The commutator, e, is arranged to prevent sparking when the brushes leave a contact piece. This is done by splitting up the brushes into several parts and inserting resistances between the part which leaves the contact piece last and the rest of the circuit. This resistance checks the current ere the final rupture of contact takes place.
Figs. 6 and 7 will explain the structure of the commutator. Here a a a are the segments or contact pieces insulated from each other, and b' b b are the collecting brushes carried on a spindle, c c'. One of these brushes, b', is connected to the spindle, c, through an electrical resistance of plumbago, arranged as shown in Fig. 7, where d e are metal cylinders, d being in contact with the brush, b', while e is in contact with the spindle, c. The space, f, between these two cylinders, d e, is filled with a mixture of plumbago and lampblack of suitable resistance, confined at the ends by ivory disks. The brush, b', is adjusted by bending till it remains in contact with any segment of the commutator for a short time after the other brushes have left contact with that segment, and thus instead of sudden break of circuit and consequent sparking, a resistance is introduced, and contact is not broken until the current has been considerably reduced.
The contact segments are supported at both ends by solid insulating disks; but they are insulated from each other by the air spaces between them, where the brushes rub upon them.
The alternating current dynamo of Drs. Hopkinson and Muirhead differs little in general construction from that we have described; except that the commutator is very much simplified, and the armature bobbins are placed opposite each other on both sides of the rim. Instead of forming the coils into complete bobbins, Dr. Muirhead prefers to wind them in a zigzag form round the grooved iron rim after the manner shown in Fig. 8, which represents a plan and section of the alternating current armature. This arrangement is simpler in construction than the bobbin winding, and is less liable to generate self-induction current in the armature. Sir William Thomson has adopted a similar plan in one of his dynamos. In Fig. 8, a is the pulley fixed to the spindle of the machine, b b is the iron rim, and c c are the zigzag coils of copper ribbon. The field magnets are also wound in a similar manner.
It will be seen from our description that Drs. Hopkinson and Muirhead have scarcely had sufficient credit given them for this interesting machine, which so closely approximates to the Ferranti. One of their alternating dynamos has been built, and was shown at the Aquarium Exhibition. It works well, and is capable of supporting 300 Swan lights, while in size and appearance it resembles the Ferranti machine in a very striking manner. Drs. Muirhead and Hopkinson have also designed a magneto-electric alternating current machine; but as it closely resembles the machines described, with the exception that permanent magnets are employed as field magnets, we need not dwell upon it further.--Engineering.
AN IMPROVED MANGANESE BATTERY.
By GEORGE LEUCHS.
The Leclanche battery is distinguished for its simplicity, its small internal resistance (0.7 to 1.0 Siemens unit), and that all chemical action ceases when the current is broken, that it is not sensitive to external influence, and by the self-renewal of the negative electrodes. But on the opposite side the action is not very great (= 1.20 or 1.48 D.), and the zinc as well as the sal ammoniac are converted into products that cannot be utilized.