Trembler coil

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A trembler coil, around 1915. The mechanism on the end is the "trembler" or interrupter. Vibrator ignition coil.png
A trembler coil, around 1915. The mechanism on the end is the "trembler" or interrupter.

A trembler coil, buzz coil or vibrator coil is a type of high-voltage ignition coil used in the ignition system of early automobiles, most notably the Benz Patent-Motorwagen and the Ford Model T. [2] Its distinguishing feature is a vibrating magnetically-activated contact called a trembler or interrupter , [3] [1] which breaks the primary current, generating multiple sparks during each cylinder's power stroke. Trembler coils were first used on the 1886 Benz automobile, and were used on the Model T until 1927. [4]



Ford Model T engine. The rectangular black box behind the engine contains the trembler coils Ford-T engine - Flickr - granada turnier.jpg
Ford Model T engine. The rectangular black box behind the engine contains the trembler coils

The trembler coil was a device called a Ruhmkorff or induction coil, widely used in the 19th century. [5] It combines two magnetic devices on the same iron-cored solenoid. The first is a transformer, used to transform low voltage electricity to a high voltage, suitable for an engine's spark plug. Two coils of wire are wound around an iron core. The primary winding carries the low voltage battery current, and the secondary winding generates the high voltage for the spark plug. Attached to the end of the coil is an interrupter or trembler, a magnetically operated switch, which repeatedly breaks the primary current to create flux changes in the transformer needed to produce high voltage.

The switch contacts are on a springy iron arm, which holds them closed. The arm is mounted near the iron core. When battery power is applied, the coil acts as an electromagnet; the magnetic field from the core pulls the springy iron arm, opening the switch contacts, interrupting the primary current. The magnetic field of the core is switched off, allowing the arm to spring back, closing the contacts again. Then the primary current is turned on again, and the magnetic field opens the contacts again. This cycle repeats many times per second, while power is applied to the coil. A similar mechanism is used in the electric bell.

As the circuit opens each time, the energy stored in the solenoid's magnetic field is released and by electromagnetic induction produces a pulse of high voltage in the secondary coil winding. This voltage is sufficient to fire a spark plug located in the engine's cylinder, igniting the petrol mixture. [2]

The difference between a trembler coil and a modern ignition coil is that in a modern coil the primary current is broken only once by the contact breaker for each cycle of the piston, creating a single spark which is precisely timed to ignite the fuel at the correct point in the cycle. In the trembler coil, in contrast, the vibrating interrupter contact breaks the current multiple times during each cycle, creating multiple pulses of high voltage and multiple sparks.

The trembler coil operates equally well from AC or DC electricity. In the Model T, a battery was used for starting, but once the engine started the supply was switched to the magneto. The Model T was unusual in being fitted with an AC alternator (a permanent magnet magneto) rather than a DC dynamo. In these early days, this was not rectified and so the alternator's output remained as AC. [2] This was entirely adequate to operate the ignition system, and after 1915 to power electric headlights, although it could not be used for battery charging. [6]


Another trembler coil Automotive vibrator spark coil.jpg
Another trembler coil

The trembler coil was an induction coil, developed during the 1880s from a device invented by Charles Grafton Page and independently by Nicholas Callan in 1836. It was widely used around the turn of the century to produce high voltage for spark-gap radio transmitters, x-ray machines, arc lights, and medical electrotherapy devices. It was simply adopted for use in automobiles.

A simpler device, the low tension coil, was already in use for stationary engines and spark-ignition gas engines. This was a simple iron-core inductor, used with a battery and a switch called a contact breaker. It had a single winding and so was not a transformer like the trembler coil. When the piston was at the right point, the contact breaker opened, breaking the battery current. Self-induction due to the collapsing magnetic field generated a high voltage pulse in the coil, which was applied to the spark plug. The drawback of the low tension coil was that the high ignition voltage was generated in the same circuit that the low voltage battery current flowed in.

Use for car ignition systems

In the Ford Model T, four trembler coils were used, one for each cylinder. This was a precursor of the modern use of individual plug-top coils, where each cylinder has its own coil, thus avoiding the need for a HT distributor. [7] They were packaged together in a single wooden box, potted with pitch for reliability and waterproofing. [6]

For correct operation, each cylinder must be fired in turn, and at the right time. Both of these tasks were conducted by the 'timer' or low-voltage distributor, a rotary switch. [2] In a four-stroke engine, the timer is driven at half-crankshaft speed, like the camshaft. The Timer switched the primary current to each coil in turn and also started the coils at an appropriate time in the cylinder's cycle. As was common for engines of this period, the ignition timing was also controlled by a manual advance and retard control. Starting a cold engine could require dexterous manipulation of the control to get the engine to start. Modern engines control such timing even more carefully; this is now automatic and not obvious to the driver.

A significant difference from modern ignitions is in the strength and number of sparks produced. A modern system produces one, large, spark at exactly the time commanded. The trembler coil systems cannot produce such a high energy spark, but it does produce a continuous stream of sparks for as long as the timer circuit is closed. Early engines, like the Model T, ran at slow speeds with large cylinders filled with weakly burning mixtures of low octane ratings. These were both less sensitive to the accuracy of ignition timing and their mixtures also benefited from having a sustained ignition source. [8]

The Model T was also available in versions tuned for kerosene or ethanol fuels. Trembler ignitions were particularly suitable for igniting these mixtures. Tremblers remained popular for kerosene and TVO tractor engines long after they were obsolete for petrol.


In time, the trembler coil system was seen as obsolete and was replaced, first by the ignition magneto and later by Kettering's battery ignition system, using battery, coil and contact breaker. These systems used a single ignition coil for the engine, supplying the spark to each cylinder in turn. A device called the distributor (previously used with magnetos) was used to switch the high-voltage current to each plug in turn. The high voltage distributor evolved from the timer and it too was a rotary switch driven at camshaft speed. The first high voltage distributors likewise used a wiping-contact, but as the current was at such a high voltage, these gave trouble with arcing and erosion of the contacts leading to a poor connection. It was realised that a jump-spark distributor would work equally well at high voltages and would be less susceptible to problems from erosion. [9]

Other uses

The wide availability of the Model T made their component parts equally widespread. Their trembler coil in particular became a popular component for electrical hobbyists and backyard tinkerers, and was one of the first factory-made electrical components to be available in such numbers. They were used as shocking coils, in either the pseudo-medical or the prank sense, [10] Model T coils were also used for some of the earliest home-made electric fences for livestock control. They were also popular with early amateur radio operators for building simple spark-gap transmitters for Morse code transmission, until the introduction of continuous-wave transmitters rendered them obsolete (and eventually banned by government agencies due to their broad-band transmissions). The Model T coils remained so popular for non-car use that they remained in production into the 1960s, years after the car itself.

Vibrator power supplies

A similar device is the vibrator power supply, used to power electronic valve radios from low-voltage batteries. Low voltage DC from the battery is chopped by a trembler circuit and this pulsed square wave used to drive a transformer, providing the 90V or so required by valves. As this application is more frequency sensitive, the vibrators were generally a separate unit, with a more stable frequency tuned reed, apart from the transformer. This transformer had output tappings for the 90V HT and also the 22V grid and 6V heater circuits required by valves. In many cases, the transformer was the same unit used for powering the radio set from domestic mains, using the same secondaries but with an additional primary winding at mains voltage.

Related Research Articles

Electromagnetic coil

An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil, spiral or helix. Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in devices such as electric motors, generators, inductors, electromagnets, transformers, and sensor coils. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely, an external time-varying magnetic field through the interior of the coil generates an EMF (voltage) in the conductor.

Ignition magneto electricity-producing device

An ignition magneto, or high-tension magneto, is a magneto that provides current for the ignition system of a spark-ignition engine, such as a petrol engine. It produces pulses of high voltage for the spark plugs. The older term tension means voltage.

Alternator Device converting mechanical into electrical energy

An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines.

Induction coil Type of electrical transformer

An induction coil or "spark coil" is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. To create the flux changes necessary to induce voltage in the secondary coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical contact called an interrupter. Invented in 1836 by Nicholas Callan, with additional research by Charles Grafton Page and others, the induction coil was the first type of transformer. It was widely used in x-ray machines, spark-gap radio transmitters, arc lighting and quack medical electrotherapy devices from the 1880s to the 1920s. Today its only common use is as the ignition coils in internal combustion engines and in physics education to demonstrate induction.

An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, rocket engines, etc. The widest application for spark ignition internal combustion engines is in petrol (gasoline) road vehicles such as cars and motorcycles.

Contact breaker

A contact breaker is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch is set so that a spark is produced at the right time to ignite the compressed air/fuel mixture in the cylinder of the engine. A mechanism may be provided to slightly adjust timing to allow for varying load on the engine. Since these contacts operate frequently, they are subject to wear, causing erratic ignition of the engine. More recent engines use electronic means to trigger the spark, which eliminated contact wear and allows computer control of ignition timing.


A distributor is an enclosed rotating shaft used in spark-ignition internal combustion engines that have mechanically timed ignition. The distributor's main function is to route secondary, or high voltage, current from the ignition coil to the spark plugs in the correct firing order, and for the correct amount of time. Except in magneto systems and many modern computer controlled engines that use crank angle/position sensors, the distributor also houses a mechanical or inductive breaker switch to open and close the ignition coil's primary circuit.

Ford Model T engine Motor vehicle engine

The Ford Model T used a 177 cu in (2.9 L) sidevalve, reverse-flow cylinder head inline 4-cylinder engine. It was primarily a gasoline engine. It produced 20 hp (14.9 kW) for a top speed of 45 mph (72 km/h). It was built in-unit with the Model T's novel transmission, sharing the same lubricating oil.

Ignition coil Automobile fuel ignition system component

An ignition coil is an induction coil in an automobile's ignition system that transforms the battery's voltage to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel. Some coils have an internal resistor, while others rely on a resistor wire or an external resistor to limit the current flowing into the coil from the car's 12-volt supply. The wire that goes from the ignition coil to the distributor and the high voltage wires that go from the distributor to each of the spark plugs are called spark plug wires or high tension leads. Originally, every ignition coil system required mechanical contact breaker points and a capacitor (condenser). More recent electronic ignition systems use a power transistor to provide pulses to the ignition coil. A modern passenger automobile may use one ignition coil for each engine cylinder, eliminating fault-prone spark plug cables and a distributor to route the high voltage pulses.

Capacitor discharge ignition

Capacitor discharge ignition (CDI) or thyristor ignition is a type of automotive electronic ignition system which is widely used in outboard motors, motorcycles, lawn mowers, chainsaws, small engines, turbine-powered aircraft, and some cars. It was originally developed to overcome the long charging times associated with high inductance coils used in inductive discharge ignition (IDI) systems, making the ignition system more suitable for high engine speeds. The capacitive-discharge ignition uses capacitor discharge current to the coil to fire the spark plugs.

Vibrator (electronic)

A vibrator is an electromechanical device that takes a DC electrical supply and converts it into pulses that can be fed into a transformer. It is similar in purpose to the solid-state power inverter.

Hit-and-miss engine

A hit-and-miss engine or Hit 'N' Miss is a type of stationary internal combustion engine that is controlled by a governor to only fire at a set speed. They are usually 4-stroke but 2-stroke versions were made. It was conceived in the late 19th century and produced by various companies from the 1890s through approximately the 1940s. The name comes from the speed control on these engines: they fire ("hit") only when operating at or below a set speed, and cycle without firing ("miss") when they exceed their set speed. This is as compared to the "throttle governed" method of speed control. The sound made when the engine is running without a load is a distinctive "Snort POP whoosh whoosh whoosh whoosh snort POP" as the engine fires and then coasts until the speed decreases and it fires again to maintain its average speed. The snorting is caused by the atmospheric intake valve used on many of these engines.


A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter.

Low tension coil

A low tension coil is an electrical device used to create a spark across the points of an ignitor on early 1900s gasoline engines, generally flywheel engines, hit and miss engines, and other engines of that era. In modern electronic terms, a low tension coil is simply a large inductor, an electrical device that stores energy for brief periods. The term "low tension" was the terminology of the day used to differentiate it from the term "high tension", and generally meant "low voltage" (tension) as opposed to "high voltage" (tension). High tension coils produce high voltages, generally meant to produce a spark across a spark plug.


An interrupter in electrical engineering is a device used to interrupt the flow of a steady direct current for the purpose of converting a steady current into a changing one. Frequently, the interrupter is used in conjunction with an inductor to produce increased voltages either by a back emf effect or through transformer action. The largest industrial use of the interrupter was in the induction coil, the first transformer, which was used to produce high voltage pulses in scientific experiments and to power arc lamps, spark gap radio transmitters, and the first X-ray tubes, around the turn of the 20th century. Its largest use was the contact breaker or "points" in the distributor of the ignition system of gasoline engines, which served to periodically interrupt the current to the ignition coil producing high voltage pulses which create sparks in the spark plugs. It is still used in this application.

High tension leads

High tension leads or high tension cables or spark plug wires or spark plug cables, colloquially referred to as HT leads, are the wires that connect a distributor, ignition coil, or magneto to each of the spark plugs in some types of internal combustion engine. "High tension lead" or "cable" is also used for any electrical cable carrying a high voltage in any context. Tension in this instance is a synonym for voltage. High tension leads, like many engine components, wear out over time. Each lead contains only one wire, as the current does not return through the same lead, but through the earthed/grounded engine which is connected to the opposite battery terminal. High tension may also be referred to as HT.

Inductive discharge ignition systems were developed in the 19th century as a means to ignite the air–fuel mixture in the combustion chamber of internal combustion engines. The first versions were low tension coils, then low-tension and in turn high-tension magnetos, which were offered as a more effective alternative to the older-design hot-tube ignitors that had been utilized earlier on hot tube engines. With the advent of small stationary engines; and with the development of the automobile, engine-driven tractors, and engine-driven trucks; first the magneto and later the distributor-type systems were utilized as part of an efficient and reliable engine ignition system on commercially available motorized equipment. These systems were in widespread use on all cars and trucks through the 1960s. Manufacturers such as Ford, General Motors, Chrysler, Citroen, Mercedes, John Deere, International Harvester, and many others incorporated them into their products. The inductive discharge system is still extensively used today.

The Delco ignition system, also known as the Kettering ignition system, points and condenser ignition or breaker point ignition, is a type of inductive discharge ignition system invented by Charles F. Kettering. It was first sold commercially on the 1912 Cadillac and was manufactured by Delco. Over time, this system was used extensively by all automobile and truck manufacturers on spark ignition, i.e., gasoline engines. Today the system is still widely used in coil-on-plug, coil-near-plug and in coil packs in distributorless ignitions. An alternative ignition system used in automobiles has been capacitor discharge ignition, which is primarily found now as aftermarket upgrade systems. Electronic ignition was a common term for Kettering inductive ignition with the points replaced with an electronic switch such as a transistor.

Magneto Electricity-producing machine

A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, although it is usually considered distinct from most other alternators, which use field coils rather than permanent magnets.

Alternator (automotive) Devices in automobiles to charge the battery and power the electrical system

An alternator is a type of electric generator used in modern automobiles to charge the battery and to power the electrical system when its engine is running.


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