allowing an external electromagnet to pull the electrode into contact with the pool; the electromagnet can also serve as the starting inductance, arranging the electromagnet to tip the bulb of a small rectifier, just enough to allow mercury from the pool to reach the starting electrode, providing a narrow neck of mercury between two pools, and by passing a very high current at negligible voltage through the neck, displacing the mercury by magnetostriction, thus opening the circuit, Passing current into the mercury pool through a bimetallic strip, which warms up under the heating action of the current and bends in such a way as to break the contact with the pool.