Moving the lever L of the rheostat to the right increases the resistance in series with the field winding, and this reduces the amount of current in that winding, thus reducing the strength of the magnet and consequently the voltage at the brushes. The contrary movement of the lever, by cutting out the resistance, produces the opposite effect. =The Compound Dynamo.=--This class of generator is designed to automatically give a better regulation of voltage on constant pressure circuits than is possible with a shunt machine. It possesses the characteristics of both the series and shunt machines, of which it is in fact a combination. [Illustration: FIG. 194.--Regulation of shunt dynamo by method of varying the field strength. A rheostat is placed in series with the field coils, and by varying the resistance, more or less current will flow through the coils, thus regulating the field strength.] The field magnets of the compound dynamo, as shown in fig. 195, are wound with two sets of coils, one set being connected in series, and the other set in parallel, with the armature and external circuit. The purpose of the series winding is to strengthen the magnets by the current supplied from the armature to the circuit, and thus automatically sustain the pressure. If the series winding were not present, the pressure at the terminals would fall as the load increased. This fall of pressure is counteracted by the excitation of the series winding, which increases with the load and causes the pressure to rise. The number of turns and relative current strengths of the series and shunt windings are so adjusted that the pressure at the terminals is maintained practically constant under varying loads. With respect to the ratio between the number of turns of the two field windings, the dynamo is spoken of as: 1. Compound; 2. Over compounded. =Ques. What is the difference between a compound and an over compounded dynamo?= Ans. In the first instance, there are just enough turns in the series winding to maintain the voltage constant at the brushes for variable load. If a greater number of turns be used in the series winding than is required for constant voltage at the brushes for all loads, the voltage will rise as the load is increased, and thus make up for the loss or drop in the transmission lines, so that a constant voltage will be maintained at some distant point from the generator. The machine is then said to be _over compounded_. =Ques. For what service is over compounding desirable?= Ans. For incandescent lighting where there is considerable length of transmission lines. =Ques. What is the usual degree of over compounding?= Ans. Generally for a rise of voltage of from five to ten per cent. In construction, the field coils are wound with a greater number of turns than actually required, the machine being accurately adjusted by a running load test after completion.