Reverse Pressure Cells.—These consist of unformed lead plates immersed in the ordinary electrolyte of dilute sulphuric acid. As they have no active material, they possess no capacity, but are capable of setting up an opposing pressure of about 2 volts each to the discharging current flowing through them, thereby cutting down the total voltage of the battery, so that the net voltage across the line depends on the number of reverse current cells in series in the battery circuit. As the voltage of the battery falls during discharge, the reverse pressure cells are cut out, successively, thus keeping the external or line voltage constant.
Fig. 1,190.—Regulation with reverse pressure cells. These cells are merely lead plates placed in an electrolyte of dilute sulphuric acid. They have no capacity but set up an opposing or reverse voltage of approximately 2 volts per cell if current be passed through them. In using these cells for controlling discharge, the total number of active cells in the battery will be the same as if the method of end cell control had been used. Reverse pressure cells represent an increase in equipment of about 8 per cent. or more. These cells, as shown, are connected in the circuit in opposition to the main battery, and conductors are run from each of them to points on a switch similar to an end cell switch. At the beginning of discharge, all the reverse cells are in circuit, acting in opposition to the main battery. As discharge proceeds and the battery voltage falls, the reverse cells are gradually cut out of circuit. The only advantage in this method of regulation is that the discharge throughout the battery is uniform, but this fact alone does not warrant such means of regulation on account of the additional expense involved, and the energy loss when discharging against reverse cells is the same as if resistance had been placed in the circuit.
It is obvious, that as these cells do not possess any capacity, the number of active cells required in the battery will be the same as when end cell control is employed. Therefore, the reverse pressure cells represent an increase in equipment, which entails an additional expense of at least 8 per cent. For this reason, and also on account of the fact that the amount of energy lost in discharging against reverse pressure cells, is the same as when the resistance methods of controlling the discharge are employed, the use of cells for this purpose is now practically obsolete.
Fig. 1,191.—Holzer-Cabot dynamotor (type K). A dynamotor is a combination of dynamo and motor on the same shaft, one receiving current, usually of different voltage, the motor being employed to drive the dynamo with a pressure either higher or lower than that received at the motor terminals. A machine of the dynamotor form, with its windings exactly alike, is often used in three wire systems to balance or equalize the two halves of the circuit as in [fig. 798].
Boosters.—In general, a booster may be defined as a dynamo inserted in series in a circuit, to change its voltage. It may be driven by an electric motor, in which case it is sometimes called a motor-booster. The function of a booster is to add to an electric pressure derived from another source.
For instance, if a storage battery be used in conjunction with one or more dynamos to supply current to an electric light installation, the battery cannot be charged from the machines which are feeding the lamps, because it requires a pressure higher than that required for the lamps to complete the charge. A small dynamo is therefore connected in series, with the main machines and the battery, acting in conjunction with the former to provide the necessary pressure.