Abstract: Apparatus for registering a chance event, particularly the passage of a ball over a closed boundary line in a game, is described wherein the ball contains an oscillatory circuit. Two parallel conductors, fed by an a.c. generator in phase opposition extend along the boundary line so that a receiving conductor extending between the transmitting conductors is subject to a resultant zero voltage until the ball approaches and passes the boundary line so disturbing the electromagnetic fields radiated by the transmitting conductors. The receiving conductor current then passes through an amplifier, phase comparison means, a pulse generator and display device. The phase comparison means is connected to distinguish the direction in which the ball crosses the line and also enables a zero setting device to accommodate prolonged extraneous signals. A compensating antenna and circuit eliminate stray field effects. The transmitting and receiving conductors can encircle a goal structure.