For what purpose did John Clay get a job with Wilson?

"Redhead Union"

The Red-Headed League by Sir Arthur Conan Doyle

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How does Mr. Wilson get the job in the Red-Headed League?  

Jabez Wilson is the victim of an elaborate scheme to get him out of his shop during the day so that his new employee John Clay, who calls himself Spaulding, can work on a long tunnel from Wilson's basement to the bank where a large amount of gold coins are being stored. Clay and his henchmen run a newspaper ad seeking a replacement for an opening in the Red-headed League, and Clay calls Wilson's attention to it. When Wilson gets to the office of this bogus Red-headed League, there are many red-headed men waiting to be interviewed; but of course Clay and his confederates have predetermined that only Wilson will get the job, which involves copying articles out of the Encyclopedia Britannica. Wilson is characterized as a gullible and simple-minded man, as he would have to be in order to fall for such a hoax. He is told that the job is his because of the exceptional color and quality of his red hair. His job is abruptly terminated when the burglars are finished with the tunnel and ready to break into the storage vault of the bank. Wilson is an amusing character who has made this one of Sir Arthur Conan Doyle's most popular Sherlock Holmes stories.