The Safety Connection at the Lamp
When using a series connection of several lamps, the mentioned switching off doesn´t guarantee a comprehensive safety. It´s then advisable to use lamps with a safety connection (German Imperial Patent), which surely prevents a damaging of the lamp by a further burning. Fig. 3 shows this device. It works in a way that a circuit parallel to the carbons, which consists of a thin tin fuse, is switched on when the carbon holder touches a contact as soon as the carbons have been burned down to a large extent. When this circuit is connected the lamp´s voltage decreases down to zero that causes the lamp to go out immediately. Shortly afterward also the safety is burning through, which cuts off the shunt circuit.
It is recommended to supply all the lamp´s connections with these safety devices, because it offers a complete protection if the extinction appliance should fail for any reason. The safety device is only activated after the failure of the other system so that it just works as a reserve.