NOTE.--The action of an equalizing bar in equalizing the load on compound dynamos run in parallel may be explained as follows: The compound winding of a dynamo raises the pressure in proportion to the current flowing through it, and if, in a system of parallel operated compound dynamos without the equalizing connection, the current given by one machine were slightly greater than the currents from the others, the pressure of that machine would increase. With this increase in pressure above the other machines, a still greater current would flow, and so raise the pressure further. The effect is therefore cumulative, and in time the one dynamo would be carrying too great a proportion of the whole current of the system. With the equalizing connection, whatever the current flowing from each machine, the currents in the various compound windings are all equal, and so the added pressure due to the compound winding is practically the same in each machine. Any inequality in output from the machines is readily eliminated by adjusting the shunt currents by means of the shunt rheostats. When compound wound dynamos are operated in parallel, the equalizer bar insures uniform distribution among the series coils of the machines. NOTE.--To secure the best results in parallel operation, dynamos should be of the same design and construction and should possess as nearly as possible the same characteristics; that is, each should respond with the same readiness, and to the same extent, to any change in its field excitation. Any number of such machines may be operated in parallel. The usual practice is to connect the equalizer and the series field to the positive terminal, though if desired, they may be connected to the negative terminal; both however, must be connected to the same terminal. The resistance of the equalizer should be as low as possible, and it must never be greater than the resistance of any of the leads from the dynamos to the bus bar. Sometimes a third wire is run to the switchboard from each dynamo and there connected to an equalizer bar, but the usual practice is to run the equalizer directly between the dynamos and to place the equalizer switches on pedestals near the machines. This shortens the connections and leads to better regulation. The positive and equalizer switches of each machine differ in pressure only by the slight drop in the series coil, and in some large stations these two switches are placed side by side on a pedestal near the machine. In such cases, the equalizer and positive bus bars are often placed under the floor near the machines, so that all leads may be as short as possible. If all the dynamos be of equal capacity, all the leads to bus bars should be of the same length, and it is sometimes necessary to loop some of them.