The two curves overlap each other, and in order to determine the effect of this it is necessary to trace the resultant curve, 3. This is easily done, as the resultant electromotive force induced at any point in the revolution of the armature is equal to the sum of the pressures induced in AA′ and BB′. Thus, at the beginning of the revolution the pressure induced in AA′ is at zero point, and in BB′ at its maximum J, hence, the resultant curve begins at the point J. Again, for any point in the revolution, as N, the height of the resultant curve is equal to NP + NT = NV. For 45° or 1/8 revolution, the resultant curve reaches its amplitude, which is equal to 2 × RZ = RW, and at 90° it again reaches its minimum, XY. =Ques. State the conditions upon which the steadiness of the current depends.= Ans. _It depends on the number of coils and the manner in which they are connected._ Comparing curves 1 and 3, in fig. 185, it will be noted that with four coils the variation of pressure or amplitude of the pulsations is less than half that obtained with two; moreover, with four coils the number of pulsations per cycle is doubled. In order to further observe the approach to continuous current obtained by increasing the number of coils, the effect of a six coil armature is shown in fig. 186, the resultant curve being obtained in the same manner as just explained. For comparison, the curves for the three cases of two, four, and six coils are reproduced under each other in fig. 187. As the number of coils is further increased, the amplitude of the pulsations decreases so that the resultant curve approaches nearer the form of a straight line. In the actual dynamo there are a great many coils, hence the amplitude of the pulsations is exceedingly small; accordingly, it is customary to speak of the current as “continuous,” although as previously mentioned such is not the case. CHAPTER XV CLASSES OF DYNAMO In order to adapt the dynamo to the varied conditions of service, its design is modified in numerous ways, giving rise to the different “types.” These may be classified with respect to: 1. Field magnets; 2. Field excitation; 3. Field winding. The first division relates to the number of magnetic poles, as unipolar, bipolar, and multi-polar dynamos; also inter-polar dynamos. Under the second division are included the following: