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. I replace the solution of sal ammoniac by one of caustic potash or soda (12 to 15 per cent.), and the thin zinc rods by zincs with larger surfaces. In this manner, I obtain a powerful and odorless battery, having all the valuable qualities of the Leclanche, and one that permits of a renewal of the potash solution as well as of the negative electrode. The electromotive power of this element may be as high as 1.8 D. The same pyrolusite (binoxide of manganese) cylinder used with the same thin rod of zinc will precipitate 75 per cent. more copper from solution in an hour when caustic potash is used than when sal ammoniac is employed. But by replacing the thin zinc rod by a zinc cylinder of large surface, 2½ times as much copper is precipitated in the same time.