the open position depicted in fig2 cable 20 connects between spring motor 14 and probe 18 through roller 25 roller 25 held to fixed shield 11 by means of brackets 26 and screws 27 seats cable 20 one end of cable 20 is securely fastened to moveable shield 12 by means of moveable shield clamp 21 as shown in fig2 a similar spring motor 14 39 probe 18 39 and cable 20 39 and moveable shield clamp 21 39 are located at the opposite edge of fixed shield 11 when contact is made with probe 18 force is applied to cable 20 which being attached to moveable shield clamp 21 causes moveable shield 12 to slide within guide slots 17 into the open position depicted in fig2 against the opposing force of spring motor 14 the opposing force exerted by spring motor 14 is selected such that contact with only one of the two probes 18 18 39 is not sufficient to overcome the opposing force of both spring motors 14 14 39 as well as the forces of friction and gravity and hence moveable shield 12 remains in the closed position depicted in dashed lines in fig2 spring motor.