=Ques. How is the degree of over compounding varied?= Ans. A rheostat is placed in shunt with the series winding so that the current passing through the winding may be regulated to control the voltage of the machine. [Illustration: FIG. 195.--Compound wound dynamo, used when better automatic regulation of voltage on constant pressure circuits is desired than is possible with the shunt machine. The compound dynamo is a combination of the series and shunt types, that is, the field magnet is excited by both series and shunt windings. With a proper selection of the number of turns in the series coils, the voltage may be kept automatically constant for wide fluctuations in the load. When the machine is _over compounded_ its characteristic is to slightly increase the voltage with increase of load, a desirable feature for long transmission lines in order to compensate for the line drop.] =Ques. How are the ends of the shunt winding of a compound dynamo connected?= Ans. There are two methods of connection, being known as the short shunt and the long shunt. =Ques. Describe the short shunt.= Ans. In the short shunt, the ends of the shunt winding are connected directly to the brushes as in fig. 196. =Ques. Describe the long shunt.= Ans. In the long shunt, one end of the shunt winding is connected to one of the brushes and the other end to the terminal connecting the series winding with the external circuit as in fig. 197. =Ques. Which is the more desirable?= Ans. Theoretically, the long shunt is preferable as being the more efficient; however, in practice, the gain is not very appreciable and the short shunt is generally used. [Illustration: FIGS. 196 and 197.--Short and long shunt types of compound wound dynamos. The distinction between the two is that the ends of the short shunt connect direct with the brush terminals, while in the long shunt type, fig. 197, one end of the shunt connects with one brush terminal and the other with the terminal connecting the series winding with the external circuit. R is the shunt field rheostat for regulating the current through the shunt.] =Ques. What may be said regarding the voltage in short, and long shunt machines?= Ans. In a short shunt machine, the shunt winding is subjected to a higher voltage than with a long shunt. The pressure applied through a shunt winding with a long shunt, for any particular load, is equal to the voltage at the brushes plus the drop in the series winding. =Ques. For what other service besides incandescent lighting are compound dynamos adapted?= Ans. They are employed in electric railway power stations where the load is very fluctuating. =Ques. What is the effect of a short circuit on a compound dynamo?= Ans. It overloads the machine, since the excessive current flowing through the series field tends to keep the voltage at its normal value.