This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
When a storage battery is recharged, the completion of the recharging is marked by the development of gas in the cell. This fact is the basis of a simple electrical device which will ring a bell when the battery is fully charged. A glass tube bent as shown and having a small bulb near its upper end is inserted in the top of the cell, a small quantity of mercury being first introduced in the bend below the bulb. Wires extend down the upper tubing to within a short distance of the mercury. These wires are connected with binding posts so mounted that they can rest on top of the tube, whereupon the bell circuit is completed as shown. Small quantities of gas may develop during the charging of the cell, but if a small hole is pierced in the tube between the mercury and the cell, this gas will not exert pressure enough on the mercury to push it up and connect it with the wires, which will take place first when the development of gas becomes very active at the completion of the recharging.
Ill: The Gas Generated by a Filled Storage Battery Will Make Electric Connections and Sound an Alarm