Fig. 75.—Gravity Cells. These consist of zinc and copper elements, immersed in a zinc-copper sulphate solution. They cannot be easily made, and are best purchased. The illustration also shows the star-shaped copper and "crowfoot" zinc element used in a gravity cell.
After the cells have once been "formed" all that they require is occasional recharging from gravity cells or from a dynamo, by connecting the positive pole of the charging current to the positive plates of the storage cells and the negative pole to the negative plates.
When the cells are fully charged, bubbles of gas will rise freely from the plates. If a dynamo is used it must be "shunt" wound and not a "series" machine. Recharging will only require about one-quarter of the time consumed in forming.
It is a very good plan to connect twelve gravity cells in series and use them to recharge the storage battery. The gravity cells can always be kept connected to the storage cells when the latter are not in use and thus remain fully charged and ready to supply their maximum current.
After the cells have been in use for some time, it is a good plan to lift out the plates and remove all sediment which has settled to the bottom of the jars.
A set of three such storage cells will have an E. M. F. of over six volts. Any number may be connected up in series in order to obtain a higher voltage.
Storage batteries are usually rated in "ampere hours." An ampere hour is the amount of current represented by one ampere flowing for one hour. A ten-ampere-hour storage battery will deliver:
One ampere for ten hours
Two amperes for five hours