TURN CO-ORDINATOR
This instrument is sometimes called a "Turn and Slip Indicator". It is, in fact, two separate instruments in one case. The balance ball section comprises of a ball free to move within a curved tube. When the aircraft is stationary and level or flying in balance, the ball rests at the bottom of the curve. This ball can be referred to during all modes and in all 
conditions of flight to ensure that the aircraft is flying in balance, and therefore efficiently.
The balance can be likened to a motorcyclist leaning in to a corner. If he leans over too far for the speed he is going, he will fall off into the inside of the turn. If he tries to take the corner too fast he may skid out of the turn, much like a speedway rider on loose gravel. In either case, when related to aircraft performance, the balance ball will not be in the centre between the two vertical bars.
The turn co-ordinator part is achieved by a gyro the properties of which mean it tends to maintain a constant position in space. The instrument does not indicate the bank angle achieved, but measures the rate at which the aircraft is changing heading. This is very important when the aircraft is flying in very poor visibility or in cloud, out of site of any external reference points. When flying in these extreme conditions, it is recommended that all turns are kept to "Rate 1". This is a turn that will achieve 360 degree of turn in exactly 2 minutes. This is a relatively gentle turn designed to prevent the pilot losing control of the aircraft by banking too steeply. If the aircraft is turned so that the "Rate 1 turn" marks represented on the face of the instrument are kept level with the aircraft symbol, then the aircraft will turn a full 360 degrees in 2 minutes.
This instrument gives no pitch information, whatsoever. Therefore it is usual to refer to the Artificial Horizon/Attitude Indicator, when making turns whilst flying in cloud. The turn co-ordinator has it's gyro spun up by an electric motor and is therefore not dependant on the engine driven air pump that powers the gyros in the Attitude Indicator and Direction Indicator. For these reasons the turn co-ordinator is mainly there as a back up system