INDUCTION COIL. A form of electrical ap paratus used for transforming an interrupted current of low potential or pressure into one of high potential and alternating in direction. The fundamentid fact of electro-magnetic induction was first stated by Michael Faraday in n paper presented to the Royal Society on November •4, 1831, in which he derived the conclusion that any change in a magnetic field will induce an elee trie current in a conductor situated in it. This basic principle of electro-niagnetie induction underlies the construction and operation of the induction coil. As regards its construction, it consists of four essential parts: A bundle of soft iron wire, AA (Fig. 1), called the core; the 'pri mary coil,' BBBB, of insulated copper wire wound on the iron core, which is usually made of comparatively large wire and short in length; a much larger 'secondary coil,' CC, of longer, finer copper wire; and a device, D (Wagner's hammer, 1830), called the rheotome or inter rupter, which alternately makes and breaks the connection of an electric current through the primary coil. In dimensions they vary from a few inches in length and a traction of a pound in weight to those of large dimensions used for wireless telegraphy described below.
The operation of the apparatus is as follows: The small spring S, being in contact with the point P, allows the current from the battery E to flow through the primary coil, converting it and its core into a powerful The production of this powerful magnetic field in and around the secondary coil induces in it a mo mentary current of high potential, and opposite in direction to the primary. This secondary current is usually able to pass as a spark between the terminals of the secondary TT. When the core AA be comes magnetic it attracts the piece of soft iron I on the spring S and draws it up, thereby breaking the con neetion between the spring S and the point P. and in terrupting the flow of cur rent in the primary. There upon the electromagnet AB loses its magnetism, and, ceasing to attract the iron I, the spring returns to ondary coil in the same direction as the primary current, and hence in the opposite direction to the current induced in the secondary upon start ing the current. in the primary. The above cycle
of operations repeats itself periodically, at a rate depending upon the spring S. Many modifi cations of the interrupter are used. some attached directly to the coil, as above described, some operating independently, but still electro-mag netically, and yet others are driven by an inde pendent motor and are purely mechanical. Fig. 2 illnstrates a form due to Foucault. One cir cuit runs from the binding post k' through the its original position, restoring the contact be tween S and P and again starting the current in the primary. The disappearance of the magnetic field on breaking the primary circuit induces a momentary current of high potential in the sec magnet D, pulling down the armature a sup ported by the spring c. and lifting the wire B out of the mercury cup B', interrnpting the current, whereupon a springs up and B enters the mer cury, again starting the current. This device may be introduced into the primary circuit of an induction coil, or the latter may be connected to the posts F, H. K. so as to use the wire A and cup A' for interrupting the current for the in duction coil, while B' operates to keep the ap paratus in motion. A commutator consisting of a hard rubber cylinder with metal contact pieces is shown at I, with binding posts at n and in. Fig. 3 shows another form of interrupter attached to an early form of tion coil. Fig. 4 shows the latest improvements—a Queen coil. with independent mechanical ter, capable of giving a 46-inch spark.
In some cases an alternating cur rent is used in the primary without an interrup ter; then the induction coil becomes practically a, transformer, and the action is essentially the same as in apparatus of this class. See Ttu.ss