1. An improved orthodontia appliance comprising: a face bow, said bow including a mouth brace adapted to be positioned within the mouth of an orthodontia patient and to engage certain teeth of the patient and band lugs on both sides of said brace adapted to remain outside the mouth of the patient when the bow is in use; an elastic band extending between said band lugs, the resiliency of said band being such as to cause said mouth brace to impart a predetermined force on said certain teeth when said brace is positioned in the mouth of the patient and the band is positioned about the head of the patient; means for adjusting the overall length of said band between said band lugs; and force determining means having at least one electrical characteristic which varies in predetermined fashion in response to the force exerted by said band on said teeth through said brace, said force determining means including a strain gauge rigidly coupled to said elastic band and aligned along the stretch axis of said band.