The spark of the matter

Flywheel magnetos can be a boon – but they also reduce sane adults to tears. While poor set-up and condition will reduce their efficiency, so too other factors including flywheel speed, limited coil and condenser size, heat and magnetism…

Words and photographs: RICHARD ROSENTHAL

Although an electrical engineer named Marcus was probably the first to build a low tension magneto, by 1898 German electrical engineering experts Simms-Bosch were among the leaders to hone the design into a workable system for the internal combustion engine.

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Initially built with a fixed H-shaped armature and rotating segments, Simms-Bosch soon reworked the concept into a unit which would still look familiar to us today, comprising a horseshoe-shaped magnet within which rotated an armature.

The low tension magneto is devoid of an ignition contact breaker set and its armature carries low tension windings only to generate low tension electrical current, and therefore cannot produce a spark in the engine’s combustion chamber with a conventional sparking plug. Instead, an igniter within the combustion chamber is employed.

Igniter design varies, but basically comprises an insulated plug having exposed contact with the tip of a movable lever pressed against it, which, with appropriate wiring, makes an electrical circuit with the low tension magneto.

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On rotation, the engine – often by means of a cam – mechanically flicks the igniter’s lever at the point of desired timed ignition to break the circuit and create a spark within the combustion chamber. And the quicker the speed of the circuit breaking, the greater the intensity of the spark.

A sight we all recognise instantly, the aluminium saucepan-shaped cover of a Villiers flywheel magneto. Slight twist here, as this is a French-assembled 1924 147cc Villiers engine, but the flywheel magneto with lighting coils is identical to its Wolverhampton-built counterpart. One assumes engine kits were initially sent out for assembly to circumnavigate then French legislation.

Low tension magnetos were a familiar fitment to stationary/industrial engines for some decades, but only enjoyed a brief life in the car and motorcycle world.

Singer employed low tension magnetos for most of their Perks and Birch-designed Motor Wheels. History informs the designers/patentees, Perks and Birch, built around 200 units.

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Most were sold to Singer, who then bought remaining stock, patents and manufacturing rights c1900 to continue production until 1903/4.

Read more and view more images in the August 2019 issue of TCM – on sale now!

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