CONTACT PHENOMENA IN TELEPHONE SWITCHING CIRCUITS 41 



ployed as early as 1926 in studies of contact sparking but it was very 

 cumbersome in use, often introduced artificial conditions into the 

 circuit of the contacts, and progress with its use was necessarily very 

 slow. During the past few years rapid advances have been made in 

 the development of glass envelope cathode ray oscillograph tubes. 

 By employing the latest types of tubes, and combining them when 

 necessary with wide band high-frequency amplifiers and with circuits 

 which permit synchronization of the tube sweep circuit with the con- 

 tact operation, it has been possible to make thousands of observations 

 in the time originally taken by a single oscillogram, and to cover the 

 entire range of currents, voltages, and frequencies involved. We now 

 have available means which will permit the visual observation of 

 transient voltages at frequencies as high as 400 megacycles per second, 

 and transient currents with components reaching 20 megacycles per 

 second. Single pulses lasting a small fraction of a microsecond, and 

 complex transients containing components as high as 5 megacycles, 

 can be clearly resolved and photographed while the envelopes of still 

 higher frequencies can be recorded. 



In order to study the transients at contacts operating at 50 volts 

 and steady currents under one ampere, in common types of telephone 

 circuits, voltages as high as 2000 and currents reaching 20 amperes 

 must be within the range of the apparatus. A detailed description of 

 the apparatus will not be attempted in this article, but the results of 

 observations made with it and photographs of the more significant 

 transient components will be presented. 



Study of the currents requires an amplifier as an impedance matching 

 device and some circuit conditions make a shielded input transformer 

 necessary. An input impedance of from 0.4 to 2 ohms, a voltage gain 

 of about seventy-five times, and a substantially flat characteristic of 

 output versus input from 20 kilocycles to 20 megacycles are usually em- 

 ployed. Lower frequencies may be observed with other amplifiers 

 and the range from zero to 10,000 c.p.s. is studied by means of the 

 "Rapid Record" oscillograph. 



With earlier cathode ray tubes, beam currents of 40 microamperes at 

 5000 volts were employed. The latest tubes give a beam current of 

 about one milliampere at this voltage. A Leica camera with an F1.5 

 Xenon lens and ultra speed panchromatic film has been used in most 

 of the photographic work. The photography is complicated by the 

 presence in a single transient photograph of some components in which 

 the beam speed may be a thousand times as fast as it is in others. 

 However, beam speeds in excess of 200 kilometers a second are photo- 

 graphed, and a continuous sine wave of 5 megacycles frequency may be 



