can supply the commercial electricity grid. A typical design is an axial-flux generator recycled from a car brake disk and hub bearing. A MacPherson strut provides the azimuth bearing to bring the turbine into the wind. The brake disk, with its attached rare-earth magnets, rotates to form the armature. A plywood disk carrying multiple axial coils is placed alongside this, with an iron armature ring behind it. In large sizes, from the 100 kW to MW range, the machines developed for modern wind turbines are termed permanent magnet synchronous generators. Bicycles One popular and common use of magnetos of today is for powering lights and USB powered devices on bicycles. Most commonly, a small magneto, termed a bottle dynamo, rubs against the tire of the bicycle and generates power as the wheel turns. More expensive and less common but more efficient is the hub dynamo that rotates neodymium magnets around a copper coil in a claw pole cage inside the hub of a wheel. Commonly referred to as dynamos, both devices are in fact magnetos, producing alternating current as opposed to the direct current produced by a true dynamo. Medical application The magneto also had a medical application for treatment of mental illness in the beginnings of electromedicine. In 1850, Duchenne de Boulogne, a French doctor, developed and manufactured a magneto with a variable outer voltage and frequency, through varying revolutions by hand or varying the inductance of the two coils, for clinical experiments in neurology. Ignition magnetos Magnetos adapted to produce impulses of high voltage for spark plugs are used in the ignition systems of spark-ignition piston engines. Magnetos are used in piston aircraft engines for their reliability and simplicity, often in pairs. Motor sport vehicles such as motorcycles and snowmobiles may use magnetos because they are lighter in weight than an ignition system relying on a battery. Small internal combustion engines used for lawn mowers, chain saws, portable pumps and similar applications use magnetos for economy and weight reduction. Magnetos are not used in highway motor vehicles that have a cranking battery, which may need more ignition timing control than a magneto system can provide, though sophisticated solid state controllers are becoming more common. Telephone Manual telephones for local battery station service in magneto exchanges were equipped with a hand-cranked magneto generator to produce an alternating voltage to alert the central office operator, or to ring the bells