I referred the Wikipedia and Advance Chemistry by Philip Matthews, for having a much comprehension on the down cell, but there was a opposition while reading the both references.
Wikipedia states that,
Although theory predicts that a potential of a little over 4 volts should be sufficient to cause the reaction to go forward, in practice potentials of up to 8 volts are used. This is done in order to achieve useful current densities in the electrolyte despite its inherent electrical resistance. The overvoltage and consequent resistive heating contributes to the heat required to keep the electrolyte in a liquid state.
But the Advance Chemistry book by Philip Matthews (page 521) states that,
A large current is passed through the cell, but at a low voltage. This has the effect of both discharging the sodium effectively and heating the mixture so that it does not crystallize
The both references have a opposition at the fact of amount of voltage to heat generating.
Which one is correct? Also I hope an answer with the reasons of how does the voltage affect on downs cell, with the heat generating. Thanks