No public utility shall grant any free service, nor charge or receive a greater or lesser or different compensation for any service rendered to any person, firm or corporation than the compensation fixed for such service by the schedules on file with the commission and in effect at the time such service is rendered.
RSA 378:14
1913, 99:2. PL 242 :15. RL 292:15. 1951, 203:46 par. 14, eff. Sept. 1, 1951.