There are two types of electric tube, one with long wires for joining up with the electric circuit and the other without external wires. The first type has two insulated wires led into the interior and attached to two insulated brass cones which are connected by a wire "bridge" of platinum silver. This bridge is surrounded by a priming composition of guncotton dust and mealed powder and the remainder of the tube is filled with powder. When an electric current passes through, the bridge is heated to incandescence and ignites the priming composition.