Julius Sumner Miller: Lesson 43 - Adventures in Electromagnetism

A - Faraday's Toroid: An iron ring - like a donut - Faraday used the ring of an anchor chain - has two coils of wire wound on it on opposite sides. The coils may have different turn-counts. These coils have no physical connection with each other - that is - they are not connected to each other. Now if we connect one coil to a seat of emf - a battery -say - and the other coil to a galvanometer - the second coil detects the magnetic field produced by the first one. Accordingly - in the second one we get an EMF of Induction. This is the principle of the transformer. The coil we energize we call the primary - the other is referred to as the secondary. B - Two coils are free of each other - one sliding in the other - and we energize the one with current from a battery. The other "feels" the magnetic influence and has induced in it an emf. This emf is enhanced by closer coupling which is accomplished by having the coils closer upon each other or by providing an iron core. C - An array of coils of different turn-count can be connected in series. The insertion of a bar magnet into a coil - one alone - or two in tandem - produces an emf. We can with this device discover that the emf is governed by the turn-count - by the polarity of the magnet -by the speed of the magnet into and out of the coil. The windings are such that some emfs add and others subtract. It is an interesting exercise to SEE what happens when certain coils are connected and then tell HOW they are wound and what their turn-count is. D - This principle of electromagnetic induction has astonishing consequences. If the primary coil is a few turns of heavy wire and the secondary many many turns of fine wire the induction can give rise to an emf across the secondary of many thousands of volts. A fat juicy heavy spark several inches long can be produced. And of the spark I like to ask: What is it that we SEE? What.is it that we HEAR? Why is the spark of some color - blue say or red or orange? All enchanting inquiries for enquiring minds.