<s>
The	O
Strowger	B-Device
switch	I-Device
is	O
the	O
first	O
commercially	O
successful	O
electromechanical	O
stepping	O
switch	O
telephone	O
exchange	O
system	O
.	O
</s>
<s>
Because	O
of	O
its	O
operational	O
characteristics	O
,	O
it	O
is	O
also	O
known	O
as	O
a	O
step-by-step	O
(	O
SXS	B-Device
)	O
switch	O
.	O
</s>
<s>
The	O
Strowger	B-Device
system	I-Device
was	O
widely	O
used	O
until	O
the	O
development	O
of	O
the	O
more	O
reliable	O
crossbar	O
switch	O
,	O
an	O
electromechanical	O
switch	O
with	O
a	O
matrix	O
of	O
vertical	O
and	O
horizontal	O
bars	O
and	O
simpler	O
motions	O
.	O
</s>
<s>
The	O
telegraph	O
keys	O
or	O
telephone	O
dial	O
creates	O
trains	O
of	O
on-off	O
current	O
pulses	B-Algorithm
corresponding	O
to	O
the	O
digits	O
1-9	O
,	O
and	O
0	O
(	O
which	O
sends	O
10	O
pulses	B-Algorithm
)	O
.	O
</s>
<s>
The	O
stepping	O
motion	O
is	O
controlled	O
by	O
the	O
current	O
pulses	B-Algorithm
coming	O
from	O
the	O
originating	O
customer	O
's	O
telegraph	O
keys	O
,	O
and	O
later	O
from	O
the	O
rotary	O
dial	O
.	O
</s>
<s>
The	O
Strowger	B-Device
switch	I-Device
has	O
three	O
banks	O
of	O
contacts	O
.	O
</s>
<s>
The	O
Strowger	B-Device
switch	I-Device
uses	O
two	O
telegraph-type	O
keys	O
on	O
a	O
telephone	O
set	O
for	O
dialing	O
.	O
</s>
<s>
The	O
first	O
set	O
of	O
incoming	O
pulses	B-Algorithm
raises	O
the	O
armature	O
of	O
an	O
electromagnet	O
to	O
move	O
a	O
shaft	O
which	O
selects	O
the	O
desired	O
level	O
of	O
contacts	O
,	O
by	O
engaging	O
a	O
pawl	O
with	O
the	O
upper	O
ratchet	O
.	O
</s>
<s>
The	O
second	O
set	O
of	O
pulses	B-Algorithm
,	O
from	O
the	O
second	O
key	O
,	O
operates	O
another	O
electromagnet	O
.	O
</s>
<s>
The	O
commercial	O
version	O
of	O
the	O
Strowger	B-Device
switch	I-Device
,	O
as	O
developed	O
by	O
the	O
Strowger	O
Automatic	O
Telephone	O
Exchange	O
Company	O
,	O
used	O
a	O
rotary	O
dial	O
for	O
signalling	O
to	O
the	O
exchange	O
.	O
</s>
<s>
Later	O
Strowger	O
(	O
SXS	B-Device
)	O
exchanges	O
often	O
use	O
a	O
subscriber	O
uniselector	B-Algorithm
as	O
part	O
of	O
the	O
line	O
equipment	O
individual	O
to	O
each	O
line	O
,	O
which	O
searches	O
"	O
forward	O
"	O
for	O
a	O
first	O
selector	O
.	O
</s>
<s>
Hence	O
exchanges	O
with	O
subscriber	O
uniselectors	B-Algorithm
were	O
usually	O
used	O
at	O
British	O
exchanges	O
with	O
a	O
high	O
proportion	O
of	O
business	O
customers	O
,	O
e.g.	O
</s>
<s>
The	O
fundamental	O
modularity	O
of	O
the	O
system	O
combined	O
with	O
its	O
step-by-step	O
(	O
hence	O
the	O
alternative	O
name	O
)	O
selection	O
process	O
and	O
an	O
almost	O
unlimited	O
potential	O
for	O
expansion	O
gives	O
the	O
Strowger	B-Device
system	I-Device
its	O
technical	O
advantage	O
.	O
</s>
<s>
In	O
modern	O
terminology	O
,	O
the	O
previous	O
systems	O
were	O
not	O
"	O
scalable	B-Architecture
"	O
.	O
</s>
<s>
From	O
1912	O
,	O
the	O
British	O
General	O
Post	O
Office	O
,	O
which	O
also	O
operated	O
the	O
British	O
telephone	O
system	O
,	O
installed	O
several	O
automatic	O
telephone	O
exchanges	O
from	O
several	O
vendors	O
in	O
trials	O
at	O
Darlington	O
on	O
10	O
October	O
1914	O
and	O
Dudley	O
on	O
9	O
September	O
1916	O
(	O
rotary	B-Device
system	I-Device
)	O
,	O
Fleetwood	O
(	O
relay	O
exchange	O
from	O
Sweden	O
)	O
,	O
Grimsby	O
(	O
Siemens	O
)	O
,	O
Hereford	O
(	O
Lorimer	O
)	O
and	O
Leeds	O
(	O
Strowger	O
)	O
.	O
</s>
<s>
The	O
GPO	O
selected	O
the	O
Strowger	B-Device
switches	I-Device
for	O
small	O
and	O
medium-sized	O
cities	O
and	O
towns	O
.	O
</s>
<s>
The	O
Director	O
systems	O
used	O
SXS	B-Device
switches	O
for	O
destination	O
routing	O
and	O
number	O
translation	O
facilities	O
similar	O
to	O
the	O
register	O
used	O
in	O
common-control	O
exchanges	O
.	O
</s>
