The ring contacts are arranged in a line in such a manner that a circular contact-piece, made of thin sheets of copper, can be forced through them in turn by means of a central screw spindle. The mains for charge and discharge are attached to the fixed disc contacts on the central screws, and the regulating cells of the battery are coupled to their respective contact rings by sockets at the back of the board. The difficulty with the battery transformer system is the introduction of 400 to 500 volts into the houses, which would be necessary without the batteries are always fixed in sub-stations from which a low-pressure current, say of 100 volts E. M. F., could only be distributed. A method has been devised by Mr. Henry Edmunds to obviate this disadvantage. He also uses the high-tension current to charge the batteries, but by means of a distributor, which is automatically worked by the current, each group of cells is charged in turn, when it is entirely cut off from the supply main to the house, through which current is perhaps being taken for lighting purposes. The system is now being adopted by the Cadogan Electricity Supply Company, Chelsea. DIRECT-CURRENT TRANSFORMERS OR DYNAMOTORS. The method of transforming by direct current without the aid of batteries is not practically at work; but, as the advantages are so obvious and its development is only a question of time, a description of the system may not be considered out of place. The electrical exhibition at Philadelphia in 1884 contained a dynamotor which was exhibited by the Van de Poele Electric-Light Company, but, as far as could be ascertained, was not worked, and, as it was simply described as an induction machine for distributing currents for the use of incandescent and other lights, it attracted little attention. The advantages of an alternating current transformer system of distribution, Class II., has been put forward in these pages, especially that of simplicity and cheapness. An alternating current dynamo for a given output is cheaper than a direct, and it takes less labour to look after it, because it has no commutator. An alternating transformer is also an exceedingly simple piece of apparatus. If originally made with due care and kept in a dry place, it never breaks down, as it has no moving parts, and so there is nothing to go wrong. The alternating system of distribution has, however, some very serious disadvantages. In the first place, it is most important that motors should be driven during the day when the lights are not in use. In the second, batteries cannot be used in an alternating current system, so any immunity from breakdown that they might ensure is wanting; and steam must be kept up all day and all night.