Tensioning device for a board for educational purposes

Claim 1

IN A BOARD FOR EDUCATIONAL PURPOSES HAVING AN ENDLESS BAND OF FLEXIBLE MATERIAL CAPABLE OF BEARING INDICIA, TWO VERTICALLY SPACED ROTATABLE ROLLERS AROUND WHICH SAID BAND IS ENTRAINED, THE LOWER OF SAID ROLLERS BEING VERTICALLY SUPPORTED BY SAID BAND, AND A FRAME INCLUDING TWO SUBSTANTIALLY PARALLEL UPRIGHTS RELATIVE TO WHICH SAID ROLLERS ARE ROTATABLY MOUNTED; A TENSIONING DEVICE COMPRISING AT LEAST ONE LATCH PLATE ROTATABLY MOUNTED ON THE LOWER ONE OF SAID ROLLERS, CONSIDERED WITH SAID BOARD IN THE OPERATIVE POSITION, SAID LOWER ROLLERS BEING OPERATIVELY URGED AWAY FROM THE UPPER ONE OF SAID ROLLERS BY THE WEIGHT OF SAID LOWER ROLLER, A RATCHET FIXEDLY MOUNTED ON AT LEAST ONE OF SAID UPRIGHT MEMBERS ADJACENT TO SAID LATCH PLATE, AND A PAWL CARRIED BY SAID LATCH PLATE, THE CENTER OF GRAVITY OF SAID LATCH PLATE BEING ON THE SAME SIDE OF AN IMAGINARY VERTICAL THROUGH THE POSITION OF ROTATABLE MOUNTING OF SAID LATCH PLATE AS SAID PAWL, CONSIDERED WITH SAID BOARD IN THE OPERATIVE POSITION, WHEREBY SAID PAWL IS URGED BY GRAVITY INTO ENGAGEMENT WITH SAID RATCHET, SAID RATCHET BEING SO MOUNTED AS TO PREVENT MOVEMENT OF SAID LOWER ROLLER TOWARDS SAID UPPER ROLLER WHEN SAID PAWL IS IN ENGAGEMENT WITH SAID RATCHET.