**MODELLING OF PHYSICAL SYSTEMS
ACCORDING TO MAXWELL'S FIRST METHOD **

by
**M. Robert Showalter**^{(1)} and **Stephen
J. Kline**^{(2)}

** Logically
rigorous attempts to write out models that reflect physically detailed
circumstances have sometimes been blocked by undefined crossterms. Physically
arbitrary assumptions have long been accepted to avoid the difficulty.
Examples of these undefined crossterms are shown, and a definition procedure
for them is set out so that more physical modelling procedures leading
to abstract equations become possible. An electrical
transmission line model is used as the example. **

The
jump between a physical system model, defined in terms of drawings, measurement
procedures and other detail, and the abstract mathematical representation
of it remains a partly mysterious one. Nonscientists
and smart students find this jump hard to follow, and hard to trust, generation
after generation. Scientists, by and large, take the jump on
faith. Korzibski's dictum is much quoted.

"**The map [system representation, or sysrep] is
not the territory [system].**"

However,
in formal systems, such as mathematics, the system and the sysrep are identical
__by construction__. In such systems, the map **is**
the territory^{(3)}." This
is a reason for residual caution about our formal models. At a basic level,
they are not self cleansing. Experience provides
other reasons for caution. Mathematical modelling of
physical systems has produced many triumphs, but also some disappointments.
For example, mathematical modelers of neural systems have not explained
the information processing that we know brains do. One reason,
we think, it that the effective inductance of the presently accepted passive
neural conduction equation is often too low by more than 10^{10}
: 1^{(4)}

. People doing theoretical work all through the pure and
practical sciences have other difficulties, often unpublished, that provide
residual reasons to doubt that our mathematical modelling procedures are
**always** right. George Johnson^{(5)} says
that

" **Scientists must constantly
remind themselves that the map** **is not the territory, that the models
might not be capturing** **the essence of the problem, and that the
assumptions built** **into a simulation might be wrong**^{(6)}.
"

We agree. We've
found Johnson's point directly applicable to mathematical modelling. We
have had difficulty deriving valid modelling systems of coupled differential
equations by conventional means, starting from finite increment equations.
Cases where we have failed to get good results by the standard
means have included a piston ring^{(7)} design
model and the passive neural conduction equation. We have been
re-examining the fit between formal mathematical mapping and the physical
territory, and examining assumptions built into that mapping.
We found ourselves retracing ground first covered by James Clerk Maxwell.
We've found an error that we believe Maxwell may have suspected.
When that error is corrected, some terms now called infinitesimal (or infinite)
are finite. The error is very often entirely unimportant.
But it can have devastating effects in some cases, including cases important
to an understanding of the brain.

This paper
shows the error, and shows how it may be fixed, proceeding from Maxwell's
perspective, which might, today, be called a "philosophical"
perspective. It presents our fix, but does not prove
that fix rigorously. Companion papers deal with the same subject matter
from other perspectives.

One paper shows that the
error comes from an difference between measurable spaces and the abstract
domain of the algebra, requiring an arithmetical restriction on our use
of the dimensional parameters we use in our physical models, so that these
models can be consistently and correctly mapped into the domain of the
algebra^{(8)}.

A second paper shows how some of our limiting arguments,
that involve dimensional parameters, are false^{(9)};

A third paper shows by numerical example that the new
interpretation of terms can be significant or insignificant depending on
the numerical size of the dimensional parameters involved^{(10)}.

Here is James Clerk Maxwell, writing a year before his
death^{(11)}:** **

**"There are two methods of
interpreting the equations relating to geometry and the other concrete
sciences.**

**"We may regard the symbols
which occur as of themselves denoting lines, masses, times &c; or we
may consider each symbol as denoting only the numerical value of the corresponding
quantity, the concrete unit to which it occurs being tacitly understood.**

**"If we adopt the first method
we shall often have difficulty in interpreting terms which make their appearance
during our calculations. We shall therefore consider
all the written symbols as mere numerical quantities, and therefore subject
to all the operations of arithmetic during the process of calculation.
But in the original equations and the final equations, in which every term
has to be interpreted in its physical sense, we must convert every numerical
expression into a concrete quantity by multiplying it by the unit of that
kind of quantity." **

According
to the first, more literal method Maxwell cites, we have "difficulty"
interpreting some (cross effect) terms, indeed we cannot interpret them
at all. We are stopped. THEREFORE we make a plausible
assumption. We make that assumption along with Maxwell, giants before
him (Newton, LaPlace, LaGuerre, and Fourier) and workers since. We
decide to act AS IF our physical quantity representing symbols may be abstracted
into simple numbers in our intermediate calculations. This __assumption__
has produced equations that fit experiment innumerable times. But it remains
a pragmatic __assumption__ with no logically rigorous basis at all.

On Maxwell's first assumption, we
have terms that are difficult (impossible) to interpret.

On Maxwell's second assumption, these terms fit readily into our calculus
apparatus, and can quite often be "shown" by a limiting argument
to be infinitesimal or infinite.

** The
second assumption may therefore be shown wrong mathematically by finding
an inconsistency in the arithmetical usages it assumes. **We
have found such an inconsistency^{(12)}.
**The second assumption may also be shown wrong empirically by data showing
that it calls finite terms infinitesimal, or calls finite terms infinities. **
As described below, neural data indicates that effective inductance is
many billions of times greater than that predicted by a conduction equation
derived by Maxwell's second method. Also, calculations
according to Maxwell's second method that accounted for the dielectric
capacitance of a conductive line calculated an infinite line capacitance,
and hence a zero velocity of conduction. That infinity
has always been ignored on practical grounds. By measuring
conduction velocities in nearly pure water, velocities that are thousands
of times less than the speed of light, one finds that the "infinite"
dielectric capacitance of the conductive water medium is finite and consistent
with the calculation procedure set out here (Showalter, in preparation)
according to Maxwell's first method.

We have found that by adding a type limitation to the
dimensional parameters we can interpret the kind of terms Maxwell calls
"difficult." When this is done, Maxwell's first method becomes
operational and new terms, now thought to be infinitesimal or infinite,
are finite.

**BIOLOGICAL MOTIVATION:**

The Kelvin-Rall, (**K-R**),
equation is the standard electrical line conduction equation stripped of
terms in electromagnetic inductance. Reasons to doubt **K-R**
have been accumulating, and are reviewed elsewhere^{(13)}.
The coherent propagation of action potentials seems inconsistent under
**K-R**, particularly if channel populations are heterogeneous or sparse.
**K-R** seems a poor fit to observations concerning neural
synchrony. **K-R** predicts that neurons are slower than
they are. Under **K-R**, the low dissipation conduction
observed in dendrites requires very special and demanding assumptions about
channel distribution and behavior. To us, the most compelling
reason to doubt **K-R** is the EEG and MEG data of David Regan^{(14)}
.

Regan
used the zoom FFT technique on electroencephalography and magnetoencephalography
data. He showed peaks organized in the integer multiple
sums and differences characteristic of resonance. The EEG and
MEG data measured population behavior over a significant volume of brain.
Even so these integrated peak bandwidths were very tight, less than .002
Hz. We were led to conclude that brain was an assembly including
large populations of very high Q resonant structures coupled by the waves
that the EEG was measuring. Consulting anatomy, we had to assume
that either short dendritic sections or dendritic spines were the resonant
elements. Regan's data, without which we would not have persevered,
implied that the effective inductance predicted by the presently accepted
Kelvin-Rall equation was too small by enormous factors (in the range of
10^{10}-10^{18}:1 for various cases.) These
same large factors fit action potential behavior. We had these
good reasons to question the derivation of Kelvin-Rall, that followed the
"second method" cited by Maxwell.

**Crossterms in derivation of coupled equations from
models**

This paper shows how the "difficult to interpret"
crossterms come about in the derivation of electrical line equations from
a physical model set out interpreting "the symbols which occur as
of themselves denoting lines, masses, times &c."

To derive a differential equation
from a physical model in classical physics, we argue as follows, and generally
do so without formal distinction between Maxwell's first and second methods:

1. We construct a model (including
a sketch, and any necessary information) at a finite scale that represents
the laws and geometry in question.

2. We derive finite increment equation(s)
that map(s) the finite model.

3. We infer the form of the finite
increment equation at smaller and smaller scales, until it is defined at
point (differential) scale. THE EQUATION AT DIFFERENTIALLY SMALL INCREMENT
SCALE IS THE DIFFERENTIAL EQUATION^{(15)}

.

Let's
proceed to infer a finite increment equation for electrical transmission
along a line, a case that is inherently coupled. We proceed
according to Maxwell's first method, using symbols and arithmetical operations
to represent a physical situation, not just manipulating abstract and disembodied
number-symbols.

**Fig. 1** shows a neural conductor (axon or dendrite
considered as a transmission line). A tubular membrane is filled with and
surrounded by an ionic fluid. The fluid inside the tube carries current
(and signal) and has resistance **R** and electromagnetic inductance
**L** per unit length. The outer fluid is grounded. The membrane separating
these conducting fluids has capacitance and leakage conductance per unit
area. We speak of the following variables and parameters:

**v** = voltage **i**
= current

**x** = position along the line delta
x= arbitrary length interval

**R** = resistance/length **L**=electromagnetic
inductance/length

**G**= membrane conductance/length
**C**=capacitance/length

**Fig. 1** shows an arbitrarily chosen length, alpha,
which we will call delta **x** because that is commonly done.
Length increment alpha, which we call delta x, is
picked from other indistinguishable lengths. (For consistency, the length
of is a number delta times the unit of measure used in the calculation
in the **x** direction (meters, cm, or whatever).) We are
giving length alpha a two-symbol name that includes a separate number.
Other entities that have numerical values in our calculations are denoted
as single symbols, and do not have separate numerical symbols (such as
delta) associated with them.

We need finite difference equations
that define, **delta v/delta x** and **delta i/delta x**.
For the finite equations, we'll be writing out terms that have
usually been understood to exist, but that have been called infinitesimal
(based on Maxwell's second method) and neglected.

Because voltage drop over a length **alpha** depends
on current, and current over length **alpha **partly depends on
charge carriers stored in capacitance or lost through membrane leakage
over **alpha**, we know that **delta v/delta x** for **delta
x= alpha** must partly depend on effects including **C**
and **G** over length **alpha=delta x**

and

Because current drop over length **alpha** depends
on voltage change over **alpha**, and voltage change over **alpha** partly
depends on **R** and **L**, we know that **delta i/delta x **
for **delta x=alpha** must partly depend on effects of **R**
and **L** over length **alpha=delta x**.

From such interactions, it follows that:

**delta i over the interval is a function of v at x and
x+delta x
which is a function of i at x
and x + delta x
which
is a function of v at x and x + delta x
which is a function of i at x and x+delta x
and
so on
and
so on . . . .**

In current practice, when we derive a differential equation from such a coupled relation, we say:

**i over the interval is a function of v at x and x+delta x**

(and
**nothing **more.)

The truncation
implied in the words "and nothing more" follows from the second
method Maxwell cites, because the crosstems are infinitesimal under that
assumption. The truncation does not follow from Maxwell's
first method.

Let's
derive voltage and current equations that include crossterms.
We'll see why the crossterms cause us "difficulty in interpretation."
We'll write our voltage and current functions as **v(x,t)** and **i(x,t)**.
We're assuming homogeneity and symmetry for our conductor.
We assume that, for small enough lengths **delta x**, the average
voltage (current) across the interval from **x **to **x+delta x**
is the average of the voltage (current) at **x** and at **x+delta x**.

Writing down voltage change as a function of the dimensional
parameters and variables that directly affect voltage, we have.

Writing down current change as a function of the dimensional
parameters and variables that directly affect current, we have.

We may equally well rewrite (1a) and 2a) going from points
**x-delta x** to **x+delta x**, so that the interval is
centered at **x**.

Note that equation (1b) includes
**i(x+x/2)** and its time derivative. **i(x+x/2)** is
defined by equation (2b). Equation (2b)
includes **v(x+x/2)** and its derivative. **v(x+x/2)** is defined
by equation (1b). Each of these equations
requires the other for full specification: each contains the other.

If the cross-substitutions specified
implicitly are explicitly made, the resulting equations will also each
contain the other. So will the generation of equations following, and the
next, and so on. This is an endless regress. Each substitution introduces
new functions with the argument **(x+x/2)**, and so there is a continuing
need for more substitutions. To achieve closure,
one needs a truncating approximation position of **x**, for current,
voltage, and their time derivatives.

We can proceed with these substitutions, associating symbols
without interpreting them numerically or physically. For example

is

which expands algebraically to

These terms would be simpler if voltages and derivatives
of voltages were taken at the interval midpoint, **x**. But even so
simplified, it is terms of this kind that are "difficult to interpret"
in Maxwell's sense. (Maxwell was an industrious analyst living in an analytically
competitive world, and when he wrote "difficult to interpret"
he meant operationally impossible.)

We are not yet concerned with the size of these terms.
That is a matter of arithmetic. We are concerned with their existance.
If one wishes to speak of expressions like those of (6), what do they mean
for finite when the symbols are considered to stand for fully physical
things, or complete models of physical things, subject to the detailed
physical rules that stand behind the model? How do you interpret them with
a sketch interpreted by measurement procedure? The second author, who has
written a standard book on dimensional analysis^{(16)}
could not interpret these expressions. ** In discussions with mathematicians,
engineers, and scientists, the first author was not (for three years) able
to find anyone who was confident of the meaning of these kinds of expressions
at finite scales (or, as a matter of logic, when length was reduced to
an arbitrarily small value in a limiting argument.)** Maxwell seems
to have had the same difficulty. The equations below shows voltage change
over an interval of length

Again, we are concerned with the formal meaning of these
terms, and not yet with their size. Their size depends on the values of
**R**, **L**, **G**, and **C** that apply to a particular physical
case.

The equation for **i(x+x/2,t)-i(x-x/2,t)** is isomorphic
to **7a** with swapping of **v** for **i**, **R** for **G**,
and **L** for **C**.

Here are the terms in **7a** where we encounter Maxwell's
"**difficulty**."

These "difficult" terms all represent combined
physical effects that integrate together over a length. When we interpreted
these terms by scaling and sketching arguments, we encountered questions
of definition, but it always appeared that the magnitude of these cross
effects must be CONSTANT per unit length. However, in currently standard
analysis, the combined effects represented in the crossproducts above vanish,
because according to that analysis these same crossterms vary with length
so that they vanish in the limit. For some time we were stalled about here,
knowing that we had a contradiction, but not knowing how to resolve it.

Maxwell describes the standard __assumption__ that
avoids the difficulty in literal, dimensional, interpretation of these
terms, and explains why he reluctantly accepted that assumption. His justification
for this assumption is strange enough to bear repeating (and strange enough
to help explain why some smart, careful students, who wish to carefully
and redundantly trace decisive stages of logic as they learn them, can
distrust mathematical modelling procedures, and can even refuse to learn
and use them.) If we consider our physical symbols as representations of
the fully dimensional things they stand as names for, and if we ask to
make physical sense of some crossterms as physical entities, we find that
we cannot interpret them at all^{(17)}. We
are stopped. THEREFORE we decide to act AS IF our physical quantity representing
symbols are NO MORE than simple numbers in our intermediate calculations.
(That is, we classify the issues that involve the details of measurement
and measurement derived definition out of existance in the map we choose
to use.) Efficient though this assumption has often been, the assumption
has no logically rigorous basis at all. This is a mapping logic that teachers
typically do

not even attempt to *teach*. Instead, it is imposed.
Students who

rebel here are lost to the more quantitative kinds of
science.

The assumption we call "Maxwell's second method"
is so convenient and has become so reflexive that we do not think to suspect
it. Once the assumption is made, dismissal of the "difficult terms"
above follows directly. We "consider __all__ the written symbols
as mere numerical quantities, and therefore subject to all the operations
of arithmetic during the process of calculation." Our "difficult"
terms may then be analyzed by a standard limiting argument. Taking the
limit as length **x** approaches 0, these terms vanish (or become infinite).
This limiting argument is longstanding and indeed reflexive practice for
working analysts. Some of the best such analysts, long accustomed to standard
practice, may find it hard to even think about the possibility that "the
written symbols" . . . might NOT be "subject to all the operations
of arithmetic" in a map that really fit the natural territory being
described. We understand these conceptual difficulties. We had them.

Even so, we also had reasons to question Maxwell's second
(and standard) method. Empirical concerns in neural modelling have been
described before and elsewhere^{(18)}. We
also had problems at the theoretical interface of physical modelling and
analysis. Looking at crossterms in coupled equations like **7a**, we
had some procedural uncertainties but we could show, by sketch-modelling,
that the crossterms must be finite (and might be large) at finite scales.
But there was an inconsistency. A coupled equation like **7a**, expressed
at length **x**, could be reduced to a differential equation. That differential
equation could then be integrated up to scale **x**. The integrated
value would be different from the value of the finite increment equation
it came from at scale **x**, by the value of the (finite) crossterms.
We found that this could be a numerically large disparity and contradiction.

We have found that the dimensional parameters, such as
**R**, **L**, **G**, **C** are not "just dimensional numbers,"
and are not "subject to all the operations of arithmetic" in
the expected way. Operation with these dimensional parameters is subject
to an additional, easy, but new rule.

To see our reasoning in connection to Maxwell's statements,
let's rewrite **7a**, substituting the symbol "length" for
**x**.

Now, suppose we shrink our length interval to a __point__.
(Not a very short interval, but a __point__.) One may have questions
about what the notion of a point means, but the length (or area, or volume)
of a point is not some finite value. A point is of limitlessly small extent,
not some numerically specifiable extent. Even so, point values of **R**,
**L**, **G**, **C**, **i**, **v**, and **t** are all
numerically well defined and familiar.

**But what do we mean by "length
at a point"?**

To advance our argument, let's use the expression **(length) _{p}**
for "length at a point" without yet defining what that will have
to mean.

The notion of a "point" is associated with conceptual
difficulties, some of them much involved with the history of mathematical
inquiry over centuries^{(19)}. An ordinary
dictionary may devote several columns to the word "point"^{(20)}.
A mathematical dictionary may refrain from defining "point" at
all^{(21)}. Even so, the usual mathematical
idea of a point is a position in some defined space, where the position
is so sharply defined that it has position but not extent. According to
this idea, a point has zero length, zero area, zero volume, and a point
in time has zero temporal extent. The notions of "length at a point"
or "volume at a point" or "area at a point" are necessarily
abstractions and generalizations of length, volume, and area over finite
extents. These are necessary notions, that are embedded in our usage of
point values of many quantities such as the following.

**pressure, density, resistance, inductance, pressure,
shear stress, viscosity, thermal conductivity**

For example, pressure at a point includes implicitly the
notion of area at a point. Density at a point, which is mass per unit volume
at a point, implicitly includes the notion of volume at a point. Resistance
per unit length at a point implicitly requires a notion of length at a
point. *However, arithmetically clear statements about what must be meant
by "length at a point," "area at a point," "volume
at a point," or "a point in time" have not been available.
Perhaps it is better to say that arithmetically clear statments about what
must be meant by "the property of length at a point," "the
property of area at a point," "the property of volume at a point,"
and "the property of time at a point in time" have been unavailable.
*

We have found that the dimensional parameters^{(22)},
such as **R**, **L**, **G**, **C** are not "just dimensional
numbers" and are not "subject to all the operations of arithmetic"
that Maxwell's second method requires. For instance, the following expressions,
taken from **7a**, are not arithmetically consistent entities as now
interpreted:

These expressions, evaluated in different units, or according to

different patterns that should be arithmetically acceptable,
yield contradictory results^{(23)} ^{(24)}.
We have been applying our ordinary arithmetic rules to entities like this,
that are subject to particular arithmetical restrictions, not knowing of
the restrictions. Doing so, we've been persuading each other by limiting
arguments that are incorrect^{(25)}. The dimensional
parameters, such as **R**, **L**, **G**, and **C**, are subject
to an additional rule that ordinary numbers do not have. Here is the rule,
applied to derivation of differential equations from finite increment physical
models^{(26)}:

**When we represent a finite increment physical SYSTEM
("Maxwell's first method represented system") in the form of
a differential equation (defined at a POINT in space and time) we must
put ALL the variables and increments into POINT FORM - it is not valid
to have all the quantities except the increments in point form, with the
increments in extensive form. The point forms of spatial quantities and
time (expressed here in cm and second units) are:**

**length at a point: (1 cm) _{p} area at a point:
(1 cm^{2})_{p}**

**volume a a point: (1 cm ^{3})_{p} a
point in time: (1 second)_{p}**

**with the UNITS of length, area, volume, and time **

**and**

**the NUMERICAL VALUES of 1 = length/length, 1= area/area**

**1 = volume/volume, and 1= time/time respectively**

__With ALL the variables and increments in our equation
representation set out in point form, algebraic simplification yields a
differential equation that validly represents our system.__

This new rule can be operationally identical to our current
limiting procedures, or radically different from those procedures, depending
on the numerical size of the dimensional parameters we happen to be dealing
with in a particular physical case.

Once our equations are represented in this way, we can
do valid arithmetic on every term (neglecting the subscript **p** that
is only a marker.) Let's rewrite **7b** with " (1 cm)_{p}
substituted for "length_{p}". We lack the space to write
out the numerical and unit parts of **R**, **L**, **G** and **C**,
which are well understood.

When we separate **R**, **L**, **G** and **C**
into numerical parts (**R**_{n}, **L**_{n}, **C**_{n},
and **G**_{n}) that are algebraically simplified together, and
unit groups that are algebraically simplified together, we can do valid
arithmetic on equation **7c**. The result, set out in semi-arbitrary
voltage-unit, charge-unit, cm, time-unit system (**v**-**Q**-**cm**-**t**
units) is:

The analogous **di/dx** equation is

These differential equations, when integrated to length
**x**, reconstruct the values that apply to that length **x**, with
no lost terms, as they should. Every term in these differential equations
passes the loop test set out in a companion paper^{(27)}.
We may map these differential equations symbol-for-symbol into corresponding
partial differential equations. We may map these differential (or corresponding
partial differential) equations symbol-for-symbol into the domain of the
algebra. These equations are different equations from the Kelvin-Rall equations,
which lack all the terms below the first line of **7a**.

With this procedure, the first method Maxwell cites becomes
operational. Symbols can be interpreted as explicitly physical quantities,
algebraically manipulated, and then, without assumption, mapped into abstract
mathematical equations. Dimensional entities, the dimensional parameters,
involve a procedural restriction that effects the algebraic simplification
of crossterms. Equations that result from a proper algebraic simplification
of such terms can then be mapped into the domain of the algebra, and used
without any further restriction on our familiar arithmetical usages.

**For MOST purposes, this new derivation is just like
the old one. The new terms produced are finite, but they are usually too
small to consider, even in the most accurate modelling work. However, for
some purposes, the new terms are important.** The relative and absolute
importance of the terms depends on the numerical values that the dimensional
parameters **R**, **L**, **G**, and **C** happen to have. These
relative and absolute importances do not change when particular values
of **R**, **L**, **G**, and **C** are changed from one unit
system to another.

Let's consider our **dv/dx** equation

for a wire (or a neuron) and see how arithmetical logic
determines the terms our modelling equations should include. We'll fill
in numerical values for equation 8, for a wire, and for a neural line.
For a 1 mm copper wire with ordinary insulation and placement, typical
values of the dimensional parameters would be:

**R = .14 x 10 ^{-4} ohm/cm C = 3.14 x 10^{-9}
farads/cm**

**G = 3.14 x 10 ^{-10} mho/cm L = 5 x 10^{-9}
henries/cm**

and the equation 8 can be written as follows (with numerical values

of the symbolic terms written below the symbolic terms
below.)

For this wire case, all the new crossterms are valid and
finite terms, but they are numerically insignificant, far too small numerically
to consider for modelling. The equation derived by the old limiting argument
is operationally right (but not mathematically perfect). In the same notation
as (8-wire) above, the reasonable equation to use is:

or, in simpler notation

or, yet more compactly

**However, the picture is starkly different when
one looks at equation 8 with numerical dimensional parameter values that
correspond to a 1 micron diameter neural dendrite.**
Assuming reasonable values of axolemma conductivity, capacitance per membrane
area, and inductance per unit length (volume conductivity 110 ohm-cm g
= 3.18 x 10

**R = 1.4 x 10 ^{10} ohm/cm L = 5 x 10^{-9}
henries/cm**

**C = 3.14 x 10 ^{-10} farads/cm G = 3.14 x 10^{-8}
mho/cm**

Note that **R** is 10^{11} larger than in the
previous case of the wire. The terms in equation 8 now have very different
numerical values. Although most of the crossterms remain too small to consider,
two crossterms are now dominant terms, and one of the primary terms, inductance,
**L**, is too small to sensibly include in a modelling equation. In
this regime, **L** is vastly smaller than the **R ^{2}C/4**
crossterm. Here is equation

**For this neural line case, the equation derived
by the old limiting**

**argument is terribly misleading.** The reasonable
modelling

equation to use is

rather than

This paper has shown a new technique for deriving differential
equations that is different from the usual one. It has put the new technique
and the standard technique into the context of J.C. Maxwell's thought.
It has illustrated the new technique, and argued for it, but has not proved
it. A companion paper works though in more formal detail why it is necessary
to use the notions of "length at a point", "area at a point",
"volume a point", and "a point in time" rather than
the incremental notions now used^{(28)}. Another
companion paper illustrates the invalidity of our limiting arguments by
example^{(29)}. A third companion paper illustrates,
by a numerical example, that new terms derived according to Maxwell's first
method can be (and very often are) far too small to matter quantitatively,
but that under some other circumstances, these new terms can be dominant
terms. In the case of neural transmission, consideration of the new terms
predicts effective inductances 10,000,000,000 and more times the effective
inductances predicted by Maxwell's second (and standard) method^{(30)}.

**Notes: **

1. Department of Curriculum and Instruction, School of Education, University of Wisconsin, Madison, U.S.A. (email: showalte@macc.wisc.edu)

2. Department of Mechanical Engineering, Stanford University, Stanford Ca. USA

3. Kline, S.J. (1995) **Conceptual Foundations for
Multidisciplinary Thinking** Stanford, Ca. Appendix C, p. 313.

4. Showalter, M.R. __A__ (1997) **Hypothesis:
dendrites, dendritic spines, and stereocilia have resonant modes under
S-K theory**.

5. See http://www.santafe.edu/~johnson and Science Forums for THE NEW YORK TIMES at http://www.nytimes.com/.

6. Johnson,
G. (1997) __Proteins Outthink Computers in Giving Shape to Life__**NEW SCIENTIST** March 25, 1997.

7. Showalter, M.R. ** Fully Hydrodynamic Piston
and Cylinder Assembly** U.S. Patent # 4,470,388, Sept 11, 1984.

8. Showaler, M.R., and Kline, S.J. __A__ (1997)
**COUPLED PHYSICAL FINITE MODELS INVOLVE DIMENSIONAL PARAMETERS AND MUST
BE SIMPLIFIED IN INTENSIVE FORM**

9. Showalter, M.R., and Kline, S.J. __B__ (1997)
**CONVENTIONAL LIMITING ARGUMENTS APPLIED TO PHYSICAL DIMENSIONAL MODELS
SOMETIMES MISINTERPRET TERMS**

10. Showalter, M.R. and Kline, S.J. __C__ (1997)
**Equations derived by Maxwell's first method restrict the range of applicability
of inferences from experiments.**

11. Maxwell, J.C. (1878) DIMENSIONS **Encyclopedia
Britannica, 9th ed. **

12. Showalter & Kline __A__ pp. 11-15.

13. **Showalter, M.R.** __B__ (**1997**)
**Reasons to doubt the current neural conduction model.**

14. Regan, D. (**1989**) ** Human Brain Electrophysiology**
(Elsevior, New York) pp. 103-110.

15. Mathematicians may prefer to say that the equation at differentially small scale is as close to a differential equation as it can be in a measurable physical domain, and that this "differential equation" can be mapped into the domain of the algebra on a symbol-for-symbol basis.

16. Kline, S.J. (1965, 1984) ** SIMILITUDE AND
APPROXIMATION THEORY** McGraw Hill, New York; Springer-Verlag, ?????.

17. Showalter & Kline __A__ pp. 12-14.

19. Boyer, C. B. (1949, 1959) ** The history
of the calculus and its conceptual development. (The concepts of the calculus)**
with a foreword by Richard Courant. -- Dover, New York.

20. **Webster's Third New International Dictionary**,
unabridged, P.B.Gove, ed, Merriam-Webster, Springfield, Mass.

21. __The International Dictionary of Applied Mathematics__
Van Nostrand, Princeton N.J. 1960.

23. Showalter & Kline __A__ pp. 12-14.

24. Showalter & Kline __B__ pp. 5-12.

26. Showalter & Kline __A__ p. 22.