INDUCTION COIL. 597 INDUCTION COIL. iron wire, AA (Fig. 1), called the core; the 'pri- niarj- coil,' BBliB, 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, liner copper wire; and a device, D (Wagner's hammer, 183'J), called the rhcotonie or inter- rupter, which alternately makes and breaks the connection of an electric current through the jirimary coil. In dimensions thej' vary from a few inches in length and a fraction of a pound in weight to those of large dimensions used for wireless telegraphy described below. Tlie 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 primarj' coil, converting it and its core into a powerful electro-magnet. 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- nection between the spring S and the point P, and in- terrupting the flow of cur- rent in the primary. There- upon the electro-magnet AB loses its magnetism, and, ceasing to attract the iron I, the spring returns to ondarj- coil in the same direction as the primary current, and hence in the opposite direction to the current induced iu the secondary upon start- ing the current in the primary. The above cycle of operations repeats itself periodically, at a rate deix^uding upon the spring H. Many modifi- cations of the interrupter are used, some attached directlj' to the coil, as above described, some operating independently, but still electro-mag- Eetically, and yet others are driven by an inde- pendent motor and are purely mechanical. Fig. 2 illustrates a form due to Foucault. One cir- cuit runs from the binding post k' through the •a« Fig. 3. .. Core of iron wire; B poMts of secondary circuit G, t«nsion spring; " L, contact point. EARLY EITCHIE INDUCTION COIL. B', binding posts of primary circuit : E, standard carrying interrupter; H, contact screw ; J, contact spring ; K, C D, binding F, armature; binding post; Fig. 2. FOUCirLT IXTERRrPXER. 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 niercury cup B', interrupting the current, whereupon « 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 ji and m. Fig. 3 shows another form of interrupter attached to an early form of induc- tion coil. Fig. 4 shows the latest improvements — a Queen cnil. with independent mechanical interrup- 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 Teams- FORMEB.
