A New German-made Magneto.
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An Accessory Which Possesses Several Good Constructional Features That Should be of Interest to Users.
We had an opportunity, during the last week, of inspecting one of the " Mafan " magnetos manufactured by Hartmann and Braun, Ltd., Frankfort-on-Main, the sole concessionnaires for this accessory for Great Britain and Ireland being Messrs. Duncan Watson and Co., 62, Berners Street, W. This German maker has had over 40 years of experience in the construction of electrical instruments generally, and the Hartmann productions are used for testing purposes by the German and many other Governments.
No attempt has been made to depart from the general outlines of accepted magneto design, but when the construction is examined many novel and serviceable features will be found to have been embodied, Firstly, the pole pieces : the maker claims that the construction here employed, which consists of a covering piece riveted to the base plate, affords additional stability. On the finished machine, the pole ends are covered by plates, which make, to all intents and purposes, a water-proof armature case. Ii urther, this construction allows of greater accuracy in the machining and grinding, it being possible to allow a smaller air space, so obtammg a greater efficiency.
The safety spark gap is constructed of steatite, a material with a great antipathy to water, which prevents the lodging of any moisture which may get on to the machine. This spark gap is made integral with the high-tension carbon brush, and any access of current generated by the machine is safely discharged by its aid. The permanent magnets are not weakened in any way by screws or rivets, but are clamped in position by means of two strong and ingeniously-constructed plate clamps. At the other end of the magneto, several novel points are incorporated. The contact breaker is of a parallel type, and dispenses with the use of a spring trembler. The slide holding the contact surfaces reciprocates in a straight line in a well-insulated and mounted frame. This accessory is easily detachable, and it can, by the turning of a bush, quickly be adapted for service with either a right or left-handed magneto. Tests by the maker have shown that, at a speed of 6000 r.p.m. the interruptions are very regular. This has been demonstrated by photographic methods. The spark is tested at the works under a pressure of seven atmospheres.
An interesting departure will be found in the provision made for the
timing or checking of the cylinder ignition. Mounted on the hightension. spindle is the usual gear wheel, and, towards the outside edge of this, two holes are drilled, which can be brought into register with two further holes drilled in a frame cross-member. When timing the ignition of a four-cylinder engine it is necessary merely to bring two of the holes into register and to insert a pin. This sets the timing of the first cylinder, the second pair of holes giving the correct armature position for the third cylinder. Engines possessing any other riumber of cylinders are supplied with magnetos which have holes suitably drilled.
The lubrication arrangement deserves a word to itself. The old spring-cap oil cup, for so long employed on locomotives and steam tractors, is again brought into ser vice. The pushing back of the cap shuts off the ports to the feed pipes, and when a charge of oil is poured into the cup, the releasing of the cap opens the ports, and the lubricant is conducted by means of pipes to the ball bearings and other running parts of the machine, so ensuring that no bearing or wearing surface shall be neglected or . allowed to run dry.
The automatic-ignition apparatus for use when starting up the engine is an interesting characteristic. It is designed to avoid the necessity on the driver's part of manipulating the firing point. Two flexible chains enclosed in a case, and having springs between the different links and the inside diameter of the case, are employed in this construction. By this means the armature speed is retarded and is suddenly speeded up prior to the ignition period. A further interesting point made on behalf of this ignition apparatus is that the timing is automatically advanced or retarded as the engine speed is increased or reduced.
hot and effective spark is produced at all speeds, and it is further claimed that, the limit of angular adjustment, which is, on the usual magneto, from 30 to 40 degrees, is increased by this means to 75 degrees. The advantages of removing any opportunity of interfering with the ignition by the driver are apparent, and with this object in view, the constructors of the " Mafan " have so designed the automatic ignition adjuster that it can be fitted to the ordinary magneto of standard type which has coned shafts. No doubt many users will be inclined to experiment.