The heart acts as a pump. When the heart beats, it pumps the blood in one direction.
One Direction was signed to Simon Cowells record label becuase when they were on the X-Facetor, he was their mentor. When they were eliminated, Simon thought they were too talented to let go of so he signed them and started their career.
A knight on a chessboard starting from the positions d2, d3, e2, and e3 can move in an L-shape pattern, either two squares in one direction and then one square perpendicular, or one square in one direction and then two squares perpendicular.
The thing that keeps tempo in music is the beat.
Sammy Kershaw
Sony music I think or it might be syco music
The heart is the organ that keeps the blood flowing. Valves in the heart ensure the flow is only in one direction.
One directional valves keep blood in the heart flowing in the right direction. There are three valves: biscupid valve, semilunar valve and tricuspid valve. These open to let the blood flow through then close to prevent the blood flowing back in the wrong direction making the heart less efficient.
They ensure that the blood flows only in one direction (back toward the heart).
These are valves, and are found in veins to prevent blood from flowing backwards.
. . . then that would be alternating current, or simply, 'AC'.
Alternating current would fit this description.
There are valves in the heart which open and close that allow the blood to be pumped in one direction only. See the related link for more information.
One can determine the direction in which current is flowing by using a device called a compass or by observing the movement of charged particles in a circuit.
Polarity
Current flowing in only one direction.
Valves
The easy answer is it allows electricity to flow in one direction but prevents it from flowing the other direction.