Name the material in which no Thomson effect is observed.
A) $Pb$
B) $Fe$
C) $Ni$
D) $Au$
E) $None$

Hint: The evolution or absorption of heat when an electric current passes through a circuit composed of a single material that has a temperature difference along its length. This transfer of heat is superimposed on the common production of heat associated with the electrical resistance to currents in conductors. If a copper wire carrying a steady electric current is subjected to external heating at a short section while the rest remains cooler, heat is absorbed from the copper as the conventional current approaches the hot point, and heat is transferred to the copper just beyond the hotpoint.

Complete step by step solution:
There are two types of Thomson effects. Positive Thomson effect and negative Thomson effect. In the positive Thomson effect, it is found that the hot end is at high potential and the cold end is at low potential. Heat is evolved when current is passed from hotter end to the colder end and heat is absorbed when current is passed from colder end to hotter end. The metals which show the positive Thomson effect are $Cu$ , $Sn$ , $Ag$ , $Cd$ , $Zn$ , etc.
In the negative Thomson effect, it is found that the hot end is at a low potential and the cold end is at a higher potential. Heat is evolved when current is passed from colder end to hotter end and heat is absorbed when current is passed from hotter end to the colder end. The metals which show the negative Thomson effect are $Fe$ , $Ni$ , $Bi$ , $Pt$ , $Hg$ , etc.
$Pb$ is the element that shows neither positive nor negative Thomson effect. If a potential difference is applied on $Pb$ then it is found that heat is neither evolved or absorbed. Therefore the element $Pb$ shows no Thomson effect. While all remaining elements Fe, Ni, and Au show the Thomson effect.

Hence the correct option is A.

Note: The amount of heat evolved or absorbed when one-ampere current flows for one second in metal between two points which differ in temperature by one degree Celsius is zero when that metal is $Pb$ . And this amount of heat for other metals which shows the Thomson effect is called the Thomson coefficient. Always remember whenever the Tomson effect is zero for metal then the metal does not show the Thomson effect.