The specimen presented here is rather small: it is 14.5 cm long and has a diameter of 7.75 cm, while the large models measure up to 65 cm in length and 25 cm in diameter. Depending on the model, the primary coil is powered by one or more piles and the connection takes place by means of a special commutator. But the current also passes through the contact between the point of an adjustable metal screw and an flexible copper reed which, having one end fixed, acts as a spring.
The free end of the reed has a hammer of soft iron which, in the rest position, almost lies on the core.