Abstract: A system for supplying power to one or more electromagnets which includes a main power rectifier source and battery system including batteries charge controls and warning devices for one or more magnets such that in the event the main power source and/or the battery charger become inoperative power will be supplied to the magnets if energized at such time until the load on the magnet can be safely removed. Visual and/or audible indications are immediately given to the operator of the failure of either the main power rectifier or of the battery charger so that corrective and safety measures can be taken. A diode is provided between the battery and the load such that power flows from the battery only if the main power source voltage falls below that of the battery.

Abstract: An air filter of the type having a porous filter medium body layer supported in a complementary unitary frame has a perforated panel supportingly engaging one face of the layer with folded up narrow panels along the edges of the layer, narrow retainer panels overlying the margins of the opposite face of the layer from the perforated panel, and each of the retainer panels being provided with spaced extensions which overlap adjacent the corners of the layer and are secured together as by means of adhesive, there also being anchoring adhesive securing two of the retainer panels to the filter layer. The overlapping extensions are perforated.An efficient method of making the air filter is provided.