The most common forms of primary cells in use are Leclanche's wet cell and the dry cell. Unlike the primary cell, the secondary cell can be used again after it has discharged all its electrical energy. There are two types of secondary cell, namely the lead-acid and the alkaline cell. Repeated charging and discharging of a battery under manufacture causes the plates to be covered in lead compounds, the negative plate with spongy lead and the positive plate with lead peroxide, these being important to the chemical action of the battery. Cells and batteries are charged by connecting them to a controlled d.c. source. This source may be obtained in several ways: rectified a.c.; motor generator set; rotary converter; and d.c. mains supply. Cells, batteries and their associated charging equipment are frequently used in installation work such as indicator and call systems in hospitals and hotels, fire-alarm and burglar-alarm systems, and emergency lighting installations.