[Illustration: 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 principal parts: 1. The field magnet; 2. The armature. [Illustration: 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 space 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 principal 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.