amount of current in the field and local circuits is in proportion to the resistance in' in each. The hi-lashes have a fixed position. There is in every dynamo a pniut of maximum connunta lion on the armature which changes with the resistance on the Imp-line, moving with the natation (toward brush e) when the resistance decreases, and ha etc when it increases. This affects the current in the local and field circuits as follows: When lights are turned out, 1h11 line resistance is • mad the maximum point moves forward and forces motr current not of lamsh c and les: out of knish a. 'flats reduces ihecurrent in t he field-magnets and the E. M. 1'., and consequently the [Hover required to operate the dynamo. but t he current on the lamp-line remains constant because the local circuit is increased proportionally to the de crease of the field circuit. The remaining lamps, therefore, retain their full brilliancy, while the current can not increase and de stroy the apparatus. The regulator is illustrated dingrammatically in It is operated by the slide, which is controlled by a solenoid. Any tenden cy of the current to increase raises the contact, and the result is a decrease of resistance in the local circuit and an increase of resistance in the field cir cuit. More current will therefore flow through the local circuit and less through the field circuit. The gener ating capacity of the dynamo is in stantly reduced, and any tendency to produce a current above the standard is overcome. Should the tendency of the current be to decrease, say by a reduction of the speed of the armature, the slide lowers, increasing the resist ance in the local and reducing it in the field circuit. More current will then flow around the field-magnets and less out on the local circuit, The gener ating capacity of the dynaino will therefore increase to maintain the current at standard.
It was stated above that regulation could be effected in a dynamo by shunting the lines of force around the armature. This has been carried out in the Golden and Troffer dynamo (Fig. 50). The double-magnet type of dynamo is very suitable for this purpose, for one half of the machine may be constantly magnetized to the desired point of saturation, while the other half acts as a keeper. The former may be called the constantly mag netized limb, and the latter tlw keeper. They are similarly wound with exciting coils. If the con stantly magnetized limb is fully excited by the usual current through its coils, and no current is passed through those of the keeper. the bitter will act as a magnetic shunt, and nearly all the lines of have will pass round and round in it closed circuit of iron little or no polarity will be found at the pole-pieces, and there will be a minimum effect on the armature. lty allowing a weak current to pass round the keeper, the diversion of the lines of force through it will be partially obstructed, and the rest will take their usual path through the armature. With a certain strength of current this obstrnetion wilt be complete, no lines will pass in either direction, and the magnetic effect is the same as though the keeper had been entirely removed from the dynamo. The strength of the field and its action on the arma
ture is the same as_though the dynamo were provided with one magnet only. If now the current through the coils on the keeper be increased beyond its neutral point, it will assist the permanent magnet. the lines of force in the former will be re versed, and finally, when the whole current passes round it, the volts are at the maximum, and the dynamo works as an ordi nary series-wound machine. It was soon found in practice that. owing, to the complete magnetic circuit. formed by the pair of magnets, the changes in the strength of the field were some what sluggish. This was remedied by making a gap in the magnetic circuit, by boring out the yoke on the side where the magnet-bars passed through, and bushing them with brass. using brass washers also on the ends of the bars and under the nuts. A practical application has been made of this method for the purpose of running Bernstein incandescent lamps arranged in series. and a dynamo con trolled on this system by an automatic regulator enables the lamps to be extinguished by short circuiting.
The Edson Dynamo.—The early form of the Edison dynamo consisted of a drum-armature built up of laminated iron, and revolving in a field consisting of one or more long cylindrical electromagnets, according, to the output of the machine. The most powerful machines of this type were those constructed for the first Edison central stations in Pearl Street, New York, 1882, and Milan, Italy, illustrated in Fig. 51.
The armatures of these machines are 27.3 in. in diameter. and El in. long. The shaft is of steel, 71 in. in diameter, and of a total length of 10 ft. 3 in. Provision is made for an air blast to keep the armature cool. The armature is driven direct by a Porter-Allen steam-engine run ning at 350 revolutions per min., and having a piston-speed of 133 ft. per min. Machines of this type are still operated in the Edison station, Milan, Italy, but their use has been abandoned in New York, on account of the in creased efficiency of the improved type of Edison machine illus trated in Fig. 52.
In this machine the magnets are greatly shortened, and con sist of circular wrought - iron cores mounted on heavy cast iron pole-pieces, which surround the armature. The ma-chine rests upon a cast-iron bed-tdate, which carries the armat ore bearings. lint in order to avoid short-circuiting of the magnetic lines around the arnoatue, which would ensue if the pole-pieces rested directly in eonmet with the iron hot plate, the latter is separatN1 from them bv the zinc castings shown. The tnanetie yoke between the tops of the field-magnets consists of a massive Mock of m1-4,11011 iron, and npon it are mounted the contacts and switch for opening and closing the el-milt to t he machine.