then to the forward position again no clock pulse is supplied to the clock input terminal 110 of the d type flip flop 104 now assume that the forward backward change over device is at the forward position the forward switch 70 is closed the backward switch 72 is open and the output terminal 182 of the d type flip flop 170 is at the high level if the forward backward change over device is moved to the neutral position the forward switch 70 is opened but the d type flip flop output terminal 182 remains at the high level then if the forward backward change over device is moved back to the forward position the forward switch 70 is closed again but the d type flip flop output terminal 182 still remains at the high level as a result no clock pulse is generated via the terminals 154 and 156 now if the forward backward change over device is moved to the forward position the forward switch 70 is closed and the backward switch 72 is opened in response to the closing of the forward switch 70 the control coil 60 of the forward contactor 40 is energized and the terminals 54 and 56 of the contactor 40 are connected on the other hand since the control coil 52 of the backward contactor 36 is in the deenergized state the terminals 46.