§ 30. Special police officers 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 patrol officer 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 police officer 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 patrol officer 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.