw on a coil will often increase the strength of the current four or five times the original amount, hence, the necessity of carefully adjusting the vibrator. When the adjustment is not properly made it causes, 1, short life of the battery, 2, burned contact points, and 3, poor running of the engine.

4. In adjusting a multi-unit coil, if any misfiring be noticed, hold down one vibrator after another until the faulty one is located, then screw in its contact screw very slightly.

5. The number of cells in the circuit should be proportioned to the design of the coil.

If the coil be described by the maker as a 4 volt coil, it should be worked by two cells of a storage battery or four dry cells. The voltage of the latter will be somewhat higher, but since their internal resistance is also greater, the current delivery will be about the same. Most coils are made to operate on from 4 to 6 volts.

6. It is a mistake to use a higher voltage than that for which the coil is designed, because it does not improve the spark and the contact points of the vibrator will be burned more rapidly, moreover, the life of the battery will be shortened.

CHAPTER XII

THE DYNAMO

The dynamo is a machine which converts mechanical energy into electrical energy by electromagnetic induction.

The word dynamo is used to designate a machine which produces _direct current_ as distinguished from the _alternator_ or machine generating an _alternating current_. In a broader sense, the word _generator_ is used to denote any machine generating electric current by electromagnetic induction; the term therefore includes both dynamos and alternators.

=Operation of a Dynamo.=--A dynamo does not create electricity, but generates or produces an _induced electromotive force_ which causes a current of electricity to flow through a circuit of conductors in much the same way as a force pump causes a current of water to flow in pipes. The electromotive force generated in the dynamo causes the current of electricity to pa.s.s from a lower to a higher potential in the machine, and from the higher back to the lower potential in the external circuit; that is, the dynamo generates electrical pressure which overcomes the _resistance_ or opposition to the current flow in the circuit. The pump produces a mechanical pressure which, for instance, may be used to force water into an elevated reservoir against the back pressure due to its weight.

[Ill.u.s.tration: FIG. 162.--Holzer Cabot type "M" dynamo. The design of the base is such that it allows the field ring or frame to drop down, lowering the center of gravity, which gives increased stability. The pedestals are bolted directly to the base. Both front and rear pedestals are removable, so that the armature may be taken out from either end without disturbing the brushes or connections. The journals are provided with oil rings which keep the oil in continual circulation around the shaft by means of oil grooves in the journal. The pole pieces are cylindrical in shape and are fitted with shoes which retain the field coils in place and a.s.sist commutation. The field coils are former wound, the insulation being reinforced with mica. They are soaked in varnish and baked for 24 hours at 225 Fahr. The armature is wound as desired, series, shunt or compound.

The armature core is of the drum type and is laminated, the discs being held by end plates locked without through bolts. The armature coils are formed of round, ribbon or bar copper, and are without joint except at the commutator; they lie in troughs of insulating material, the upper layers being insulated from the lower layers; they are retained in place by maple wedges secured by binding wires, soldered throughout their length. The commutator segments are drop forged in the smaller, and hard drawn in the larger sizes. Radial brushes are used. The efficiency of this type machine is stated by the maker at from 80% to 90%, according to size.]

The point to be emphasized is that _the dynamo does not create electricity_ (nor the pump water) _but sets into motion something already existing by generating sufficient pressure to overcome the opposition to its movement_.

[Ill.u.s.tration: FIG. 163.--General Electric 16 KW multi-polar dynamo designed to operate at moderate and slow speeds. The outer structure of the machine consists of a magnet frame having feet in one casting.

Adjustment is provided for moving the machine on its bed plate to tighten the belt. The field coils are former wound and the series windings permit of any degree of compounding up to 10% by the use of suitable German silver shunts connected across the series field.]

=Essential Parts of a Dynamo.=--The dynamo in its simplest form consists of two princ.i.p.al parts:

1. The field magnet; 2. The armature.

[Ill.u.s.tration: FIG. 164.--General Electric dynamo with end shield and armature removed showing construction. The core of the armature consists of laminations keyed to spider with s.p.a.ce blocks inserted at intervals to provide ventilating ducts for cooling the core and windings. The armature is _former wound_--that is, the inductors are bent to the proper shape on a form; they are, therefore, interchangeable.]

=Ques. What is the object of the field magnet?=

Ans. To provide a field of magnetic lines or lines of force to be _cut_ by the armature inductors as they revolve in the field.

=Ques. What is an armature?=

Ans. A collection of _inductors_ mounted on a shaft and arranged to rotate in a magnetic field with provision for collecting the currents induced in the inductors.

A simple loop or turn or wire may be considered as the simplest form of armature.

=Ques. How do armatures and field magnets differ in dynamos and alternators?=

Ans. A characteristic feature is that in the dynamo the field magnet is the stationary part and the armature the rotating part, while in the alternator the reverse conditions usually obtain.

=Ques. With respect to this feature, what names are sometimes given to the armature and field magnet?=

Ans. The _stator_ and the _rotor_ depending on which moves.

=Ques. What is the real distinction between an armature and a field magnet?=

Ans. The name field magnet is properly given to that part which, whether stationary or revolving, _maintains its magnetism steady during operation_; the name armature is properly given to that part which, whether revolving or fixed, _has its magnetism changed in a regularly repeated fashion when the machine is in motion_.

=Construction of Dynamos.=--In the make up of a dynamo, as actually constructed, there are five princ.i.p.al parts, as follows:

1. Bed plate; 2. Field magnets; 3. Armature; 4. Commutator; 5. Brushes.

CHAPTER XIII

THE DYNAMO: BASIC PRINCIPLES

A dynamo is a machine for converting mechanical energy into electrical energy, by means of electromagnetic induction, the amount of electric energy thus obtained depending upon the mechanical energy originally supplied.

The word dynamo is properly applied to a machine which generates[14]

direct current, as distinguished from the alternator, which generates alternating current.

=Ques. Define a dynamo with respect to its principle of operation.=

Ans. A dynamo is _a machine for filling and emptying conducting loops with magnetic flux, and utilizing the electromotive force thus induced in them for the production of current in the external circuit_.

The fitness of this definition is apparent, having in mind the principles of electromagnetic induction.

=Ques. What are the three essential parts of a dynamo?=

Ans. The field magnet, armature, and commutator.