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. =Ques. What is the object of the field magnet?= Ans. To provide a magnetic field, through which the conducting loops arranged on a central hub and forming the _armature_ are carried, or the flux carried through them, so that they are successively filled and emptied of magnetic lines. =Ques. What is a commutator?= Ans. A device for causing the alternating currents generated in the armature to flow in the same direction in the external circuit. =Ques. Upon what does the voltage depend?= Ans. Upon the _rate_ at which each conducting loop is filled and emptied of lines of force and the number of such loops with their grouping or connection. =Ques. How is the operation of a dynamo best explained?= Ans. By considering first the action of the simplest form of current generator, or elementary alternator. =Ques. Describe an elementary alternator.= Ans. It consists, as shown in fig. 165, of a single rectangular loop of wire A B C D, one end being attached to a ring F and the other to the shaft G, and arranged so as to revolve around the axis X X′, which is located midway between the two poles of the magnet. Two metallic strips or _brushes_ M and S connected with the external circuit, bear on the ring F and shaft G, respectively, in order to “collect” the current generated in the armature when the machine is in operation. The long, straight, horizontal arrows joining the two poles of the magnet, represent the _lines of force_ which make up the magnetic field between the poles. The field is here assumed to be uniform, as indicated by the equal spacing of the arrows. =Ques. What happens when the loop is rotated?= Ans. According to the law of electromagnetic induction, when the loop is rotated around its horizontal axis in the direction indicated by the curved arrow, an electromotive force will be induced in the loop, the magnitude of which depends on the _rate_ of change of the number of lines of force threading through, or embraced by the loop.