Ans. The series coils of each are connected as in fig. 685, and the shunt coils are connected as a single shunt as in fig. 687, which may either extend simply across the outer brushes of the machines, so as to form a double short shunt, or may be a shunt to the bus bars of external circuit, so as to form a double long shunt. [Illustration: Fig. 689.--Coupling compound dynamos in series; short shunt connection. The dotted lines indicate the changes that would be made for long shunt connection.] Compound Dynamos in Parallel.--Machines of this type will not run satisfactorily together in parallel unless all the series coils are connected together by an equalizing connection, as in series dynamos. The method of arranging the connections as adopted in practice, being illustrated in fig. 690. By means of it idle machines are completely disconnected from those at work. Ques. How is the equalizer connected? Ans. The equalizer is connected direct to the positive brushes of all the dynamos, a three pole switch being fitted for disconnecting it from the circuit when the machine to which it is connected is not working. The two contacts of the switch are respectively connected to the positive and negative conductors, while the central contact is connected to the equalizer. [Illustration: Fig. 690.--Diagram showing method of coupling compound dynamos in parallel.] Switching a Compound Dynamo Into and Out of Parallel.--If the characteristics of all the dynamos be similar, and the connections arranged as in figs. 690, or 691, the only precaution to be observed in switching a new machine into parallel is to have its voltage equal, or nearly equal to that of the bus bars previous to closing the switch. If this be the case, the new machine will take up its due share of the load without any shock.