that the secondary should give a considerable range of pressure. This may be accomplished by means of an ordinary fixed ratio motor generator of which the secondary pressure is varied by means of a resistance inserted in the armature circuit on the primary side. The method is a wasteful one, and its use is chiefly confined to small batteries. A very suitable arrangement is a variable ratio motor generator in which the secondary armature revolves in a field having an auxiliary magnet, the pressure being varied by altering the field strength. When a very large amount of variation is necessary, a motor and dynamo coupled together mechanically are employed and the fields of both varied. The booster is so connected that the secondary armature is in series with the ordinary mains; it is, therefore, evident that only a small proportion of the energy put into the battery is transformed, so that, on the whole, there is not much loss by raising the pressure in this way. The difficulties met with in consequence of the unequal amount of work on the regulating cells have brought about a tendency to dispense with their use. In traction work, the commonest way of connecting up the battery is to join it simply across the circuit and to use a plain shunt-wound machine as a generator. When the load is normal, the battery will be charged; but on any sudden access of load, the pressure from the machine will fall slightly, and the cells will then discharge into the line. Some makes of cell are better suited to this purpose than others; the smaller the difference between the pressure they require to charge them and their discharging pressure the better. For lighting, the arrangement hardly gives sufficient regulation of pressure. It would probably be better to arrange to discharge the battery as a whole, and make up for its fall in pressure during working by means of a booster. Incidentally, the variation of the pressure by steps of two volts is objectionable, and if, as is often the case, several cells are placed between each pair of contacts of the regulating switch, the effect on the light is most unpleasant. In order to obtain a large variation in pressure by small steps, without having an undue number of contacts on the regulating switches and of connections to the battery, Messrs Verity & Co. have recently introduced a combination of two switches geared together, one of which alters the number of cells by steps of four, or other desired number, while the other varies the number one at a time. To examine the working, suppose twenty different pressures are required. One switch has five contacts, to which four groups, each of four cells, are connected, while the other has four contacts, to which three single cells are connected. The arrangement is shown diagrammatically in fig. 117. The switches are so geared that the five-contact one moves forward one step for each complete revolution of the other. The five-contact switch, which is actually circular, though shown developed in the diagram, has two distinct arms, insulated from one another, each with its own contact ring, but both passing over the same row of contacts. By this ingenious arrangement, the contact is always. broken while no current is passing, hence avoiding all trouble from the slow motion imparted by the gear. The four-contact switch is provided with the ordinary arrangement of resistance for bridging across two adjacent contacts, already explained. In the position shown in the figure, the last single cell is in circuit, and contact y is making connection to the second group of four cells. The next movement will cut out the single cell, leaving the four connected through x. Contact y then carries no current, and, as the motion continues, breaks connection with the first four cells, and next makes connection with the next y FIG. 117.-Special switches giving fine adjustment with small number of contacts. group of four. The continued motion then causes connection to be broken with the ring connected with x, and y again comes into use, the three cells on the four-contact switch being in series with the group. These are cut out one by one, and the whole cycle is then repeated. Another interesting battery switch invented by the late Dr John Hopkinson for automatically controlling the regulating cells may be referred to here. There are two varieties of the apparatus: one is employed for charging the battery, automatically cutting out the regulating cells as they become charged, the other for maintaining a constant pressure at any desired point. To maintain constant pressure, the following arrangements are adopted : The switch that actually varies the number of cells differs only in detail. from the ordinary type that is operated by hand, the resistance employed for bridging the contacts taking the form of lead plates in water and being connected between two concentric rings, one that on which the main arm bears, the other in connection with the subsidiary arm. The switch is actuated by a motor geared to it. This motor has its field excited from a single cell, usually the end one, the armature by four or five cells, the direction of the current through it being determined by the controlling mechanism. This comprises a relay and a 're-setting' drum. The relay consists of two magnets, each of which has an armature contained in a brass box; one armature rests normally on the bottom, being kept down by gravity; the other on the top, in virtue of the magnetic attraction. In their normal positions the circuits operated by the armatures are open. The windings of the relays, each with an extra resistance in series with it, are connected in parallel between the two points between which constant pressure is to be maintained, the extra resistance of one relay intervening between it and one point, that of the other between it and the other point. The resetting drum, of ebonite, carries contact-making surfaces which alter the connections of three brushes, so arranged that as the drum moves to one or other side of its normal position it either short-circuits the magnet of one relay or increases the current in that of the other. When the standard pressure exists, the relay armatures are in their normal positions. Suppose the pressure to fall, the armature that is normally held up drops and allows current to pass through the motor, at the same time releasing a brake disc controlling a train of wheels connected with the switch. The motor revolves in such a sense as to add cells to the circuit, thus raising the pressure. When the switch has moved on one contact the resetting drum increases the strength of the relay, as already explained, and breaks the motor circuit. A special device causes the brake to stop the motor at the exact instant that the switch arm is centrally situated on the contact. If the pressure be too high, the other relay acts and operates the motor in the opposite direction, so as to take cells out of circuit. It is reset by the drum, which of course moves in the opposite direction. The form of relay adopted gives very firm contact and the switch works well. The switch when applied to automatically cutting out regulating cells as they become charged is similar in general design, but necessarily differs somewhat in arrangement. The series of operations need not be described in detail. In designing a battery sub-station, it is as important to arrange that everything shall be fire-resisting as in every other kind of building in connection with central station work. Furthermore, great care must be taken to use only such materials as are not greatly affected by acids, because the amount of acid spray given off during charging is considerable. The floors and so much of the walls as is near enough to be splashed by the spray should be covered with asphalte. A thoroughly efficient system of ventilation should be provided, the plenum system being used in order to avoid the acid-laden air passing through the fans. A plentiful supply of water for swilling down the floors and for washing out the cells during repair is necessary, the drain pipes being laid with a view to their carrying off acid. Conductors should be of bare metal supported on porcelain, all metallic objects being painted with acid-resisting paint or enamel. Where bare conductors cannot be used, the cables should be lead covered throughout their length. Switchboards and boosters should be kept in a separate chamber from the cells. A large battery sub-station, designed by the Author in accordance with the principles just enunciated, will serve as an illustration of this class of work. The battery is placed about a mile away from the generating station, and is fed from the ordinary distributing network, a five-wire one, into which it also discharges. The method of charging adopted is by means of a separate motor mechanically coupled to a dynamo. Regulating cells are used to vary the pressure on the mains during discharge. Three intermediate points in the battery are connected to the mains. The battery consists of 224 cells, each having a capacity of 1500 ampere hours and a maximum discharge rate of 600 amperes. These are divided into four groups of forty-four cells each, forming four main sub-batteries, A, B, C, and D, and four groups of twelve each, forming the regulating cells for A, B, C, and D respectively. The four sub-batteries, each of which has its own regulating cells, are always connected in series, but they can be arranged in any order, so that any one can be discharged into any one of the four pairs of mains forming the five-wire network. The motor-driven generator will give any pressure up to 180 volts, and is connected in series with the 400 volts pressure given by the mains, giving 580 volts together for charging the whole of the cells in series. If one of the sub-batteries require considerably more charging than the remainder, it can be charged by itself, the whole of the current being then transformed by the booster. The switchboard is of the open skeleton type advocated in Chapter XVII. It is shown diagrammatically in fig. 118, and a perspective view is given in fig. 119. There are seven vertical bars, four being connected through single pole switches to four of the conductors of the five-wire network, the next two through a two-way switch by means of which either one or the other, but not both simultaneously, can be connected to the remaining conductor of the network. One of these two bars, the sixth from the end, is connected to one pole of the generator side of the booster, and the seventh bar to its other pole, a double pole switch being inserted between the machines and the bars. There are eight horizontal bars crossing these vertical bars, forming four pairs. One bar of each pair is connected to one pole of a sub-battery; the other is prolonged to form the regulating switch for the same sub-battery. The regulating switches consist each of a flat bar; parallel with this is a series of twenty-five contacts, the end ones being broad, the remainder alternately narrow and broad. The broad contact nearest the main board is connected to the end of the sub-battery and to the first regulating cell, the remaining broad ones to the junctions between successive regulating cells. The narrow contacts are connected through a resistance, each to the next broad |