The third diagram shows a metadyne connected to two separate motors, and this arrangement was often used for the control of traction motors on electric trains. Connecting them in this way reduces the effective loading on the Metadyne, and enables a smaller machine to be installed. The Metadyne acts as a "positive or negative booster". If Vcc is the supply voltage, and V2 is the output voltage of the Metadyne, then the total voltage across the load can vary from 0 to 2·Vcc, as V2 varies between −Vcc and +Vcc. Although the system is prone to the currents in the two halves of the load becoming unbalanced, this can be corrected by the provision of extra series windings, which act like an additional circuit resistance.