Improvements in Direct Call Telephone Systems.
Year: 1898

page_text: Improvements in Direct Call Telephone Systems.
I, HERMANN OPPENHEIMER, Electrical Engineer, of 55, Redcross Street,
Barbican, in the City of London, do hereby declare the nature of this invention
to be as follows:-
The use of line selectors for telephonic intercommunication has the one dis-
advantage, that while intercommunication can be had without the means of a
central switchboard, any of the side stations may overhear a conversation that
is going on between 2 other stations. In a good many instances, more especially
in large warehouses, factories, etc., it is sometimes found essential that conversa-
tion should be carried on in absolute secrecy, and for this purpose I have con-
structed a line selector, which fulfils these requirements without great complica-
tions.
To this end, I employ small springjacks having 2 contact springs, the con-
nection of which is arranged in series. The line selector which I show diagram-
matically consists of a jack b, and a spring f which latter is in metallic connection
with the former when at rest, or it may be formed out of 2 metal pieces, one
part of which is brought into contact with the conducting cord of the plug
when the latter is inserted, while the other part is insulated therefrom. The
lines are so arranged that each is brought at n into a permanent connection
with the terminal E of the corresponding telephone apparatus, and the spring-
jack of the line is so connected that that part of the springjack which ought
to come into contact with the cord of the plug, i.e. the. jack b, is directed
towards n.
The spring f however is connected towards the opposite direction,
and the last jack is connected with the common return K, which at the same
time is in connection with one pole of the calling battery K Z
The zinc pole of the battery is, as generally done with line selector installa-
tions, carried right through the whole system, and branched off at each station
to the terminal marked W Z. This arrangement necessitates that the line of
the intermediate stations must be carried back partly throughout the whole
system in order to obtain the connecting in series, as will be seen in diagram
at stations II and III where the line 1.2" or 1.3" is carried back from the ter-
minus of the whole installation.
The main line 1.2 passes in such case by the corresponding apparatus or as
shown in II without having any connections. This system of line selectors
allows installations with earth return or with metallic return, but when a single
line (earth return) is used, a third station is still able to overhear conversation
going on between 2 other side stations; whereas when metallic circuit is used,
overhearing is absolutely impossible.
If, for instance, station IV puts the plug into springjack I, in order to
establish a connection with apparatus I, the current comes from station IV
through springjacks III, and II, to the point I" and the return plug S¹, to the

page_text: common return K with which station IV is connected by means of its own
terminal marked E, and through the line 1.4 at station I.
If station No. 2 wishes to overhear, this can be done like an ordinary line
selector system. In order however to prevent this intrusion on the part of a
third station, it is necessary that the called-up station (in the present case
No. I) should withdraw the plug S from the jack which is connected to the
common return K. This withdrawal causes the home station to be disconnected
from the common return K, and when such withdrawn plug is inserted into
springjack No. 4 at station I, the line 1.4 and with it. the station IV, is also
disconnected from the common return K.
If now any of the side stations wish to overhear the conversation going on,
by tapping the plug holes, say for instance station II through tapping the plug
holes I or IV, it will be of no avail because the 2 corresponding stations have
no connection whatever with the common return K, from whence alone the inter-
fering station can get current.
If the interfering station should go a step further, and should try to insert
the plug into springjack I or IV, the connection between I and IV would thereby
be interrupted, and the result would be that conversation between the 2
stations would be stopped at once, and any attempt to overhear on the part of
a third station would be frustrated.
$
Dated this Nineteenth day of April 1898.
Improvements in Direct Call Telephone Systems.
I, HERMANN OPPENHEIMER, Electrical Engineer, of 55, Redcross Street,
Barbican, in the City of London, do hereby declare the nature of this invention
and in what manner the same is to be performed, to be particularly described
and ascertained in and by the following statement:-
The use of line selectors for telephonic intercommunication has the one dis-
advantage that while intercommunication can be had without the means of a
central switchboard, any of the side stations may overhear a conversation that
is going on between 2 other stations. In a good many instances, more especially
in large warehouses, factories, etc., it is sometimes found essential that conversa-
tion should be carried on in absolute secrecy, and for this purpose I have con-
structed a line selector which fulfils these requirements without great complica-
tions.
To this end, I employ small springjacks having 2 contact springs, the con-
nection of which is arranged in series. The line selector which I show diagram-
matically consists of a jack b, and a spring f which latter is in metallic connection
with the former when at rest, or it may be formed out of two metal pieces, one
part of which is brought into contact with the conducting cord of the plug
when the latter is inserted, while the other part is insulated therefrom. The
lines are so arranged that each is brought at n into. a permanent connection
with the terminal E of the corresponding telephone apparatus, and the spring-
jack of the line is so connected that that part of the springjack which ought
to come into contact with the cord of the plug, i.e. the jack b, is directed
towards n. The spring however is connected towards the opposite direction,
and the last jack is connected with the common return K, which at the same
time is in connection with one pole of the calling battery K Z.
The zinc pole of the battery is, as generally done with line selector installa-

page_text: tions, carried right through the whole system, and branched off at each station
to the terminal marked W Z. This arrangement necessitates that the line of
the intermediate stations must be carried back partly throughout the whole
system in order to obtain the connecting in series as will be seen in diagram
at stations II and III where the line 1.2" or 1.3" is carried back from the ter-
minus of the whole installation.
The main line L³ passes in such case by the corresponding apparatus or as
shown in II without having any connections. This system of line selecters
allows installations with earth return or with metallic return, but when a single
line (earth return) is used, a third station is still able to overhear conversation
going on between 2 other side stations; whereas when metallic circuit is used,
overhearing is absolutely impossible.
If, for instance, station IV puts the plug into springjack I, in order to
establish a connection with apparatus L, the current comes from station IV,
through springjacks III, and II, to the point I" and the return plug $1, to the
common return K with which station IV is connected by means of its own
terminal marked E, and through the line L at station I.
If station II wishes to overhear, this can be done like an ordinary line
selector system. In order however to prevent this intrusion on the part of a
third station, it is necessary that the called-up station (in the present case
Station I) should withdraw the plug S¹ from the jack k which is connected to the
common return K. This withdrawal causes the home station to be disconnected
from the common return K, and when such withdrawn plug is inserted into
springjack No. 4 at station I, the line L and with it the station IV, are also
disconnected from the common return K.
If now any of the side stations wish to overhear the conversation going on,
by tapping the plug holes, say for instance station II through tapping the plug
holes I or IV it will be of no avail because the 2 corresponding stations have
no connection whatever with the common return K, from whence alone the inter-
fering station can get current.
If the interfering station should go a step further, and should try to insert
the plug into springjack I or IV, the connection between I and IV would thereby
be interrupted, and the result would be that conversation between the two
stations would be stopped at once, and any attempt to overhear on the part of
a third station would be frustrated.
The drawings I refer to in this Complete Specification are those filed with
the Provisional Specification.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is:-
1. A line selector with springjacks having two contact springs, the connec-
tions of which are arranged in series, and which are connected up in the manner
and for the purpose as fully described, thereby attaining the result as fully
described and for the purpose set forth.
Dated this 2nd day of December 1900
HERMANN OPPENHEIMER.