<s>
A	O
telephone	B-Algorithm
magneto	I-Algorithm
is	O
a	O
hand-cranked	O
electrical	O
generator	O
that	O
uses	O
permanent	O
magnets	O
to	O
produce	O
alternating	O
current	O
from	O
a	O
rotating	O
armature	O
.	O
</s>
<s>
In	O
early	O
telegraphy	O
,	O
magnetos	B-Algorithm
were	O
used	O
to	O
power	O
instruments	O
,	O
while	O
in	O
telephony	O
they	O
were	O
used	O
to	O
generate	O
electrical	O
current	O
to	O
drive	O
electromechanical	O
ringers	O
in	O
telephone	O
sets	O
and	O
activate	O
signals	O
on	O
operator	O
consoles	O
.	O
</s>
<s>
Telegraphy	O
pre-dated	O
telephony	O
and	O
magnetos	B-Algorithm
were	O
used	O
to	O
drive	O
some	O
of	O
the	O
early	O
printing	O
telegraph	O
instruments	O
.	O
</s>
<s>
The	O
later	O
automatic	O
and	O
printing	O
instruments	O
,	O
such	O
as	O
the	O
Wheatstone	O
ABC	O
telegraph	O
,	O
required	O
greater	O
currents	O
that	O
could	O
be	O
delivered	O
by	O
a	O
hand-cranked	O
magneto	B-Algorithm
.	O
</s>
<s>
Many	O
early	O
manual	O
telephones	O
had	O
an	O
attached	O
hand-cranked	O
magneto	B-Algorithm
that	O
produced	O
an	O
alternating	O
current	O
(	O
AC	O
)	O
at	O
50	O
–	O
100	O
V	O
for	O
signaling	O
to	O
ring	O
the	O
bells	O
of	O
other	O
telephones	O
on	O
the	O
same	O
(	O
party	O
)	O
line	O
,	O
and	O
to	O
alert	O
an	O
operator	O
at	O
the	O
local	O
telephone	O
exchange	O
.	O
</s>
<s>
The	O
telephone	O
instrument	O
obtained	O
talking	O
current	O
by	O
powering	O
a	O
carbon	O
microphone	O
with	O
a	O
local	O
battery	O
,	O
consisting	O
of	O
"	O
N°	O
6	O
"	O
zinc	B-Algorithm
–	I-Algorithm
carbon	I-Algorithm
dry	O
cells	O
.	O
</s>
<s>
By	O
around	O
1900	O
,	O
large	O
racks	O
of	O
motor-generator	O
sets	O
in	O
the	O
telephone	O
exchange	O
could	O
supply	O
this	O
ringing	O
current	O
remotely	O
instead	O
and	O
the	O
local	O
magneto	B-Algorithm
was	O
often	O
no	O
longer	O
required	O
,	O
but	O
their	O
use	O
continued	O
into	O
the	O
mid-20th	O
century	O
.	O
</s>
<s>
Telephone	B-Algorithm
magnetos	I-Algorithm
featured	O
a	O
large	O
gear	O
rotated	O
by	O
hand	O
with	O
a	O
handle	O
,	O
that	O
drove	O
a	O
much	O
smaller	O
gear	O
on	O
the	O
armature	O
rotor	O
,	O
providing	O
a	O
high	O
gear-ratio	O
to	O
increase	O
the	O
rotational	O
speed	O
of	O
the	O
magneto	B-Algorithm
armature	O
.	O
</s>
<s>
A	O
mechanical	O
switch	O
on	O
the	O
output	O
terminals	O
engaged	O
only	O
when	O
the	O
rotor	O
was	O
turning	O
,	O
so	O
that	O
the	O
magneto	B-Algorithm
was	O
normally	O
disconnected	O
from	O
the	O
telephone	O
circuitry	O
.	O
</s>
<s>
Ringing	O
current	O
magnetos	B-Algorithm
were	O
used	O
in	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
as	O
late	O
as	O
the	O
1980s	O
,	O
when	O
they	O
were	O
still	O
used	O
in	O
manual	O
private	O
branch	O
exchanges	O
(	O
PBXs	O
)	O
,	O
small	O
business	O
switchboards	O
worked	O
by	O
operators	O
.	O
</s>
<s>
Magnetos	B-Algorithm
were	O
used	O
when	O
such	O
a	O
facility	O
was	O
not	O
available	O
,	O
or	O
as	O
a	O
backup	O
source	O
when	O
the	O
facility	O
failed	O
.	O
</s>
<s>
Unlike	O
in	O
the	O
public	O
telephone	O
network	O
,	O
which	O
has	O
a	O
standard	O
ringing	O
cadence	O
(	O
the	O
repeating	O
pattern	O
of	O
ringing	O
and	O
silence	O
)	O
,	O
the	O
ringing	O
cadence	O
when	O
using	O
a	O
magneto	B-Algorithm
depended	O
on	O
the	O
skill	O
of	O
the	O
operator	O
.	O
</s>
<s>
Linesman	O
's	O
test	O
sets	O
also	O
included	O
a	O
magneto	B-Algorithm
,	O
for	O
use	O
when	O
ringing	O
out	O
to	O
either	O
the	O
exchange	O
or	O
the	O
subscriber	O
,	O
from	O
anywhere	O
along	O
the	O
line	O
.	O
</s>
