496 FARM MOTORS 



INDUCTION COILS AND TRANSFORMERS 



666. Self-induction. Self-induction is defined as the 

 cutting of a wire by the lines of force flowing through 

 the wire. When a current begins to flow through a wire 

 magnetic whirls spring outward from the wire and cut it. 

 This cutting of the wire with only its own magnetic lines 

 oi force induces an E.M.F. for an instant. But the 

 E.M.F. which it does induce has an opposite direction to 

 the E.M.F. which causes the current to flow. Hence the 

 E.M.F. will be retarded for an instant by its own induced 

 E.M.F. and will not flow until this is overcome. When the 

 current flowing through the wire is stopped the lines of 

 force again cut the wire but in an opposite direction, 

 hence this time they tend to retard the cessation of flow 

 of the current. The effects of self-induction are 

 rarely noticeable in a straight wire, but when the wire is 

 wound into coils in the form of a helix the magnetic 

 field of every turn cuts many adjacent turns and the 

 E.M.F. is greatly increased, being proportional to the 

 current, the number of turns and the magnetic lines 

 through the coil. If an iron core is placed within the 

 coil the effects of self-induction are very much greater. 

 By snapping the wires from a battery after passing 

 through such a coil as described above a brilliant spark 

 will be produced. This is the simple coil (Fig. 372) used 

 in make-and-break ignition on gasoline engines. 



667. Induction coil. If two coils entirely separate 

 from each other be wound around an iron core and con- 

 nected up as in Fig. 373 every time the current is started 

 in coil a there will be a deflection of the galvanometer 

 needle in b. If the current is broken in a the needle b 

 will again be deflected, but in an opposite direction. 

 From this it is seen that the magnetic lines of force which 

 surround the wire in coil a induce a current in the coil b. 



