To show the Action of Levers.—With a light but stiff wooden bar, a spring balance, and a wedge-shaped fulcrum, show:
1. The position of the weight, the fulcrum, and the power in the different classes of levers, and also the weight-arm and the power-arm in each case.
2. The direction moved by the power and the weight respectively in the use of the different classes of levers.
3. That when the power-arm and weight-arm are equal, the power equals the weight and moves through the same distance.
[pg 263]4. That when the power-arm is longer than the weight-arm, the weight is greater, but moves through a shorter distance than the power.
5. That when the weight-arm is longer than the power-arm, the power is greater and moves through a shorter distance than the weight.
To show the Loss of Power in the Use of the Body Levers.—Construct a frame similar to, but larger than, that shown in Fig. 120, (about 12 inches high), and hang a small spring balance (250 grams capacity) at the place where the muscle is attached. Fasten the end of a lever to the upright piece, at a point on a level with the end of the balance hook. (The nail or screw used for this purpose must pass loosely through the lever, and serve as a pivot upon which it can turn.) The lever should consist of a light piece of wood, and should have a length at least three times as great as the distance from the hook to the turning point. Connect the balance hook with the lever by a thread or string, and then hang upon it a small body of known weight. Note the amount of force exerted at the balance in order to support the weight at different places on the lever. At what point is the force just equal to the weight? Where is it twice as great? Where three times? Show that the force required to support the weight increases proportionally as the weight-arm and as the distance through which the weight may be moved by the lever. Apply to the action of the biceps muscle in lifting weights on the forearm.
A Study of the Action of the Biceps Muscle.—Place the fingers upon the tendon of the biceps where it connects with the radius of the forearm. With the forearm resting upon the table, note that the tendon is somewhat loose and flaccid, but that with the slightest effort to raise the forearm it quickly tightens. Now transfer the fingers to the body of the muscle, and sweep the forearm through two or three complete movements, noting the changes in the length and thickness of the muscle. Lay the forearm again on the table, back of hand down, and place a heavy weight (a flatiron or a hammer) upon the hand. Note the effort required to raise the weight, and then shift it along the arm. Observe that the nearer it approaches the elbow the lighter it seems. Account for the difference in the effort required to raise the weight at different places. Does the effort vary as the distance from the tendon?