<s>
Twistor	B-General_Concept
memory	I-General_Concept
is	O
a	O
form	O
of	O
computer	B-General_Concept
memory	I-General_Concept
formed	O
by	O
wrapping	O
magnetic	O
tape	O
around	O
a	O
current-carrying	O
wire	O
.	O
</s>
<s>
Operationally	O
,	O
twistor	B-General_Concept
was	O
very	O
similar	O
to	O
core	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
Twistor	B-General_Concept
could	O
also	O
be	O
used	O
to	O
make	O
ROM	B-Device
memories	I-Device
,	O
including	O
a	O
re-programmable	O
form	O
known	O
as	O
piggyback	O
twistor	B-General_Concept
.	O
</s>
<s>
Introduced	O
by	O
Bell	O
Labs	O
in	O
1957	O
,	O
the	O
first	O
commercial	O
use	O
was	O
in	O
their	O
1ESS	B-Device
switch	I-Device
which	O
went	O
into	O
operation	O
in	O
1965	O
.	O
</s>
<s>
Twistor	B-General_Concept
was	O
used	O
only	O
briefly	O
in	O
the	O
late	O
1960s	O
and	O
early	O
1970s	O
,	O
when	O
semiconductor	B-Architecture
memory	I-Architecture
devices	O
replaced	O
almost	O
all	O
earlier	O
memory	O
systems	O
.	O
</s>
<s>
The	O
basic	O
ideas	O
behind	O
twistor	B-General_Concept
also	O
led	O
to	O
the	O
development	O
of	O
bubble	B-General_Concept
memory	I-General_Concept
,	O
although	O
this	O
had	O
a	O
similarly	O
short	O
commercial	O
lifespan	O
.	O
</s>
<s>
In	O
core	B-General_Concept
memory	I-General_Concept
,	O
small	O
ring-shaped	O
magnets	O
-	O
the	O
cores	O
-	O
are	O
threaded	O
by	O
two	O
crossed	O
wires	O
,	O
X	O
and	O
Y	O
,	O
to	O
make	O
a	O
matrix	O
known	O
as	O
a	O
plane	O
.	O
</s>
<s>
The	O
basic	O
operation	O
in	O
a	O
core	B-General_Concept
memory	I-General_Concept
is	O
writing	O
.	O
</s>
<s>
In	O
core	B-General_Concept
memory	I-General_Concept
,	O
a	O
third	O
wire	O
-	O
the	O
sense/inhibit	O
line	O
-	O
is	O
needed	O
to	O
write	O
or	O
read	O
a	O
bit	O
.	O
</s>
<s>
This	O
means	O
that	O
core	O
is	O
a	O
non-volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
To	O
increase	O
memory	B-Device
density	I-Device
one	O
had	O
to	O
use	O
smaller	O
cores	O
,	O
which	O
greatly	O
increased	O
the	O
difficulty	O
of	O
wiring	O
them	O
onto	O
the	O
lines	O
.	O
</s>
<s>
An	O
early	O
iteration	O
of	O
the	O
Twistor	B-General_Concept
comprised	O
a	O
twisted	O
ferromagnetic	O
wire	O
threaded	O
through	O
a	O
series	O
of	O
concentric	O
solenoids	B-Algorithm
(	O
see	O
attached	O
photo	O
of	O
a	O
test	O
rig	O
for	O
a	O
single	O
"	O
bit	O
"	O
)	O
.	O
</s>
<s>
The	O
longer	O
solenoid	B-Algorithm
is	O
the	O
SENSE	O
coil	O
,	O
the	O
shorter	O
one	O
the	O
WRITE	O
coil	O
.	O
</s>
<s>
At	O
one	O
end	O
of	O
the	O
stretched	O
wire	O
was	O
the	O
READ	O
solenoid	B-Algorithm
-	O
when	O
pulsed	O
it	O
sent	O
an	O
acoustic	O
wave	O
through	O
the	O
wire	O
.	O
</s>
<s>
Twistor	B-General_Concept
was	O
similar	O
in	O
concept	O
to	O
core	B-General_Concept
memory	I-General_Concept
,	O
but	O
replaced	O
the	O
circular	O
magnets	O
with	O
magnetic	O
tape	O
to	O
store	O
the	O
patterns	O
.	O
</s>
<s>
The	O
tape	O
was	O
wrapped	O
around	O
one	O
set	O
of	O
the	O
wires	O
,	O
the	O
equivalent	O
of	O
the	O
X	O
line	O
,	O
in	O
such	O
a	O
way	O
that	O
it	O
formed	O
a	O
45-degree	O
helix	B-Application
.	O
</s>
<s>
The	O
Y	O
wires	O
were	O
replaced	O
by	O
solenoids	B-Algorithm
wrapping	O
a	O
number	O
of	O
twistor	B-General_Concept
wires	O
.	O
</s>
<s>
The	O
magnetic	O
tape	O
was	O
specifically	O
selected	O
to	O
only	O
allow	O
magnetization	O
along	O
the	O
length	O
of	O
the	O
tape	O
,	O
so	O
only	O
a	O
single	O
point	O
of	O
the	O
twistor	B-General_Concept
would	O
have	O
the	O
right	O
direction	O
of	O
field	O
to	O
become	O
magnetized	O
.	O
</s>
<s>
The	O
original	O
twistor	B-General_Concept
system	O
used	O
permalloy	O
tape	O
wrapped	O
around	O
a	O
3	O
mil	O
copper	O
wire	O
.	O
</s>
<s>
Several	O
such	O
twistor	B-General_Concept
lines	O
were	O
laid	O
side-by-side	O
and	O
then	O
laminated	O
into	O
a	O
PET	O
film	O
plastic	O
sheet	O
,	O
with	O
the	O
twistors	B-General_Concept
and	O
their	O
return	O
wires	O
about	O
1/10th	O
of	O
an	O
inch	O
apart	O
.	O
</s>
<s>
A	O
typical	O
tape	O
might	O
have	O
five	O
twistor	B-General_Concept
wires	O
and	O
their	O
returns	O
,	O
so	O
the	O
sheet	O
was	O
just	O
over	O
an	O
inch	O
wide	O
.	O
</s>
<s>
The	O
solenoid	B-Algorithm
was	O
similarly	O
constructed	O
,	O
consisting	O
of	O
a	O
number	O
of	O
0.15	O
inch	O
wide	O
copper	O
tapes	O
laminated	O
into	O
a	O
plastic	O
tape	O
of	O
the	O
same	O
basic	O
dimensions	O
as	O
the	O
twistor	B-General_Concept
.	O
</s>
<s>
Unlike	O
a	O
traditional	O
solenoid	B-Algorithm
with	O
many	O
turns	O
of	O
wire	O
around	O
an	O
open	O
core	O
,	O
this	O
system	O
was	O
essentially	O
nothing	O
more	O
than	O
single	O
wires	O
in	O
a	O
sheet	O
of	O
plastic	O
.	O
</s>
<s>
To	O
build	O
the	O
complete	O
memory	O
system	O
,	O
a	O
sheet	O
of	O
the	O
solenoid	B-Algorithm
was	O
laid	O
out	O
flat	O
,	O
say	O
along	O
the	O
X	O
direction	O
,	O
and	O
then	O
a	O
sheet	O
of	O
the	O
twistor	B-General_Concept
was	O
laid	O
on	O
top	O
at	O
right	O
angles	O
to	O
it	O
along	O
the	O
Y	O
axis	O
.	O
</s>
<s>
The	O
solenoid	B-Algorithm
tape	O
was	O
then	O
folded	O
over	O
,	O
so	O
that	O
it	O
wrapped	O
the	O
twistor	B-General_Concept
sheet	O
,	O
producing	O
a	O
series	O
of	O
U-shaped	O
solenoids	B-Algorithm
.	O
</s>
<s>
Now	O
another	O
layer	O
of	O
the	O
solenoid	B-Algorithm
tape	O
is	O
laid	O
over	O
the	O
first	O
,	O
the	O
twistor	B-General_Concept
tape	O
folded	O
over	O
so	O
it	O
now	O
runs	O
along	O
the	O
negative	O
Y	O
axis	O
across	O
the	O
top	O
of	O
the	O
new	O
solenoid	B-Algorithm
tape	O
,	O
and	O
then	O
the	O
solenoid	B-Algorithm
tape	O
is	O
folded	O
over	O
to	O
form	O
a	O
second	O
set	O
of	O
loops	O
.	O
</s>
<s>
This	O
process	O
continues	O
until	O
the	O
twistor	B-General_Concept
strip	O
is	O
"	O
used	O
up	O
"	O
,	O
forming	O
a	O
compact	O
cube	O
of	O
memory	O
.	O
</s>
<s>
Along	O
one	O
side	O
of	O
the	O
memory	O
,	O
connected	O
to	O
each	O
of	O
the	O
solenoid	B-Algorithm
loops	O
,	O
was	O
a	O
series	O
of	O
small	O
cores	O
used	O
solely	O
for	O
switching	O
(	O
their	O
original	O
purpose	O
,	O
development	O
as	O
a	O
memory	O
came	O
later	O
)	O
.	O
</s>
<s>
The	O
main	O
reason	O
for	O
Bell	O
's	O
development	O
of	O
twistor	B-General_Concept
is	O
that	O
the	O
process	O
could	O
be	O
highly	O
automated	O
.	O
</s>
<s>
Although	O
the	O
folding	O
process	O
that	O
completed	O
the	O
twistor	B-General_Concept
might	O
be	O
carried	O
out	O
by	O
hand	O
,	O
the	O
layup	O
and	O
laminating	O
of	O
the	O
sheets	O
was	O
easily	O
handled	O
by	O
machine	O
.	O
</s>
<s>
Improved	O
versions	O
of	O
twistor	B-General_Concept
also	O
wrapped	O
the	O
section	O
of	O
bare	O
copper	O
initially	O
used	O
solely	O
for	O
the	O
return	O
path	O
,	O
thereby	O
doubling	O
density	O
without	O
any	O
changes	O
to	O
the	O
production	O
techniques	O
.	O
</s>
<s>
Writing	O
to	O
twistor	B-General_Concept
was	O
effectively	O
identical	O
to	O
core	O
;	O
a	O
particular	O
bit	O
was	O
selected	O
by	O
powering	O
one	O
of	O
the	O
twistor	B-General_Concept
wires	O
and	O
one	O
of	O
the	O
solenoid	B-Algorithm
loops	O
to	O
one	O
half	O
of	O
the	O
required	O
power	O
,	O
such	O
that	O
the	O
required	O
field	O
strength	O
was	O
created	O
only	O
at	O
the	O
intersection	O
of	O
the	O
two	O
.	O
</s>
<s>
Unlike	O
core	O
,	O
twistor	B-General_Concept
did	O
not	O
have	O
a	O
sense/inhibit	O
line	O
.	O
</s>
<s>
Instead	O
,	O
it	O
used	O
a	O
larger	O
current	O
in	O
the	O
solenoid	B-Algorithm
,	O
large	O
enough	O
to	O
flip	O
all	O
of	O
the	O
bits	O
in	O
that	O
loop	O
,	O
and	O
then	O
used	O
the	O
twistor	B-General_Concept
wires	O
as	O
the	O
read	O
line	O
.	O
</s>
<s>
Twistor	B-General_Concept
was	O
thus	O
read	O
and	O
written	O
one	O
plane	O
at	O
a	O
time	O
,	O
rather	O
than	O
in	O
core	O
,	O
where	O
only	O
one	O
bit	O
per	O
plane	O
could	O
be	O
used	O
at	O
once	O
.	O
</s>
<s>
Twistor	B-General_Concept
could	O
be	O
modified	O
to	O
produce	O
a	O
ROM	B-Device
that	O
could	O
be	O
easily	O
re-programmed	O
.	O
</s>
<s>
To	O
do	O
this	O
,	O
one-half	O
of	O
each	O
solenoid	B-Algorithm
loop	O
was	O
replaced	O
with	O
an	O
aluminum	O
card	O
into	O
which	O
tiny	O
vicalloy	O
bar	O
magnets	O
were	O
embedded	O
.	O
</s>
<s>
As	O
the	O
solenoids	B-Algorithm
have	O
to	O
be	O
complete	O
circuits	O
in	O
order	O
for	O
current	O
to	O
flow	O
through	O
them	O
,	O
they	O
were	O
still	O
inserted	O
as	O
folded	O
sheets	O
,	O
but	O
in	O
this	O
case	O
the	O
loop	O
was	O
inserted	O
between	O
the	O
folds	O
of	O
twistor	B-General_Concept
instead	O
of	O
around	O
them	O
.	O
</s>
<s>
This	O
allowed	O
the	O
single	O
sheet	O
to	O
act	O
as	O
one	O
half	O
of	O
a	O
solenoid	B-Algorithm
loop	O
for	O
two	O
folds	O
of	O
the	O
twistor	B-General_Concept
,	O
above	O
and	O
below	O
.	O
</s>
<s>
To	O
complete	O
the	O
loop	O
,	O
the	O
card	O
of	O
magnets	O
was	O
placed	O
on	O
the	O
other	O
side	O
of	O
the	O
twistor	B-General_Concept
tape	O
.	O
</s>
<s>
Reads	O
were	O
performed	O
by	O
powering	O
the	O
solenoid	B-Algorithm
to	O
a	O
point	O
about	O
half	O
of	O
that	O
needed	O
to	O
produce	O
a	O
write	O
.	O
</s>
<s>
If	O
the	O
bit	O
was	O
beside	O
an	O
unmagnetized	O
bar	O
magnet	O
in	O
the	O
card	O
,	O
the	O
field	O
was	O
not	O
opposed	O
and	O
the	O
flip	O
caused	O
a	O
current	O
pulse	O
in	O
the	O
twistor	B-General_Concept
wire	O
,	O
reading	O
a	O
"	O
1	O
"	O
.	O
</s>
<s>
However	O
,	O
by	O
magnetizing	O
the	O
bar	O
at	O
that	O
bit	O
,	O
the	O
bar	O
magnet	O
opposed	O
the	O
field	O
being	O
created	O
by	O
the	O
solenoid	B-Algorithm
current	O
,	O
causing	O
it	O
to	O
be	O
below	O
the	O
write	O
strength	O
,	O
and	O
preventing	O
the	O
flip	O
.	O
</s>
<s>
The	O
permanent	O
magnet	O
twistor	B-General_Concept
(	O
PMT	O
)	O
was	O
re-programmed	O
by	O
removing	O
the	O
plates	O
and	O
placing	O
them	O
over	O
a	O
custom	O
writer	O
.	O
</s>
<s>
The	O
PMT	O
that	O
was	O
used	O
in	O
the	O
1ESS	B-Device
system	O
used	O
modules	O
with	O
128	O
cards	O
with	O
2818	O
magnets	O
(	O
for	O
64	O
44-bit	O
words	O
)	O
on	O
each	O
.	O
</s>
<s>
Another	O
form	O
of	O
twistor	B-General_Concept
ROM	B-Device
replaced	O
the	O
permanent	O
magnet	O
cards	O
with	O
a	O
second	O
magnetic	O
tape	O
wrapped	O
around	O
the	O
first	O
on	O
the	O
twistor	B-General_Concept
lines	O
,	O
in	O
a	O
"	O
piggyback	O
"	O
configuration	O
.	O
</s>
<s>
Writes	O
were	O
slightly	O
more	O
complex	O
,	O
due	O
to	O
the	O
fact	O
that	O
piggyback	O
twistors	B-General_Concept
all	O
featured	O
the	O
magnetic	O
tape	O
along	O
the	O
entire	O
length	O
of	O
the	O
X	O
wire	O
.	O
</s>
<s>
This	O
meant	O
that	O
any	O
one	O
solenoid	B-Algorithm
was	O
wrapping	O
both	O
the	O
bit	O
that	O
is	O
being	O
written	O
as	O
well	O
as	O
the	O
one	O
on	O
the	O
section	O
of	O
return	O
wire	O
.	O
</s>
<s>
To	O
set	O
the	O
one	O
both	O
and	O
not	O
the	O
other	O
,	O
the	O
solenoid	B-Algorithm
was	O
first	O
powered	O
in	O
one	O
direction	O
and	O
then	O
the	O
other	O
,	O
while	O
the	O
current	O
in	O
the	O
twistor	B-General_Concept
line	O
remained	O
constant	O
.	O
</s>
<s>
Twistor	B-General_Concept
was	O
used	O
in	O
a	O
number	O
of	O
applications	O
.	O
</s>
<s>
Much	O
of	O
the	O
development	O
funding	O
was	O
supplied	O
by	O
the	O
US	O
Air	O
Force	O
,	O
as	O
twistor	B-General_Concept
was	O
to	O
be	O
used	O
as	O
the	O
main	O
memory	O
in	O
the	O
LIM-49	O
Nike	O
Zeus	O
project	O
.	O
</s>
<s>
In	O
the	O
United	O
States	O
the	O
Bell	O
System	O
(	O
American	O
Telephone	O
&	O
Telegraph	O
)	O
also	O
used	O
twistors	B-General_Concept
with	O
permanent	O
magnets	O
as	O
the	O
"	O
Program	O
Store	O
"	O
or	O
main	O
memory	O
in	O
their	O
first	O
electronic	O
telephone	O
switching	B-Device
system	I-Device
,	O
the	O
1ESS	B-Device
as	O
well	O
as	O
others	O
in	O
the	O
ESS	B-Device
series	O
of	O
electronic	O
telephone	O
switches	O
,	O
and	O
did	O
so	O
up	O
to	O
the	O
4ESS	B-Device
switch	I-Device
introduced	O
in	O
1976	O
and	O
sold	O
into	O
the	O
1980s	O
.	O
</s>
<s>
In	O
addition	O
,	O
twistor	B-General_Concept
was	O
used	O
in	O
the	O
Traffic	O
Service	O
Position	O
System	O
(	O
TSPS	O
)	O
,	O
Bell	O
's	O
successor	O
to	O
cord	O
telephone	B-Device
switchboards	I-Device
which	O
controlled	O
call	O
handling	O
and	O
coin	O
collection	O
for	O
local	O
and	O
international	O
calls	O
.	O
</s>
<s>
By	O
2017	O
all	O
remaining	O
TSPS	O
and	O
ESS	B-Device
installations	O
used	O
to	O
provide	O
telephone	O
service	O
in	O
rural	O
areas	O
of	O
the	O
United	O
States	O
had	O
been	O
removed	O
.	O
</s>
