August 1, 1SS8.] 



♦ KNOWLEDGE ♦ 



233 



being brought out through porcelain insulators. The 

 brackets are then bolted to the gun-metal supporting ring, 

 the coils being thus rigidly and securely held in position by 

 metal supports, which in no part come between the poles, 

 and are, in fact, almost entirely out of the magnetic held. 

 By this me;xns eddy currents are almost entirely avoided, 

 any loss from this cause being still further reduced by the 

 use of CJerman silver for the brackets and bolt^^ ; the high 

 resistance of this alloy preventing the generation of local 

 currents of more than a very small amount. The gun-metal 

 supporting ring, which is bolted to the bed-plate of the 

 machine, is in two portions, being divided in a vertical 

 diametrical Hue. These two parts, after having received the 

 coils, are bolted together and to the bed-plate, the tield 

 magnet being tirst placed in position. It will be seen that 

 this design provides ample fiicilities for repairs. It allows 

 not only of single coils of the armature being quickly and 

 easily removed and replaced, but also renders it easy to take 

 out one half or the whole of the armature. We need not 

 say that it is of the first importance in all elejtric lighting 

 work, especially for central stitions, that the machinery 



may be dispensed with, and the exciting coil as well as 

 the armature may be stationary; but it is preferable, for 

 mechaniail reasons, to attach the winding to the rotating 

 magnet. As will at once be seen, this form of field magnet 

 is very simple. A single exciting coil suthces for a machine 

 of any size, speed, or number of alternations. Besides its 

 pecuharity of form, it differs from the usual arrangement 

 in that it has poles of one sign only on each side of the 

 armature ; thus the magnetic leakage between adjacent 

 poles on each side is absolutely yiil, a condition widely 

 different from that of machines having the usual arrange- 

 ment of alternate polarities, in which the leakage is very 

 considerable. By revolving the field magnet instead of the 

 more delicate armature, safety and steadiness of running are 

 secured, the heavy magnet acting as an excellent flywheel, 

 and effectually neutralising any pulsations due to irre- 

 gularity in the stroke of the engine. This is a point of 

 some moment where slow speed engines are used. Further, 

 as the parts revolving at the highest velocity are simple 

 solid iron masses of the strongest description (not laminated), 

 not subject to heating, and having no copper wire on or 



Fig. 2.— Field Magnet 



should be planned, not only with a view to the prevention 

 of a breakdown, but also to" permit of any necessary repairs 

 being quickly carried out, and it will be seen that in this 

 armature both these ends have been very satisfactorily 

 attaine.d. 



The field magnet (fig. 2) cannot be said to bear any re- 

 semblance to any form of field magnet with which electric 

 engineers are familiar. It consists of a single electro-maguet 

 built up as follows :— A short cylinder of iron, through the 

 axis of which the shaft passes, forms the core of the mignet, 

 and is wound with the exciting coil. Against each end of 

 this cylinder Ls placed a cast-iron piece of peculiar form, 

 which will be best understood from fig. 2. Each casting 

 has a number of horns or arms — nine in the machine illus- 

 trated—which radiate from the shaft and central part of the 

 casting, and then bend over, forming nine pole pieces on 

 each side of the armature. These horns on one side, as will 

 be seen, approach within a very short distance of those on 

 the other side of the armature, and in this very narrow 

 polar gap or slit the armature is held, the entire field magnet 

 revolving with the shaft on which it is mounted. The ends 

 of the exciting coil are connected to " collector " riuK on 

 the shaft, which are shown to the right of the figure. These 



iloRDEV .Vlti:exatoe. 



near to expand and fly out, the electrical and mechanical 

 considerations which in ordinary dj'namos usually renders 

 low speed advisable do not here ajjply. The armature being 

 stationary, the coils have to be supported onlj' with a view 

 to resisting the tangential drag of the field. This renders 

 insulation a matter of comparative simplicity, and is of gi-eat 

 importance in high tension work, such as this machine is 

 primarily designed for. 



The Mordey alternator is shown in its complete form in 

 fig. .3. The field magnet is almost entirely hidden by sheet 

 metiil shields, which have for their object the prevention of 

 air disturbance by the horns, which otherwise would act as 

 the blades of a fan, and cause a perceptible waste of power. 

 The fanning action is, however, permitted to a suflicient 

 extent to ensure good ventilation and cool working. The 

 armature terminals, which are not seen in the figure, are at 

 the upper part of the supporting ring. The thrust bearing 

 is adjustable longitudirually foi the purpose of enabling the 

 field to be placed exactly symmetrical with regard to the 

 armature. 



Fig. i shows also the little Victoria dynamo used as an 

 exciter, and a small adjustable resistance, which is all that 

 is required for regulation. The alternator is, in fact, very 



