Make a little horshoe electro-magnet out of a piece of 1/4 in. round soft iron rod, each limb being 1 in. long and yoke also 1 in., Fig. 1 a. Wind this with six turns of No. 20 d. c. c. Over this pivot a soft iron armature 1 in. long, b having a lever extension at the pivoted end o, and a curved piece of bare No. 16 copper wire, d, at the other. The poles of the magnet must be perfectly level, and to prevent the armature adhering when the current is off should have a disc of stamp paper stuck on the upper surface of each. A little metallic cup, in which a drop or two of mercury is placed, is fixed just under copper wire so that when the armature is pulled down by the magnet the copper wire should dip into and make contact with the mercury.
This mercury cup should be connected to a terminal leading by a stout wire to the + terminal of accumulator. The lever end of the armature is hooked on a small spiral springe of sufficient tension to pull the armature away from the poles of the magnet, when the voltage of the battery "falls to 5 volts, but not powerful enough to detach it as long as the volts exceed 5. One end of the magnet wire is connected directly to the iron of the magnet, say at f; the other end goes to the terminal G, to which the positive pole of the charging battery H is connected.
Finally, the negative pole of the battery is connected to the negative terminal of the accumulator. Now the battery and accumulator being duly coupled up to the switch, as described, the armature is gently depressed, so as to cause the curved copper wire to dip into the mercury cup. Now the current passes from the battery, round the magnet wires, through the pivot to the mercury cup ; thence through the accumulator, completing its circuit to the battery; and as long as the voltage exceeds 5v., retains the armature down, and the copper wire in the mercury. Directly the voltage falls below, the pull of the spring overcomes the pull of the magnet, and releasing the armature, breaks the circuit.