Full Idea
Mill believed in an open vote. People should be held accountable for how they vote, and therefore it should be a matter of public record.
Gist of Idea
How people vote should be on public record, so they can be held accountable
Source
report of John Stuart Mill (Representative Government [1861]) by Jonathan Wolff - An Introduction to Political Philosophy (Rev) 3 'Representative'
Book Reference
Wolff,Jonathan: 'An Introduction to Political Philosophy (Rev)' [OUP 2006], p.97
A Reaction
Nowadays it is a mantra that voting should be secret, because coercion is an obvious problem, but MPs vote publicly, and are held accountable for their voting records. People like the mafia seem to make open public voting impossible.