Abstract: A Faraday transducer for a marine speed indicator is sealed in a cylindrical housing so that it can be mounted in a boat hull with its base flush with the hull. A horseshoe magnet sealed within the housing has its poles located against the base surface and with 4 electrodes in the base surface located one on each side of each magnetic pole. The electrodes are connected together in such a way as to cancel out stray magnetic fields and to increase the Faraday emf produced by the relative movement of water flowing across the base of the transducer.

Abstract: An electronic switch has a pair of light dependent resistors forming part of a potential divider with the hysteresis of the circuit being high enough to prevent equally applied changes in light level from effecting the switch action. The divider controls the voltage to an operational amplifier which is switched off by covering the other resistor. The switch may be combined with an opto-isolator or a relay to control power circuits.