Fig. 19.—Induction coil of du Bois Reymond. "a", primary coil; "b", secondary coil; "c", bunch of wires in centre of primary coil for increasing intensity of induction currents; "d", binding screw for attachment of wire from galvanic element. The current passes up the pillar "d", along steel spring to "e", thence to the screw, the point of which touches the back of the spring at "e"; from "f" through wire of primary coil to "i", round the two pillars of soft iron "i", which it renders magnetic, and thus draws down the head of the spring "k"; this interrupts the current at "e", thus breaking the contact of the spring at the screw point. When the current is thus interrupted, the spring flies up by its elasticity and again establishes the circuit at "e". This interruption was originally invented by P. Wagner.