Arm 3 turns about fixed point B and is connected by turning pair D to arm 2 which has recording wheel 1 turning about axis c-c . When tracing point E is moved around the perimeter of closed curve a, the angle of revolution of recording wheel 1 is proportional to area A within closed curve a A = k(b₁— b₀); k= lr where b₀ = initial reading on the wheel in beginning to trace the figure; b₁ = final reading when the figure has been traced; k = proportionality factor; l = length of tracing arm 2 from tracing point E to pivot point D; r = radius of the recording wheel.
$505$SL,MO$