These telephones do not require a source of electric power for calling and communication. Rotating handle will generate electric power to ring the other station, and accumulated power is used to amplify acoustic sounds to make conversation loud and clear. They are roughly classified into two systems; Direct Telephone and Selective Telephone. The latter allows maximum 12/24 telephones to configure (one conversation link). And they are possible to connect to Intrinsically Safe Types, shown in the layout drawing below. Each telephone has relay inside terminal so that optional devices such as bells can be equipped.