An arc AB of a sector of a circle subtends an angle 140° at the centre of the circle. Find the angle subtended by the arc from the circumference.

Step 1: Identify the central angle.
The chord AB subtends an angle of $140^\circ$ at the center O of the circle.

Step 2: State the theorem.
The angle subtended by an arc at the center of the circle is twice the angle subtended by the same arc at a point on the remaining circumference:
$\theta_center = 2 \times \theta_\text{circumference}$.

Step 3: Set up the formula for the angle at the circumference.
$\theta_circumference = \frac{1}{2} \theta_\text{center}$

Step 4: Substitute the given value.
$\theta_circumference = \frac{1}{2} \times 140^\circ$

Step 5: Simplify.
$\theta_circumference = 70^\circ$

Final answer: $\boxed{70^\circ}$