that the magnetic flux due to the field magnet is caused to change its direction through the circuit of the armature. This may be done by causing the armature to revolve whilst the field magnet is stationary, or the armature may be stationary and the field magnet may revolve. In some cases both field magnets and armature may revolve. In some cases both field magnets and armature are stationary. In order, then, to create the change of flux through the armature, an intermediate piece of iron has to revolve which directs the flux one way or the other through the armature core. Machines of this class are called Inductor Machines. A comprehension of their mode of working may be obtained from a consideration of the simple model shown in Fig. 118. In this case two semicircular electromagnets are fixed with the line joining their poles at right angles to each other. One of these is the fixed field magnet F, and is excited by a current. The other of these is the fixed armature A. To a revolving shaft ab are fixed two quadrantal-shaped segments of iron I, I, which revolve in between the poles of the two magnets, as close as possible but just not touching the poles. When these pieces of iron are in the position shown in the figure they conduct the magnetic flux of the field magnet through the armature in the direction as represented by the dotted line, but when they have moved round through a quarter of a turn they conduct the flux round through the armature in the opposite direction. Thus the necessary reversal of flux is made through the coils of the armature without any movement of the iron core of the armature itself. It will then be observed that we may classify dynamoelectric machines as follows (i) Continuous current machines; (ii) Alternating current machines, or alternators; according to whether they furnish in the external circuit a continuous or an alternating current. Both classes may be either (a) Fixed field and revolving armature machines, (1) Single-phase alternators, giving one single alternating current. (2) Two-phase alternators, giving two alternating currents with a fixed difference of phase between them. (3) Polyphase alternators, giving several alternating currents having different phases. The output of a machine is always reckoned in kilowatts. Hence a 30-kilowatt (K.W.) machine is one which can produce an electrical power in the external circuit of 30,000 watts. The efficiency of a dynamo is the ratio, expressed as a percentage, between the power given out by it in the external circuit and the power required to drive it round, both powers being measured in the same units. In the case of continuous current machines, or of single phase alternators working on a non-inductive current, the output of the machine in watts can be obtained by measuring the potential difference of the brushes in volts, and the outgoing current in amperes, and then multiplying these two values together. §3. Excitation.-In the model machine described in the last section, we considered that the magnetic flux was created by an electromagnet energised by a separate electric current. There are, however, three ways in which this field flux can be created. In the first place, the field magnet may be a permanent steel magnet once for all magnetised. Next, the field magnet may be an electromagnet, and the current required to create it may be obtained from a separate or external source of current, such as a battery. Thirdly, the principle of self-excitation may be employed. In this latter case either a part or the whole of the current produced in the armature may be led through the circuit of the field magnets. If the field magnets are slightly magnetised initially in the right direction, then beyond a certain speed of revolution, the electromotive force set up in the armature will be sufficient to generate, and increase up to a limit, an exciting current which, when led through the coils of the field magnet, will fully magnetise the field magnet cores. This is called self-excitation. The self-exciting machine is the one to which the name of dynamo was originally given. Permanent field magnets and separately excited field magnet machines were previously called magneto-electric machines. Hence we have the following classification of machines depending on the method of the production of the field flux (1) Magneto-machines. Permanent magnet fields. (2) Separately excited machines. fields. (3) Self-excited machines. (a) Series wound fields. (b) Shunt wound fields. (c) Compound wound fields. Electromagnet The last class (3) can obviously only be continuous current machines. The sub-classification of self-excited machines de The method of self-excitation was devised by S. A. Varley in 1866, and soon after independently by Siemens and Wheatstone, pends upon the mode of winding of the field magnets. If the whole current from the armature goes through the field magnet coils, the machine is called a Series machine. If the ends of the field magnet circuit, and also the ends of the external circuit, are both joined to the brushes of the machine, that is, to the terminals of the armature circuit, the machine is called a Shunt machine. If a shunt machine has in addition field magnet coils which carry the whole external current, it is called a Compound wound machine. (See Fig. 119.) Shunt wound Series wound Compound wound Fig. 119.-Types of Field Magnet Winding. The power reckoned in watts taken up in the exciting coils of the field magnets is called the exciting power of the machine. § 4. General Design of Continuous Current Dynamos. From the remarks in the previous section it will have been made clear to the student that a continuous current dynamo essentially consists of two electromagnets, one of which carries a commutator, and revolves in front of, or between, the poles of the other. Experience has shown that these electromagnets should have very different forms, and with some slight modifications most continuous current machines now in use consist of a field magnet having massive round iron or mild steel legs united by a yoke, and furnished at the opposite ends with curved pole pieces nearly embracing the armature. (See Fig. 120.) The other electromagnet or armature takes the form of a drum, or ring, or cylinder of iron, wound in a particular way with coils of insulated wire. The field magnet cores are constructed of cast steel' of the highest permeability, or else of wrought iron, and the wire windings on them consist of double cottoncovered copper wire, insulated with paper and shellac varnish. The core of the armature is always built up of stampings of thin sheet iron or sheet steel. The proper |