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wiring of the plates is simplified by evaporating
one or two conducting strips passing through every
hole in the plate13. Additional wires can be put in by
threading insulated copper wires through a whole
stack of plates.
A big problem in the use of ferrite plates is the
disturbances introduced in the read wire by
half-pulsed cores in a coincident current memory.
This noise can be cancelled by using two holes (one
in each of two separate plates) per bit, or by using
a 4-plate arrangement as proposed by C S Warren1-’.
There are still many problems to overcome before
the plates can be produced with the same degree
of accuracy as the ferrite cores. It has been found
difficult to get a uniform plate material and the
mechanical difficulties with the handling of the
plates are still very serious. A breakage can very
easily occur near the edges of the plate.
The twistor
Magnetic wires with a length to diameter ratio of
100 or more have been found to have very low
demagnetization factors, and hence have an almost
square hysteresis loop. This has been utilized in
the twistor and the tensor, both products of the
Bell Laboratories13.
The twistor was first made of a massive nickel
wire, twisted to produce a stress in the material
and thereby defining an easy direction for the
magnetic flux along a helix path around the wire. It
was, however, found that these wires had a rather
high switching coefficient14, and also that is was
difficult to maintain a constant stress in the
material for a long time. A newer development utilizes
a copper wire, around which a thin nickel band is
wrapped in a helix. Many such wires can be molded
together in plastic and form bands with e.g. 20
parallell twistor wires.
A twistor buffer store15 can be made by wrapping
a coil around the band, fig. 1, and representing each
word by a certain coil. If the band is composed of 20
wires the word obviously contains 26 bits.
Writing a "1" can be done by sending coincidently
a current through the central copper wire and a
current through the actual word coil, whereby the
flux in the magnetic nickel band will be aligned
with the resultant magnetic field. A "0" is
represented by no current in the central copper wire,
in which case the field produced by the word coil
cannot alone switch the flux in the nickel band.
Reading is accomplished by pulsing the word coil
with currents opposite the write currents and of
sufficient amplitude to switch the flux.
In a similar way the twistor can be used in a
coincident current store. It may be remarked that
the field produced by a current through the central
copper wire is very effectively coupled to the nickel
band and currents of 0.1 A are sufficient for
complete switching, compared with the 2 ampere-turns
required in the word coil.
The switching speed of the twistor is of the order
of microseconds. A 4 us memory cycle time has been
realized.
The twistor is an open magnetic circuit and is
therefore sensitive to external magnetic fields. In
buffer storage applications this influence of external
Fig. 1. Twistor memory. Reading is accomplished by the
Ix current, writing by coincident lx und ly currents.
The figure shows: a typical arrangement, b
waveforms, c the representation of "1" and "0".
fields can be diminished by the use of a bias field.
The packing densities of the information on the wire
can be made of the order of V-i bit/cm wire.
The main advantage of the twistor memory element
is its low cost and the possibilities for easy
fabrication of memories. The output voltages are smaller
than for the ferrite cores but the temperature
characteristics are somewhat better.
The tensor
The tensor10 gives the possibility of non-destructive
readout for the same theoretical reasons as the thin
films (quadrature read-out). The output voltage is
small.
Multiaperture devices
The multiaperture devices have mostly been used
as logical elements10-17,18 but the fluxor13 is an example
of a multiaperture device with applications in very
fast coincident current memories.
The fluxor consists of a ferrite plate with three
holes. In one of the holes there is a signal wire,
in the other two there are ordinate wires. One of
.90 ELTEKNIK 1959
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