(1) When adjusting the lever, move the balance nuts at both ends to keep the lever in a horizontal position. If the spring dynamometer is not used, the power and resistance should be on both sides of the lever respectively.

(2) As can be seen from the figure, there are only two forces acting on the object, and the magnitude of the two forces can be displayed by the indicator of the dynamometer. Two forces are in opposite directions, not in a straight line, acting on an object; According to the condition of balance of two forces, we know that two forces need to be equal in magnitude and opposite in direction, acting on a straight line and an object. So this diagram explores the relationship between the size, direction and action line of object C at rest.

So the answer is: balance nut; Second; c； Route of action.