Carefully synchronized with this is the set of contact points that is doing the make-and-break work that actually causes the spark, but it is doing so at a very precise time at each compression stroke. In a car that has either the usual Kettering distributor or a magneto the spark is generated by a 'centralized' high voltage source which is then distributed (hence the name 'distributor') to the appropriate spark plug. So, when your Ford is hard to start, runs roughly, won't idle and lacks power it is quite likely the fact that the coils are really struggling to function even though they may be buzzing quite happily. Unfortunately for us, the insulation that was used on the wire and in the capacitor was not very good 100 years ago, and age has deteriorated it further. When the Ford coil was factory-new it would have been constructed with the best quality materials that were then available. To fully explain the function of the capacitor is beyond the scope of this article, but suffice to say that it is exactly the same job that the capacitor in a Kerrering distributor does: increases the output voltage somewhat and reduces burning of the points. This oscillation is effectively an alternating current which is exactly what a transformer requires to do its job of stepping up the voltage. The answer to this was to simply use the primary coil to generate a strong magnetic field in the iron core to open electrical contacts which in turn interrupt the primary current, and thus the magnetic field collapses allowing the contacts to come together again and start the whole cycle anew resulting in an on-off-on-off oscillation. The complicated part of all this arises when one realizes that the power supplied by the car's battery is Direct Current which cannot pass through a transformer. The input coil (called the 'primary') has the lower number of turns and the output coil (called the 'secondary') has a much higher number of turns. In very basic terms, a transformer is two insulated windings of wire around a shared iron core. Your typical 'wall wart' transformer that is used for charging your cell phone or such is doing the opposite: dropping the house line voltage down to the very low voltage for th ephione's battery management system. There is a story that Thomas Edison possessed one when he was a teenager and enjoyed giving unsuspecting people rather nasty shocks because it is essentially a transformer that 'steps up' the low voltage of a battery (six volts or so) to the very high voltage required by the spark plug (around 12,000 volts or more). The Ford coil is a very close relative of the classic induction or Ruhmkorff coil, and as such has its origins in the early 1800s. This has the advantage that if the engine is really worn and has little compression, there is a good chance that it will still run.ĭuring the years when the Model T Ford was very commonplace, the coil was known by many names (some not very polite when they were misbehaving) but the most common of these is 'buzz' or 'trembler' coils. The Ford 'T' coil, when selected by the timer, will give a continuous shower of sparks. the engine must be turning) for any spark to be generated. One other major difference is that both the Kettering distributor and high tension magneto need to be moving (I.e. This timer is vaguely similar to a more conventional distributor, except that it is operating at low voltage rather than at spark plug voltage as in a Kettering system (which is simply the much more familiar high tension distributor with points) or a high tension magneto. One of the many fascinating design aspects of the Model T Ford is its ignition system which consists of a separate, easily removed sparking coil for each cylinder and a 'timer' that is driven directly off the end of the camshaft and which selects the appropriate coil for the cylinder that is about to fire. Similar but not the Same The Model T Ford Coil and its Ignition System
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