Figs. 186 and 187.—File fitted into the American chuck.
It must, however, be insisted upon that the principle of giving a passive rôle to the artificial hand is to be preferred.
(b) Appliances with Mobile Joints.—In the course of work the direction of the wrist is changing at every instant, flexion, extension, pronation, and supination occurring, sometimes in order to move around the object, sometimes in order to maintain a suitable position when the movements of the shoulder and elbow vary the direction of the forearm.
The consequence of this is that the worker learns to turn the piece of work around with his sound hand. To get over this difficulty passive joints are inserted at the wrist, which allow the appliance to move when it is pressed against the piece of work and to take up the direction which suits the inclination of the forearm.
One of the simplest mechanisms—and one of the oldest, because it was designed by Gripouilleau—is that of the agricultural labourer's ring, intended to grip and manipulate the handle of a wheelbarrow or a plough. The ring is mounted on a transverse axis and moves in a horseshoe which in its turn revolves on a shank which is screwed into the forearm. The ring is provided with a screw, which may be tightened upon the handle if desired, but which is, however, rarely used (Figs. 188 to 190).
Figs. 188 to 190.—Agricultural hook and ring. (Gripouilleau.)
