Abstract: The friction face of an electromagnet is formed by a relatively thin sheet of friction material and by magnetic inserts which are secured to the friction material and which define the pole faces of the magnet. The friction face is adapted to be easily removed from the magnet and replaced with a new friction face when the friction face becomes worn.

Abstract: An electromagnet in which an oblong block of friction material captivates a coil on an oblong plate which is stamped from sheet metal. One pole of the magnet is formed by a stud which is secured to the plate while the other pole or poles either are formed by flanges bent from the plate or by an additional stud or studs secured to the plate.

Abstract: A control system of the type which provides "breakaway" safety action to energize the electrically actuatable coils of brakes in a trailer pulled by a towing vehicle, the coils being energized under a breakaway condition from an auxiliary battery carried in the trailer. The present system is characterized by a voltage threshold indicator which is connected across the auxiliary battery and the coils and which enables the driver to test the auxiliary battery and clearly determine if the battery has sufficient storage capacity to keep the coils adequately energized while load current is being drawn from the battery and supplied to the actual coils themselves. The indicator preferably is incorporated in a pull-apart breakaway switch unit which automatically connects the brake coils with the auxiliary battery in the event of a breakaway.