Question: The Joule effect was named after the physicist who studied the nature of what?

Context paragraphs:
James Prescott Joule: James Prescott Joule FRS HFRSE DCL LLD ( ; (24 December 1818 11 October 1889) was an English physicist, mathematician and brewer, born in Salford, Lancashire.  Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy).  This led to the law of conservation of energy, which led to the development of the first law of thermodynamics.

Joule–Thomson effect: In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin effect, Kelvin–Joule effect, or Joule–Thomson expansion) describes the temperature change of a "real" gas or liquid (as differentiated from an ideal gas) when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment.  This procedure is called a "throttling process" or "Joule–Thomson process".  At room temperature, all gases except hydrogen, helium and neon cool upon expansion by the Joule–Thomson process; these three gases experience the same effect but only at lower temperatures.

Linderman effect: The Linderman effect is a direct statistical correlation between the effect of a third party acting on multivariable data versus the mean difference of the standard deviation from known reference values.  It is assumed that the Linderman effect, acting independently, does not cause noticeable changes to the standard deviation.  The Linderman effect occurs mainly in groups of two or three individuals working within a high stress environment.  It has been noted from preliminary tests, that the Linderman effect mainly occurs in academic fields such as chemistry, biology, and physics.  The Linderman effect has not yet been studied in other high stress environments such as a rescue worker or a person in the medical profession.

Joule effect: Joule effect and Joule's law are any of several different physical effects discovered or characterized by English physicist James Prescott Joule.  These physical effects are not the same, but all are frequently or occasionally referred to in literature as the "Joule effect" or "Joule law" These physical effects include:

Joule: The joule ( ); symbol: J), is a derived unit of energy in the International System of Units.  It is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre (1 newton metre or N⋅m).  It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.  It is named after the English physicist James Prescott Joule (1818–1889).

Cis effect: In inorganic chemistry, the cis effect is defined as the labilization (making unstable) of CO ligands that are "cis" to other ligands.  CO is a well-known strong pi-accepting ligand in organometallic chemistry that will labilize in the "cis" position when adjacent to ligands due to steric and electronic effects.  The system most often studied for the "cis" effect is an octahedral complex M(CO)X where X is the ligand that will labilize a CO ligand "cis" to it.  Unlike "trans" effect, where this property is most often observed in 4-coordinate square planar complexes, the "cis" effect is observed in 6-coordinate octahedral transition metal complexes.  It has been determined that ligands that are weak sigma donors and non-pi acceptors seem to have the strongest "cis"-labilizing effects.  Therefore, the "cis" effect has the opposite trend of the "trans"-effect, which effectively labilizes ligands that are "trans" to strong pi accepting and sigma donating ligands.

Gough–Joule effect: The Gough–Joule effect (a.k.a. Gow–Joule effect) is originally the tendency of elastomers to contract when heated if they are under tension.  Elastomers that are not under tension do not see this effect.  The term is also used more generally to refer to the dependence of the temperature of any solid on the mechanical deformation.

Units of energy: Because energy is defined via work, the SI unit for energy is the same as the unit of work – the joule (J), named in honor of James Prescott Joule and his experiments on the mechanical equivalent of heat.  In slightly more fundamental terms, 1 joule is equal to 1 newton metre and, in terms of SI base units

Theory of heat: In the history of science, the theory of heat or mechanical theory of heat was a theory, introduced in 1798 by Sir Benjamin Thompson (better known as 'Count Rumford'), and developed more thoroughly in 1824 by the French physicist Sadi Carnot, that heat and mechanical work are equivalent.  It is related to the mechanical equivalent of heat.  Over the next century, with the introduction of the second law of thermodynamics in 1850 by Rudolf Clausius, this theory evolved into the science of thermodynamics.  In 1851, in his "On the Dynamical Theory of Heat", William Thomson outlined the view, as based on recent experiments by those such as James Joule, that “heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect.”

Pockels effect: The Pockels effect (after Friedrich Carl Alwin Pockels who studied the effect in 1893), or Pockels electro-optic effect, changes or produces birefringence in an optical medium induced by an electric field.  In the Pockels effect, also known as the linear electro-optic effect, the birefringence is proportional to the electric field.  In the Kerr effect, the refractive index change (birefringence) is proportional to the square of the field.  The Pockels effect occurs only in crystals that lack inversion symmetry, such as lithium niobate or gallium arsenide and in other noncentrosymmetric media such as electric-field poled polymers or glasses.

Supporting paragraphs: Joule effect James Prescott Joule
Answer: heat