In power plants serving large offices, the demand upon the storage battery is great enough to require large plate areas in each cell. The internal resistance, therefore, is small and considerable fluctuations may exist in the charging current without their being heard in the talking circuits. The amount of noise to be heard depends also on the type of charging generator. Increasing the number of armature coils and commutator segments increases the smoothness of the charging current. The shape of the generator pole pieces is also a factor in securing such smoothness.

If, with a given machine and storage battery, the talking circuits are disturbed by the charging current, relief may be obtained by inserting a large impedance in the charging circuit. This impedance requires to be of low resistance, because whatever heat is developed in it is lost energy. This means that the best conditions exist when the resistance is low and the inductance large. These conditions are satisfied by using in the impedance coil many turns of large wire and an ample iron core.

Dynamotors are not generally suitable for charging purposes. Not only is the difficulty in regulating their output a disadvantage, but the fact that the primary and secondary windings are so closely associated on the armature core makes them carry into the charging current, not only the commutator noises of the generator end, but of the motor end as well.

Mercury-Arc Rectifiers. In common-battery offices serving a few hundred lines, and where the commercial supply is alternating current, it is good practice to transform it into direct-battery charging current by means of a mercury-arc rectifier. It is a device broadly similar to the mercury-arc lamp produced by Peter Cooper Hewitt. It contains no moving parts and operates at high efficiency without introducing noises into the telephone lines. It requires little care and has good length of life.

Fig. 415. Mercury-Arc Rectifier Circuits
[View full size illustration.]

The circuit of a mercury-arc rectifier charging outfit is shown in Fig. 415. The mercury-arc rectifier proper consists of a glass bulb containing vacuum and a small amount of mercury. When its terminals are connected, as indicated—the two anodes across an alternating-current source and the cathode with a circuit that is to be supplied with direct current—this device has the peculiarity of action that current will flow alternately from the two anodes always to the cathode and never from it. The cathode, therefore, becomes a source of positive potential and, as such, is used in charging the storage battery through the series reactance coil and the compensating reactances, as indicated. The line transformer shown at the upper portion of Fig. 415, is the one for converting the high-potential alternating current to the comparatively low-potential current required for the action of the rectifier. The transformer below this has a one-to-one ratio, and is called the insulating transformer. Its purpose is to safeguard the telephone apparatus and circuits against abnormal potentials from the line, and also to prevent the ground, which is commonly placed on the neutral wire of transformers on commercial lighting circuits, from interfering with the ground that is commonly placed on the positive pole of the central-office battery.

Provision Against Breakdown. In order to provide against breakdown of service, a well-designed telephone power plant should have available more than one primary source of power and more than one charging unit and ringing unit.

Duplicate Primary Sources. In large cities where the commercial power service is highly developed and a breakdown of the generating station is practically impossible, it is customary to depend on that service alone. In order to insure against loss of power due to an accident to portions of the distributing system, it is the common custom to run two entirely separate power leads into the office, coming, if possible, from different parts of the system so that a breakdown on one section will not deprive the telephone exchange of primary power. In smaller places where the commercial service is not so reliable, it is usual to provide, in addition to the commercial electric-power service, an independent source of power in the form of a gas or steam engine. This may be run as a regular source, the commercial service being employed as an emergency or vice versâ, as economy may dictate. In providing a gas engine for driving charging dynamos, it is important to obtain one having as good regulation as possible, in order to obtain a charging current of practically constant voltage.

Duplicate Charging Machines. The storage batteries of telephone exchanges are usually provided of sufficient capacity to supply the direct-current needs of the office for twenty-four hours after a full charge has been given them. This in itself is a strong safeguard against breakdown. In addition to this the charging machines should be in duplicate, so that a burnt-out armature or other damage to one of the charging units will not disable the plant.