The Magartic in the construction of the armature it is desirable to reduce the length of the conductors as imich as possible and thus the electric resistance; in a similar manner it is advisable to reduce the length of the magnetic circuit. The magnetic circuit in a dynamo is matte up of two principal components, namely, the iron and the air-gap. The modern theory of construction of the dynamo is based largely on the recognition of this important fact : As the magnetic resistance of the air is over 'CO times greater than that of soft iron, it is of the highest importance to bring the iron of the field-magnets as close as possible to that of the armature-core. Constructors are evidently limited in this direction by the clearance between the revolving armature covered with conductors and the pole pieces. To reduce this clearance, sonic constructors place the conductors of the armature below the periphery of the armature, so that the latter can be run to within Sig in. of the pole piece. Another general method of reducing the magnetic resistance is by the employment of pole-pieces. The latter, frequently made of east iron, encircle a greater or less portion of the armature, and afford a proper path for the passage of the lines of force. ]n all cases the grain of the wrought iron employed should lie in the direction of the path of the lines of force, pass ing through it, and joints in the magnetic circuit should as far as possible be avoided.
Forms of design of field-magnets permits of great yarn° ions, and the machines of different constructors are characterized principally by the type or shape of field-magnet adopted in their machines. The principles of construction above enumerated may be taken as a general guide. While it has been pointed out that, theoretically, it is better to employ one magnetic circuit instead of two or more. modern constructors are largely employing multipolar machines both for continuous and alternating current work, the object being to reduce the speed of the machines. especially those of higher powers. Again, in many Cases the double circuit or consequent pole type of field-magnet is preferable from a mechani cal standpoint.
The accompanying engravings (Figs. 12, 13, and 14) show the principal forms of magnets employed at the present time (Thompson). No. 1 in these illustrations shows the form adopted by Wilde for use with the shuttle-wound armature of Siemens. Two slabs of iron are con nected at the top by a yoke, and arc bolted below to two massive pole-pieces. There are four joints in the magnetic circuit, in addition to the armature-gaps. and the yoke is insufficient. No. 2 shows the form adopted in the latest Edison dynamos (American pattern). The upright cores are stout cylinders. The yoke is of immense thickness, the pole-pieces are massive, lint their useless corners are cat away. There are as many joints as in Wilde's form, but such a circuit world possess a far higher magnetic connectivity than Wilde's, owing to the greater cross-section. One difficulty with such single circuit forms is, how to mount them upon a suitable bed-plate. If mounted on a bed-plate of iron, a considerable fraction of the magnet ism will be short-circuited away from the armature; hence, an intermediate bed-plate of zinc some inches deep is interposed. In the larger form (No. 10) used by Edison in his "steam dynamos" Old type) this difficulty is partially obviated by turning the magnets on one side. The favorite type of field-magnet, having a double magnetic circuit with closed poles, is represented in No. 3: it was introdneed by Gramme. It maybe looked upon as the combina tion of two such forms as No. 1, with common pole-pieces. Nos. 3 to 9 may be looked upon as modifications of a single fundamental idea. No. 4 gives the form used in the Brush dyna mo, the two magnetic circuits being separated by the ring-armature. The diagram will serve equally for many forms of flat-ring machine; but in most of these time poles at the two flanks of the ring are joined by a com mon hollow pole-piece. embracing a portion of the periphery of the ring. No. 5 shows the well-known form of Siemens, with arched ribs of wrought iron, having conse quent poles at the arch. The circuit is here of insufficient cross section. No. 6 depicts the form adopted by Weston ; and very similar forms have been used by Crompton, and by Paterson and Cooper. There is a better section here. No. 7 is a form used by Iliirgin and Crompton, and differs but slightly from the last. It has one advantage, that the number of joints in the circuit is reduced. No. 8 is a form used by Crompton, Kapp. and by Paterson and Cooper. No. 9 is the form adopted in the little Griscom mo tor. No. 18 is a further modifica-. tion due to Kapp. No. 19. which also has consequent poles, is used by McTighe, by Joel, and by I I op kinson (" Manchester " dynamo) (see below), by Clark, Muirhead & Co. (- Westminster" dynamo), by 0. E. Brown (0erlikon) (see be
low), by Blakey, Emmott & Co., and in some of Sprague's motors, but with slight differences in pro portions of the details. The main difference between No 19 and No. 6 lies in the position selected for placing the coils, No. 19 requiring two, No. (; four. No. 20, which is the design of Elwell and Parker, is a further modification of No. 3. In No. 3 (Gramme) it is usual to east the pole-pieces and end-plates, but to use wrought iron for the longitudinal cores. The requisite polar surface must be got by some means, and, when the core was made thin, the two courses open were either to fasten upon the core a massive pole-piece (Nos. 1, 3, 4, 6, 7, 19, 20), or else to arch the core No. 5 so that its lateral surface was available as a pole. Now, however, that it is known that massive cores are an advantage, the requisite polar surface can be obtained without adding any polar expansion or "piece," hut by merely shaping the core to the requi site form (No. 8). This must not be regarded as a mere thinning of the magnet ; for, though mere reduction of croYms-seetion at any part of the circuit would reduce. the magnetic conduc tivity, reduction of the thickness for the pnrpose of bringing the armature more closely into the circuit will have quite the opposite effect. Nos. 11 to IS illustrate forms of field-magnet having salient as distinguished from consequent poles. No.11 is the double Gramme machine designed by Deprez. Nos. 12 and 13 are two of the innumerable patterns due to himself. These are both of east iron : and it will be noticed that in No. In there arc no joints, it being east in one piece. No. 14 is the form used by Iloehhausell, and is practically identical with 21, save in the position of the axis of rotation. The iron flanks of No. 14, however, tend to produce a certain short-circuit ing of the magnetism by their proximity to the poles. No. 15. need by Van Depoele, is similar. No. 16 is the form used by Sylvanus Thompson in small motors, and is cast in one piece. The semicircular form adopted for the core was in tended to reduce the magnetic circuit to a minimum length. No. 17 illustrates the form used by Jiirgensen, having salient poles re-en forced by other electromagnets within the Nu. 21 shows in section the double tubular magnets of the Thomson-llonston dynamo. the spheriell alThatilre being placed. as in Nos. 12, 14, and 15. bet ween two sdient poles. There is a curious analogy between Nos. 21 and 19; but they differ entirely in the position of I he coils. No. 22 is a design by Kapp, in which there are I wo salient poles of similar polarity, and two eonsequent poles between them, one pair of coils suffieing to magnetize the Mode quadruple circuit. Almost identical forirm. have been employed by Kennedy (" iron-clad " dynamo), and by Lahmeyer and by Wenstrino. No. 23 (Pig. 13) is a type which, used long ago by Sawyer and IT I 10 n I in, 11:1A recently become a favorite one, having been revived almost simultaneously by G1111111110 type SIIIV•rit.111'"), by by Siemens (" 1 " type), by Cabella Technomasio "1, and lately by Paterson and Cooper. No. 2.1 is Brown's very massive form. No. 25 is a design by Kennedy, known as the " iron-chul " dynamo ; the iron cores are forged to shape. No. 26 is designed by Prof. George Forbes. The iron-work is in two halves ; the coils, which are entirely inclosed, are so placed as to magnetize the armature directly, one coil occupying all the available space between the field-magnet and the upper half of the armature, the other the similar space around the other half. No. 27 is the 4-pole form adopted by Elwell and Parker in some of their larger machines. No. 28 is a multipolar form used by Wilde, Gra-mme, and others, the poles which surround the ring being alternately of opposite sign. In No. 29, a modification of this design by Thury, for use with a drum-arma ture, the six inwardly directed poles are magnetized by coils wound upon the external hex agonal frame. No. 30 is a sketch of the latest form adopted by Siemens and Halske, wherein an external ring rotates outside a very compact and substantial 4-pole electromagnet (see below). A similar 6-pole machine has been designed by Ganz, of Buda-Pesth, and a 4-pole also by Fein. Another recent form of field-magnet is shown in No. 31. This, which is a single horseshoe with but one coil upon it, was designed by S. P. Thompson early in 1886: and a similar form was independently designed by Messrs. Goolden and Trotter about the same time. One-coil machines have also been recently designed by Schorch, of Darmstadt, and by R. Kennedy, of Glasgow, by Inunisch, and by J. G. Statter & Co. No. 32 represents also a machine requiring but one coil, and is of the iron-clad type. It was devised by MeTighe in 1882. and has been recently revived by Messrs. Stafford and Eaves. No. 33 represents the latest machine of Messrs. Fein, of Stuttgart, with inward-pointing poles.