The brushes, therefore, were not getting their proper difference of potential; and in part of the coils the currents were actually being forced against an opposing electromotire force. In a badly - arranged dynamo giving such a diagram as Fig. 76, a second pair of brushes, applied at the points, showing maximum and minimum potential, could draw a good current without interfering greatly with the current flowing through the existing brushes.

Fig. 74.

Fig 75.

Fig. 76.

Fig. 77.

Curves similar to those given can be obtained from the collectors of any dynamo of the first class - Gramme, Siemens, Edison, etc. - saving only from the Brush machine, which, having no such collector, gives diagrams of a different kind. It is not needful, in taking such diagrams, that the actual brushes of the machine should be in contact, or that there should be any circuit between them, though in such cases the field - magnets must bo separately excited. Also, the presence of brushes, drawing a current at any point of the collector, will alter the distribution of potential in the collector; and the manner and amount of such alteration will depend on the position of the brushes, and the resistance of the circuit between them.

## Brash Dynamo

Its armature - a ring in form, not entirely overwound with coils, but having projecting teeth between the coils like the Pacinotti ring - is unique. Though it resembles Pacinotti's ring, it differs more from the Pacinotti armature than that differs from those of Siemens, Gramme, Edison, burgin, etc.; for in all those, the successive sections are united in series all the way round, and constitute, in one sense, a continnous bobbin. But in the Brush armature is no such continuity. The coils are connected in pairs, each to that diametrically opposite it, and carefully isolated from those adjacent to them. ,For each pair of coils is a separate commutator, so that for the ordinary ring of 8 coils are 4 distinct curator each pair of coils. The brushes are arranged so as to touch at the same time the commot tors of 2 pairs of coils, but never of 2 adjacent pairs; the adjacent commutators being always connected to 2 pairs of coils that lie at rightangles to one another in the ring. The arrangement is given in Fig. 78.

Fig. 78.

In this figure, the 8 coils are numbered as 4 pairs, and each pair has its own commutator, to which pass the outer ends of the wire of each coil, the inner ends of the 2 coils being united across to each other (not shown). In the actual machine, each pair of coils, as it passts through the position of least action (i.e., when its plane is at right angles to the direction of the lines of force in the field, and when the number of lines of force passing through it is a maximum, and the rate of change of these lines of force a minimum) is cut out of connexion. This is accomplished by causing the 2 halves of the commutator to be separated from one another by about 1 of the circumference at each side, In the figure, the coils marked 1,1, are "cut out." Neither of the 2 halves of the commutator touches the brushes. In this position, however, the coils 3, at right angles to 1, are in the position of best action, and the current powerfully induced in them flows out of the brush a (which is therefore the negative brush) into a'. This brush is connected across to brush b, where the current re-enters the armature.

Now, the coils 2 have just left the position of beat action, and the coils 4 are beginning to approach that position.

Through both these pairs of coils, therefore, a partial induction will be going on. Accordingly, it is arranged that the current on passing into B, splits, part going through coils 2 and part through 4, and re - uniting at the brush B, whence the current flows round the coils of the field magnets to excite them, and then round the external circuit, and back to the brush A. (In some machines it is arranged that the current shall go round the field - magnets after leaving brush 1', and before entering brush B; in which case the action of the machine is sometimes, though, not correctly, described as causing its coils, as they rotate, to feed the field - magnets and the external circuit alternately.) The rotation of the armature will then bring coils 2 into the position of least action, when they will be cut out, and the same action is renewed with only a slight change in the order of operation. The following table summarizes the successive order of connexions during a half - revolution: -

First position. (Coils 1 cut out.). A - 3 - A'; B < 42 > B';

Field - magnets - External circuit - A.

Second position. (Coils 2 cut out.) . A < 13 > A'; B - 4 - B';

Field - magnets - External circuit - A.

Third position. (Coils 3 cut out.) A - l - A';. B < 24 > B'i.

Field - magnets - External circuit - A. .

Fourth position. (Coils 4 cut out.)

- A < 31 - > A'; B - 2 - B;

Field - magnets - External circuit - A.

Whichever pair of coils is in the position of best action is delivering its current direct into the circuit; whilst the 2 pairs of coils which occupy the secondary positions are always joined in parallel, the same pair of brushes touching the respective commutators of both.

One consequence of the peculiar arrangement thus adopted is, that measuring the potentials round one of the commutators with a voltmeter, gives a wholly different result from that obtained with other machines. For J of the circumference on either side of the positive brush, there is no sensible difference of potential. Then comes a region in which the potential appears to fall off; but the falling - off is here partly due to the shorter time during which the adjustable brush connected with the voltmeter and the fixed positive brush are both in contact with the same part of the commutator. Farther on is a region in which the voltmeter gives no indications corresponding to the cut - out position; and again, on each side of the negative brush, is a region where the polarity is the same as that of the negative brush. Fig. 79 is a diagram of a 6 - light Brush taken at one commutator, the main + brush being, however, allowed to rest (as in its usual position), in contact with both this commutator and the adjacent one.

From the foregoing considerations, it is clear that the 4 pairs of coils of the Brush machine really constitute 4 separate machines, each delivering alternate currents to a commutator, which commutes them to intermittent uni - direc - tional currents in the brushes; and that these independent machines are ingeniously united in pairs by the device of letting one pair of brushes press against the commutators of 2 pairs of coils. Further, that these . paired machines are then. connected in series by bringing a connection round from brush A to brush b.

## Dynamos Of Second Class

In these, coils are carried round to different parts of a magnetic field, whose intensity differs in different regions; or one, in different parts of which the lines of force run in opposite directions. In the early machine of Pixii, a single pair of coils was mounted so as to pass in this fashion through parts of the field where the magnetic induction was oppositely directed. Such a machine gives alternate currents, unless a commutator be affixed to the rotating axis. Niaudet's dynamo, which may be regarded as a compound Pixii machine, having the separate anmature coils united as those of Gramme and Siemens into one continuous circuit, is furnished with a radial collector mentioned above. In the Wallace - Farmer dynamo is a pair of poles at the top arranged so that the N, faces the S. pole, and another pair at the bottom where the S. faces the N. pole. The coils are carried round, their axis being always parallel to the axis of rotation upon a disk; there being 2 sets of coil. on opposite faces of 2 iron disks set back to back. They are united precisely as in Niaudet's dynamo, and each disk has its own collector. Each bar of the collector is, moreover, connected, as in the Pacinotti, Gramme, Siemens, etc, with the end of one coil and the beginning of the next.

In fact, the Wallace - Farmer is merely a double Niandet with cylindrical collectors. There is a serious objection to the employment of solid iron disks such as these: in a very short time they grow hot from the eddying Foucault currents engendered in them as they rotate. This waste reduces the efficiency. In the Hopkinson and Muirbead dynamo, the disk - armature takes a more reasonable shape. Instead of a solid iron disk to support the coils, is a disk built up of a thin iron strip wound spirally round.a wooden centre. The coils, of approximately quadrangularshape and flat form, are wound upon the sides of this compound disk. The Ball dynamo (so - called " Arago - disk ") is similar in many respects, but has no iron cores to the armature coils.

Fig. 79.