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The CGS systems define units that measure the same thing but may have
conflicting dimensions. Furthermore, the dimensions of the
electromagnetic CGS units are never compatible with SI.
But if you measure charge in two different systems you have measured the
same physical thing, so there is a *correspondence* between the
units in the different systems, and `units`

supports conversions
between corresponding units. When running with SI, `units`

defines all of the CGS units in terms of SI. When you select a CGS
system, `units`

defines the SI units and the other CGS system
units in terms of the system you have selected.

(Gaussian) You have: statA You want: abA * 3.335641e-11 / 2.9979246e+10 (Gaussian) You have: abA You want: sqrt(dyne) conformability error 2.9979246e+10 sqrt_cm^3 sqrt_g / s^2 1 sqrt_cm sqrt_g / s

In the above example, `units`

converts between the current
units statA and abA even though the abA, from the EMU system, has
incompatible dimensions. This works because in Gaussian mode, the abA
is defined in terms of the statA, so it does not have the correct
definition for EMU; consequently, you cannot convert the abA to its EMU
definition.

One challenge of conversion is that because
the CGS system has fewer base units, quantities that have different
dimensions in SI may have the same dimension in a CGS system. And
yet, they may not have the same conversion factor. For example, the
unit for the *\(E\)* field and *\(B\)* fields are the same in the
Gaussian system, but the conversion factors to SI are quite
different. This means that correct conversion is only possible if you
keep track of what quantity is being measured. You cannot convert
statV/cm to SI without indicating which type of field the unit
measures. To aid in dimensional analysis, `units`

defines
various dimension units such as ‘`LENGTH`’, ‘`TIME`’, and ‘`CHARGE`’ to be the
appropriate dimension in SI. The
electromagnetic dimensions such as ‘`B_FIELD`’ or ‘`E_FIELD`’ may be useful
aids both for conversion and dimensional analysis in CGS. You
can convert them to or from CGS in order to perform SI conversions
that in some cases will not work directly due to dimensional incompatibilities.
This example shows how the Gaussian system uses the same units for all
of the fields, but they all have different conversion factors with
SI.

(Gaussian) You have: statV/cm You want: E_FIELD * 29979.246 / 3.335641e-05

(Gaussian) You have: statV/cm You want: B_FIELD * 0.0001 / 10000

(Gaussian) You have: statV/cm You want: H_FIELD * 79.577472 / 0.012566371

(Gaussian) You have: statV/cm You want: D_FIELD * 2.6544187e-07 / 3767303.1

The next example shows that the oersted cannot be converted directly to the SI unit of magnetic field, A/m, because the dimensions conflict. We cannot redefine the ampere to make this work because then it would not convert with the statampere. But you can still do this conversion as shown below.

(Gaussian) You have: oersted You want: A/m conformability error 1 sqrt_g / s sqrt_cm 29979246 sqrt_cm sqrt_g / s^2 (Gaussian) You have: oersted You want: H_FIELD * 79.577472 / 0.012566371

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