Electrolysis.  The means of protection against the danger due to chemical action, set forth in the preceding chapter, form such a distinct phase of the subject of guarding property against electrical hazards as to warrant treatment in a separate chapter devoted to the subject of electrolysis.

[Illustration:  MAIN EXCHANGE, CLEVELAND, OHIO.  Largest Four-Party Selective Ringing Switchboard in the World.  Kellogg Switchboard and Supply Co.]

CHAPTER XX

GENERAL FEATURES OF THE TELEPHONE EXCHANGE

Up to this point only those classes of telephone service which could be given between two or more stations on a single line have been considered.  Very soon after the practical conception of the telephone, came the conception of the telephone exchange; that is, the conception of centering a number of lines at a common point and there terminating them in apparatus to facilitate their interconnection, so that any subscriber on any line could talk with any subscriber on any other line.

The complete equipment of lines, telephone instruments, and switching facilities by which the telephone stations of the community are given telephone service is called a telephone exchange.

The building where a group of telephone lines center for interconnection is called a central office, and its telephonic equipment the central-office equipment.  The terms telephone office and telephone exchange are frequently confused.  Although a telephone office building may be properly referred to as a telephone exchange building, it is hardly proper to refer to the telephone office as a telephone exchange, as is frequently done.  In modern parlance the telephone exchange refers not only to the central office and its equipment but to the lines and instruments connected therewith as well; furthermore, a telephone exchange may embrace a number of telephone offices that are interconnected by means of so-called trunk lines for permitting the communication of subscribers whose lines terminate in one office with those subscribers whose lines terminate in any other office.

Since a given telephone exchange may contain one or more central offices, it is proper to distinguish between them by referring to an exchange which contains but a single central office as a single office exchange, and to an exchange which contains a plurality of central offices as a multi-office exchange.

In telephone exchange working, three classes of lines are dealt with—­subscribers’ lines, trunk lines, and toll lines.

Subscribers’ Lines.  The term subscriber is commonly applied to the patron of the telephone service.  His station is, therefore, referred to as a subscriber’s station, and the telephone equipment at any subscriber’s station is referred to as a subscriber’s station equipment.  Likewise, a line leading from a central office to one or more subscribers’ stations is called a subscriber’s line.  A subscriber’s line may, as has been shown in a previous chapter, be an individual line if it serves but one station, or a party line if it serves to connect more than one station with the central office.