Abstract: An array of reed switches (13) is assembled on a circuit board (11) having a plurality of through-apertures (12) with one reed switch (13) disposed in each aperture with the axis of the switch envelope (14) extending perpendicularly to the plane of the board. A plurality of coils (16) is provided on the board so that each switch (13) is surrounded by a respective coil, the coils being connected to conductors on the board for operation of the switches. Also disposed is a control circuit wherein the reed switch drive coils (26) are connected in series across a constant-current voltage source and a respective solid-state switch TR1, TR2 . . . connected across each coil, with divers for each solid-state switch to permit operation of a selected reed switch.

Abstract: A variable resistor network has a coarse resistance network and a fine resistance network connected in series with the coarse network. The coarse resistance network comprises a chain of series-connected resistors any one or more of which may be switched out of the series by individual parallel-connected relays. The fine network comprises a shunt resistor together with a fixed resistor and a series-connected adjustable resistor together connected in parallel with the shunt resistor. A control circuit is arranged to control the relays for the switching of the resistors of the coarse network and also to control adjustment of the adjustable resistor. A chosen resistance value can be set by appropriate switching of the coarse network and adjustment of the adjustable resistor.