Section 30. Special policemen of corporation operating signal systems

The
  police department or board of police of any city may, in addition to the
  police force now authorized by law, appoint a  number  of  persons,  not
  exceeding  two  hundred,  who  may  be  designated  by  any  corporation
  operating a system of signaling by telegraph to  a  central  office  for
  police  assistance,  to act as special patrolmen in connection with such
  telegraph system. And the persons so appointed shall, in and about  such
  service,  have  all  the  powers possessed by the members of the regular
  force, except as they may be limited by and subject to  the  supervision
  and  control  of  the police department or board of police of such city.
  No person shall be appointed such special policeman who does not possess
  the qualifications required by such police department or board of police
  for such special service; and persons so appointed shall be subject,  in
  case of emergency, to do duty as part of the regular police force of the
  city.  The  police  department  or  board  of police shall have power to
  revoke any such appointment at any  time,  and  every  person  appointed
  shall  wear a badge and uniform, to be furnished by such corporation and
  approved by the police department or board of police; such uniform shall
  be designated at the time of the first  appointment  and  shall  be  the
  permanent uniform to be worn by such special police, and the pay of such
  special patrolmen and all expenses connected with their service shall be
  wholly  paid  by  such corporation, and no expense or liability shall at
  any time be incurred or paid by the police department or board of police
  of any city, for or by  reason  of  the  services  of  such  persons  so
  appointed.