First progress report on a multi-channel magnetic drum inner ...

III-2
head must be able to record or read at the rate of one spot every
five microSecorxds. Suppose that the first and fifth microsecond be
assigned as "no action" zones (between recorded signals) and suppose
that the current through the head be required to rise at a rate
corresponding to ten milliamperes in one microsecond, then to
dwell for one microsecond, and to recede in about one microsecond.
If the recording head had a winding inductance of $0 millihenries
(actual value of a typical commercial sound recording head) a signal
of 500 volts peak would have to be appliedl This calculation iS, of
course, extremely crude; the type of multiturn winding ordinarily used
on an iron core in commercial recording heads has appreciable inter-turn
capacitance as well as inductance, and to estimate the response rate
of the useful magnetic flux of such a distributed-parameter system is
a pernicious task. Commercial recording heads usually resonate well
below the 200 Kc repetition rate used in these approximations; below
this they may be expected to behave inductively with exponential current
rise; above this value they may be expected to act capacitatively.
3) The attribute of high terminal impedance which characterizes
commercial sound-recording heads is but one aspect of their unsuitability
for digit recording and reading at high repetition fates as in the present
aDplication. These heads usually consist of a pair of windings of from
50 to 500 turns wound about a ferro-magnetic core of high permeability;
the core is usually about the circumference of a penny, laminated, and