recruit vs sign on

recruit

verb
  • To enroll or enlist new members or potential employees on behalf of an employer, organization, sports team, the military, etc. 

  • To become an adult or breeding-age member of a population. 

  • To prompt a protein, leucocyte. etc. to intervene in a given region of the body. 

  • To supply with new men, as an army; to fill up or make up by enlistment; also, to muster 

noun
  • A hired worker 

  • A person enlisted for service in the army; a newly enlisted soldier. 

  • A new adult or breeding-age member of a certain population. 

  • A supply of anything wasted or exhausted; a reinforcement. 

sign on

verb
  • To join something, after signing. 

  • To begin broadcasting a radio or television signal, usually at the beginning of a broadcasting day and after being off the air for several hours. 

  • To commit oneself, as to a project, a goal, an organization, a cause. 

  • To apply to receive unemployment benefits. 

  • To log on; to start using a computer, radio, etc., or to start talking. 

noun
  • The time of day when a radio or television station begins broadcasting, usually after being off the air for several hours. 

How often have the words recruit and sign on occurred in a corpus of books? (source: Google Ngram Viewer )