How is the ARC REACTOR USED IN IRON MAN?

ARC REACTOR Is a Important Robotic Heart that makes the IM Suit to work properly and that makes the suit to understand that the Tony Stark is correctly saying to it, So this reactor is most appeared in the following movies:

Now when the Tony uses his own Holographic technology to take into his own human hand and then so with that he looks at it because when the reactor is placed inside the suit it makes it perfectly for when he is inside to it to flyby all around the city and defeat his enemies.
 * Iron Man I
 * Iron Man II
 * Iron Man IV
 * The Avengers