1918.] 
The N.Z. Journal of Science and Technology. 
339 
of about 50 volts potential located in the exchange building. The battery 
is charged by a 65-volt generator, run either by an electric motor attached 
to the city mains or by a petrol engine. 
Special Services. 
In any exchange equipment it is necessary to give several kinds of 
special service in addition to the regular service of connecting one sub¬ 
scriber’s line to another. For instance, in some districts more than one 
subscriber has his telephone connected to the same line, such an 
arrangement being called a “ party line.” For this service it is neces¬ 
sary to differentiate between the parties when a call is completed to 
the line. In automatic service this is accomplished at the final machine 
by means of a certain form of current being connected to ring the 
called subscriber’s bell, this form of current depending upon the number 
called for. 
Another class of service is that of private branch exchanges, consisting 
of several lines coming to one subscriber’s premises—for instance, to a 
large office or works or shop ; and in the event that one of these lines 
should be busy it is desirable that the connection should be completed 
over another one of the lines without the subscriber having to call another 
number. This is also taken care of in automatic systems at the final 
machine by the subscriber calling only one number, and in the event that 
the first line called for is found to be busy the final machine automatically 
tests over the whole group of lines leading to the subscriber’s premises- 
until one which is not in use is selected, and the connection is then com¬ 
pleted over this free line. Of course, in the event that all these lines should 
be engaged the “ busy ” tone before mentioned would be given to the calling 
subscriber. 
For toll service the subscriber calls a certain number, which connects 
him to one of a group of lines running to the toll operator, who then 
handles his call as in a manual equipment. This same operation is used 
in case the subscriber wishes to obtain information or make complaints, 
another number, of course, being used in making the call, and the con¬ 
nection is secured to the information and complaint operator. 
Automatic Telephone Area. 
In a small exchange area up to about two thousand lines it is customary 
to have all the switching equipment in one central office. In most larger 
areas this is not found to be economical, as such an arrangement would 
mean running a pair of wires from this one exchange to a great distance- 
in some cases for each subscriber, and the mileage of subscribers’ wires 
would be much greater than if two or more exchanges should be used. 
On the other hand, with more than one exchange wires are needed for 
trunks between the various exchanges ; also extra buildings are needed, 
and a slight extra maintenance expense is incurred on account of having 
the equipment decentralized. These extra charges have to be weighed 
against the saving in subscribers’ wire in determining the location and 
size of exchanges As typical cases : The Wellington area has been laid 
out to provide for exchanges located as follows when the automatic system 
