The ModifyConstant( descriptor, value_eqn, opts ) command modifies the definition of a derived or nonderived physical constant in the ScientificConstants package producing a nonderived constant for the current session.

The ModifyConstant( descriptor, derive_eqn, derive_opts ) command modifies the definition of a derived or nonderived physical constant in the ScientificConstants package producing a derived constant for the current session.

To update a constant for all future Maple sessions, add the ModifyConstant command to your Maple initialization file. For more information, see Create Maple Initialization File.

In the derive_eqn equation of the form 'derive'=derive_obj, the derive_obj expression is typically a product of rational powers of numerics, Maple constants, and physical constant identifiers (for example, symbols). Exceptions are the abs function, and a sum with dimensionally consistent summands.

The derive_opts and opts arguments can contain one or more of the following equations that set the physical constant definition.

'check' = true or false

When 'check'=true, ModifyConstant checks whether the given symbol matches the name or symbol of a physical constant in the package. If it does, an error is returned.  The default value of 'check' is true.  When 'check'=false, ModifyConstant attempts to remove from the package any conflicting physical constant.  This is unsuccessful if any conflicting constant is currently used in the definition of any derived physical constants.

'symbol' = symbol

This option defines the symbol of the physical constant. It cannot match the name or symbol of a physical constant in the ScientificConstants package (unless the 'check=false' option is given, see below). If the $'\mathrm{symbol}'$ is modified, the descriptor must be the full name of the physical constant.

The opts argument can also contain one or more of the following equations that set the nonderived physical constant definition.

'uncertainty' = uncertainty_obj

This option redefines the precision to which the physical constant's value is known.

If the pre-existing value of the $'\mathrm{uncertainty}'$ is not undefined, this equation is required.

If the constant was previously defined as a derived constant, this equation is not required. The default value of the uncertainty is undefined.

The uncertainty_obj option must be of type constant (or be so after Constant() objects evaluate) or a list of the form $\left[\mathrm{uncer}\,\mathrm{uncertainty\_opt}\right]$ where uncertainty_opt is 'relative' or 'uld' and uncer is of type constant (or is after Constant() objects evaluate).

If no uncertainty option uncertainty_opt is included, the value represents the absolute uncertainty of the physical constant. That is, the value is measured in units determined by the 'units' option.

If uncertainty_obj is of the form $\left[\mathrm{uncer}\,'\mathrm{relative}'\right]$, uncer is the relative uncertainty in the physical constant's value.  The quantity uncer*value_obj is the absolute uncertainty of the physical constant.

If uncertainty_obj is of the form $\left[\mathrm{uncer}\,'\mathrm{uld}'\right]$, uncer is the uncertainty in "units in the least digit" in the physical constant's value.  The quantity $\mathrm{uncer}\mathrm{SFloatExponent}\left(\mathrm{value\_obj}\right)$ is the absolute uncertainty of the physical constant.  This form of uncertainty cannot be used with a non-float value_obj.

'units' = units_obj

This option redefines the units in which the value and uncertainty are measured. The units_obj option can be an expression that Units[Unit] interprets as a unit or a Unit() standard form. For more information, see Units.

This equation is optional.

$\mathrm{with}\left(\mathrm{ScientificConstants}\right)\:$

$\mathrm{GetConstant}\left(c\right)$

$\mathrm{speed\_of\_light\_in\_vacuum},\mathrm{symbol}=c,\mathrm{value}=299792458,\mathrm{uncertainty}=0,\mathrm{units}=\frac{m}{s}$

$\mathrm{ModifyConstant}\left(\mathrm{speed\_of\_light\_in\_vacuum}\,\mathrm{symbol}=c\left[0\right]\right)$

$\mathrm{GetConstant}\left(c\left[0\right]\right)$

$\mathrm{speed\_of\_light\_in\_vacuum},\mathrm{symbol}=c_0,\mathrm{value}=299792458,\mathrm{uncertainty}=0,\mathrm{units}=\frac{m}{s}$

$\mathrm{AddConstant}\left(\mathrm{Hooke1}\,\mathrm{symbol}=\mathrm{k1}\,\mathrm{value}=2.3\,\mathrm{units}=\frac{N}{m}\right)$

$\mathrm{GetConstant}\left(\mathrm{k1}\right)$

$\mathrm{Hooke1},\mathrm{symbol}=\mathrm{k1},\mathrm{value}=2.3,\mathrm{uncertainty}=\mathrm{undefined},\mathrm{units}=\frac{N}{m}$

$\mathrm{ModifyConstant}\left(\mathrm{k1}\,\mathrm{value}=2.3\,\mathrm{units}=\frac{N}{m}\,\mathrm{uncertainty}=\left[0.1\,\mathrm{relative}\right]\right)$

$\mathrm{GetConstant}\left(\mathrm{k1}\right)$

$\mathrm{Hooke1},\mathrm{symbol}=\mathrm{k1},\mathrm{value}=2.3,\mathrm{uncertainty}=0.23,\mathrm{units}=\frac{N}{m}$

$\mathrm{AddConstant}\left(\mathrm{mass\_ratio}\,\mathrm{symbol}=\mathrm{`m[e]/m[p]`}\,\mathrm{derive}=\frac{m\left[e\right]}{m\left[p\right]}\right)$

$\mathrm{evalf}\left(\mathrm{Constant}\left(\mathrm{`m[e]/m[p]`}\right)\right)$

$0.0005446170214$

$\mathrm{ModifyConstant}\left(\mathrm{mass\_ratio}\,\mathrm{symbol}=\mathrm{`m[p]/m[e]`}\,\mathrm{derive}=\frac{m\left[p\right]}{m\left[e\right]}\right)$

$\mathrm{evalf}\left(\mathrm{Constant}\left(\mathrm{`m[p]/m[e]`}\right)\right)$

$1836.152674$