UNIYOLAR DYNAMOS.—These machines are constructed so that the conductor in which the currents are generated (armature) effects a continuous increase in the number of magnetic lines cut, by arranging one part of the con ductor to slide on or around the magnet. Sturgeon's wheel and Faraday's disk are types of these machines. Messrs. Siemens & IIalske have constructed a unipolar dynamo for electro-deposition (Fig. 81). In this re markable dynamo there are two cylinders of copper. both slit longitudinally to obviate eddy-currents, each of which rotates round one pole of a U-shaped electromagnet. A second eleetro-magnet, placed between the rotating cylinders, has protruding pole-pieces of arching form, which embrace the cylinders above .and below. Each cylinder, therefore, rotates between an internal and an external pole of opposite polarity, and consequently cuts the lines of force continuously by slid ing upon the internal pole. The currents from this machine are of very great strength, but of only a few volts of electromotive force. To keep down the resistance, many collect ing-brushes press on each end of each cylin der. This dynamo has been used at Oker for electroplating.
he Forbes Dynamo has also attracted considerable attention, on account of its enormous current output for a given weight. Originally Prof. Forbes began by employing an iron disk which rotated between two cheeks of opposite polarity, the current being drawn from its periphery. lle then doubled the parts. The next stage was to unite the two disks into one common cylinder• as shown at A in Fig. 82. Here the coils lying in their cases are shown in section. the dotted lines indicating the direction of the lines of magnetic force induced in the iron. These are practically closed on themselves, so that there is no external field at all. For this reason the inventor prefers to call this type of dynamo " non-polar." A rubbing contact, for which purpose Prof. Forbes at one time used carbon " brushes," and at another a number of springy strips of metal-foil, is maintained at the two extremities of the periphery. One of
the earlier forms of machine, with a single disk 1st in. in diameter, was stated to give 3,117 amperes at a potential of 5.8 volts when running at 1,500 revolutions per min. One of the later machines, in which the armature" is a cylinder of iron 0 in. hi diameter and S in. long• is designed to give, at 1,000 revolutions per min., a current of 10,000 amperes at a potential of 1 volt. The electromotive force of such machines increases as the square of the diameter. The theory of the unipolar disk machine has been given by Sir NV. Thomson, who has shown that such a machine is not self-exciting except above a certain critical speed, dependent on the resistance of the circuit: AurEaxATiso-CuaRENT MAcnixEs.—In general the methods of mechanical construction adopted in these machines do not differ materially from those of the continuous machine. In the alter nating inaelffne, however, the use of the CoMmu tator becomes superfluous, the current generated in the armature being led merely to a pair of rings attached to the shaft, and upon which the brushes hear. Special precautions, however, are necessary to avoid piercing of the insulation on the armittures of such machines. and for that purpose mica is now almost exclusively employed.
In these machines, also, thorough lamination is imperative.
According to Kapp, if we calculate out the E. M. F. of an alternate-current machine by applying to it the now well-known formulme for continuous-current dynamos. there will then be a certain numerical coefficient by which the E. M. P. thus found must be multiplied in order to obtain the actual mean alternating E. M. F. of the machine. The value of this co efficient, K. depends chiefly upon the relative width of the field-ma-gnet poles and space be tween, and also upon the amount of the surface of the armature which is covered with wire. The following table gives the value of K for different cases: According to the ordinary sine formula, the coefficient is K = 2'220, and this agrees fairly well with ease 5, which is the one most frequently met with in actual practice.