=Dobrowolski Three Wire Dynamo.=--This type of dynamo was designed to operate a three wire system of distribution without a balancer. The armature is provided with insulated slip rings connected to suitable points in the armature winding and (by means of brushes) with choking coils meeting at a common point, to which the neutral wire of the system is connected, the main terminals being connected with the outside wires. The machine is capable of feeding unbalanced loads without serious disturbance of the pressure on either side of the system. The principle of the Dobrowolski three wire dynamo is illustrated in fig. 199. The armature A is tapped at two points, B and B′, and connected to slip rings C C′. A compensator or reactance coil D, between the two halves of which there is minimum magnetic leakage, is connected to C and C′ by brushes, and has its middle point tapped and connected to the neutral wire E. [Illustration: FIG. 200.--Armature of Westinghouse three wire dynamo. Collector rings are mounted at one end of the armature as shown, and the leads to them with the armature winding are similar to those employed on the alternating current side of a rotary converter armature. The connections from the armature to collector rings may be either single phase, two phase, or three phase. The two phase connection with four collector rings and two balance coils is used in the Westinghouse three wire dynamo.] It is clear, from the symmetry of the arrangement, that the center point of the coil must always be approximately midway in pressure between that of the brushes, and hence any unbalanced current will return into the armature, dividing equally between the two halves of the coil. The arrangement forms a cheap and effective substitute for a balancer set, but lacks the adjustable properties of the latter. There are various modifications of the arrangement. Thus more than two slip rings may be used. The compensator windings, however, should always be arranged so that the magnetizing effect of the neutral current is self-neutralized in the windings, as otherwise saturation occurs causing a very heavy alternating magnetizing component. CHAPTER XVI FIELD MAGNETS The object of the field magnet is to produce an intense magnetic field within which the armature revolves. It is constructed in various forms, due in a large measure to considerations of economy, and also to the special conditions under which the machine is required to work. Electromagnets are generally used in place of permanent magnets on account of: 1, the greater magnetic effect obtained, and 2, the ability to regulate the strength of the magnetic field by suitably adjusting the strength of the magnetizing current flowing through the magnet coils.