Patents.
3,712,198; 3,718,397; 3,714,879; 3,719,416; 3,720,457; 3,721,170; 3,805,051; 3,805,052; 3,820,870; 3,837,727; 3,837,732; 3,838,910; 3,838,911; 3,840,290; 3,840,701; 3,840,741; 3,841,734; 3,841,736; 3,843,235; 3,844,640.