voter vs voting slip

voter

noun
  • Someone who votes. 

voting slip

noun
  • a piece of paper with a list of the candidates at an election (sometimes with an indication of the candidate's party); the voter puts an X in the corresponding place against the candidate of his choice, folds the slip and places it in a ballot box provided. 

How often have the words voter and voting slip occurred in a corpus of books? (source: Google Ngram Viewer )