In a turning piece, the perpendicular let fall from its connected point upon its axis of rotation is the arm or crank-arm.
If at any instant the direction of one of the crank-arms coincides with the line of connection, the common perpendicular of the line sf connection and the axis of that crank-arm vanishes, and the 3irectional relation of the motions becomes indeterminate.
The position of the connected point of the crank-arm in question at such an instant is called a dead-point.
If a connected point belongs to a turning piece, the direction of its motion at a given instant is perpendicular to the plane containing the axis and crank-arm of the piece.
The centre of the eccentric is its connected point; and its eccentricity, or the distance from that centre to the axis of the shaft, is its crank-arm.