nt involves not only
the change of switchboards, but also the change of subscribers'
instruments that are otherwise good, and this consideration alone often,
in our opinion, justifies the replacing of an old magneto board with a
new magneto board, even if the exchange is of such size as to demand a
small multiple board.

Where the plant to be established is of such size as to leave doubt as
to whether a magneto or a common-battery switchboard should be employed,
the questions of availability of the proper kind of power for charging
the batteries, the proper kind of help for maintaining the batteries and
the more elaborate central-office equipment, the demands and previous
education of the public to be served, all are factors which must be
considered in reaching the decision.

It is not proper to say that anything like all exchanges having fewer
than 500 local lines, should be equipped with magneto service. Where all
the lines are short, where suitable power is available, and where a good
grade of attendants is available--as, for instance, in the case of
private telephone exchanges that serve some business establishment or
other institution located in one building or a group of buildings--the
common-battery system is to be recommended and is largely used, even
though it may have but a dozen or so subscribers' lines. It is for such
uses, and for use in those regular public-service exchange systems where
the conditions are such as to warrant the common-battery system, and yet
where the number of lines and the traffic are small enough to be handled
by such a small group of operators that any one of them may reach over
the entire face of the board, that the simple non-multiple
common-battery system finds its proper field of usefulness.

=Line Signals.= The principles and means by which the subscriber is
enabled to call the central-office operator in a common-battery system
have been referred to briefly in Chapter III. We will review these at
this point and also consider briefly the way in which the line signals
are associated with the connective devices in the subscribers' lines.

_Direct-Line Lamp._ The simplest possible way is to put the line signal
directly in the circuit of the line in series with the central-office
battery, and so to arrange the jack of the corresponding line that the
circuit through the line signal will be open when the operator inserts a
plug into that jack. This arrangement is shown in Fig. 307 wh