Some difficulty was experienced in securing electrical apparatus suited to the searching exigencies of lighthouse engineering, and the designers made one stipulation, which at first appeared to baffle fulfilment. This was the placing of the positive carbon below, instead of above, so as to enable the strongest light to be thrown upwards, to be dealt with by the upper part of the dioptric apparatus, whereby it could be used more effectively. One firm struggled with this problem for many months, and then was compelled to admit defeat, as time for further experimenting was unavailable, since the lighthouse was almost completed. Accordingly, the designing engineers had to revise their plans, and had to acquire alternate-current De Meriten machines, which, although more expensive and less powerful than those originally intended, yet were, and are still, wonderfully steady in working, while they had previously proved highly efficient for lighthouse service. Two generators of this description were secured, and they constituted the largest that had been made up to this period, each plant weighing about 4½ tons. Each machine has sixty permanent magnets, disposed in five sets of twelve each, while each magnet is made up of eight steel plates. The armature makes 600 revolutions per minute, and develops an average current of 220 ampères. The installation is so designed that one-, two-, three-, or four-fifths, or the whole, of the current can be sent from each unit to the distributor for transmission to the lantern, or the two machines may be coupled and the full current from both utilized. The current is conveyed to the lantern through copper rods 1 inch in diameter, and this was the first occasion on which such conductors were utilized for lighthouse work. There are three lamps of a modified Serrin-Berjot type, one being in service, and the other two held in reserve. By means of a by-pass, or shunt, a large percentage of the current is sent direct to the lower carbon, only a sufficient amount to regulate the carbons being sent through the lamp. The carbons used are about 1½ inches in diameter, though two-inch carbons can be employed when both machines are running, and the rate of consumption is 1¼ inches, or, including waste, 2 inches, per hour. The power of the arc thus obtained with the current fed from one generator is between 12,000 and 16,000 candles. In the event of the electric installation breaking down, a three-wick paraffin oil lamp is kept in reserve, ready for instant service, and it can be brought into use within three minutes. [Illustration: _By permission of Messrs. Siemens Bros. & Co., Ltd._ THE ELECTRIC SEARCHLIGHTS OF THE HELIGOLAND LIGHTHOUSE. On the lower level are three projectors spaced 120 degrees apart. Above is a fourth searchlight revolving three times as rapidly as those below.]