Of special importance to the police service was the fact that by introducing this apparatus, it was not necessary to build a new network because now it became possible to rent normal telephone lines using single current, 40-milliampere, 60-volt current only. Siemens and Halske engineers, in 1925, had designed a teleprinter using the five-unit-code, start-stop principle. This was manufactured and improvements added, including the new start-stop method developed by Morkrum-Kleinschmidt. The Creed company, which had started manufacture of start-stop teleprinters before 1930, having purchased several Morkrum printers before the consolidation of that company with the Kleinschmidt company, also designed a teleprinter for the new five-unit-code, start-stop system. In another write-up, Dr. Grimsen states that after the Siemens company came into the picture, a fully automatic teleprinter network was started in 1933 with a trial installation between Berlin and Hamburg for about forty private subscribers. The results were so encouraging that the German Reichspost continued the work, and five years later the TELEX system contained about 10,000 subscribers. TELEX service was introduced in Great Britain in 1932 and was worked over the telephone network, using a single-tone voice frequency carrier signal which was keyed on and off by the teleprinter transmitter. (From Freebody’s _Telegraphy_.)[12] Interconnection for TELEX service was made by dialing a subscriber as in telephone operation. TELEX directories then, as now, gave the call numbers of subscribers. Types of answerback devices were designed in both England and Germany which finally developed into an arrangement whereby the calling subscriber would, after dialing a number, press a special key, the shifted D key—named the “Who-Are-You?” button—which would cause the transmission of a signal code automatically to activate the called teletypewriter mechanism into transmitting its TELEX number. Further apparatus, a device to punch the code in a tape and a punched-tape-controlled transmitter, was soon added. The Siemens company also developed and built completely automatic switching equipment for the TELEX system which is used by many telegraph administrations. Through the cooperation of the companies who were manufacturing teleprinters and associated equipment, the many problems that appeared were eventually solved to improve the apparatus and bring the TELEX system to eventual perfection for worldwide telegraph communications. (At the time of this writing, according to statistics, there are nearly 1,000,000 TELEX subscribers throughout the world.)