Ques. What direct current pressure is usually employed on traction lines?

Ans. 500 volts.

Ques. Mention another important service performed by a mixed system.

Ans. If the generator furnish alternating current it must be converted into direct current in order to charge storage batteries.

CHAPTER LVI
AUXILIARY APPARATUS

For the proper control of the alternating current in any of the numerous systems described in the previous chapter, various devices, which might be classed as "auxiliary apparatus," are required. These may be grouped into several divisions, according to the nature of the duty which they perform, as

1. Switching devices;