set globali a parameter?

My prod_server version is 5.1.47 and want to set the slow query log dynamically for certain period of time. I refered one site(http://ift.tt/1F7DAjm) there is mentioned below 5.1.6 we need to set the variables into the my.cnf, but my prod_server is critical can't take my server restart is there any way we set variables dynamically for certain period of time?

Thank u in advance please help.