July 13, 1883.] 



♦ KNOWLEDGE ♦ 



25 



current of the highest efficiency by virtue of the electrical 

 resistance offered by them. 



In recently-constructed machines, an insulating block 

 similar to C is fixed on the opposite side of the commutator 

 ring. This addition does not increase the duration of the 

 disconnection, which, as each block, C, measuring one-eighth 

 of the ring's circumference, passes under the brushes twice 

 in each revolution, amounts in the aggregate to 25 per 

 cent, of the time the machine is running. The object 

 attained by the adoption of two insulating blocks is to pre- 

 vent a " static " charge in the coils, and so to materially 

 reduce the sparking, which in the Brush machine is mainly 

 due to static effects. Reverting to Fig. 6, coils 1, 1 pass 

 next to the positions 4, i, one going through the north 

 field and the other through the south field. The coils 



for the current to split at B, part going through 2, 2, and 

 part through 4, 4, reuniting at the brush B'. Thence 

 the current passes through the field magnets in the 

 direction indicated by the arrows, and, exciting them, 

 emerges finally at +, which becomes the positive pole 

 of the machine, the external current being con- 

 nected to — and -I-. In Fig. 4 (Kxowledge, Xo. 81) 

 the cvirrent is shown to enter the field magnets 

 after leaving the brush A' and before entering the brush B, 

 in which case B' becomes the positive pole. It is clear 

 that " the four pairs of coils constitute in reality four 

 separate machines, each delivering alternate currents to a 

 commutator, which commutes them to intermittent uni- 

 directional currents in the brushes ; and that these inde- 

 pendent machines are ingeniously united in pairs by the 



thus approach the zone of greatest action, 3, 3, simul- 

 taneously, but in opposite phases. Similarly, as they pass 

 the position 2, 2, they both recede from the maximum and 

 approadi the minimum zone simultaneously and in opposite 

 phases. 



Regarding the armature at a moment when the various 

 coils are in the position indicated in Fig. G, the coils 3, 3 

 arc in the region of maximum action, one coil being con- 

 nected to the brush A, which becomes the negative pole of 

 the machine, the other coil at the same time communi- 

 cating its positive charge to the brush A', and thence by 

 wire to the brush B. The coils 2, 2 have left the position 

 of best action, and the coils 4, 4 are at the same time 

 approaching that position. The induction in each case is 

 only partial, and, accordingly, an arrangement is made 



device of letting one pair of brushes press against the 

 commutators of two pairs of coils. Further, that these 

 paired machines are then connected in series by bringing a 

 connection round from brush A' to brush B." The 

 Brush dynamo thus involves some of the prettiest 

 ideas hitlierto conceived ; and multitudinous as has 

 been the number of recently-born rivals, it remains the 

 best where a large number of lamps are required to be 

 worked in series. With the 40-light machine the objection 

 to an electro-moti\e force of 2,000 volts in the conductors 

 is, of course, a great one. !>o, also, is the objection to 

 enormous gas-mains, as numy living in tlie region of 

 Tottenham Court-road a year or two since have cause to 

 remember. Wliere, therefore, such immense currents are 

 employed, it is imperative that the engineering should be 



