406 BELL SYSTEM TECHNICAL JOURNAL 



total length was 15.4 meters. A photo of the sliding bridge unit is 

 given in Fig. 1. 



The method of using such a Lecher system requires consideration. 

 If we set up a pair of parallel wires and feed energy from a generator 

 into them, we shall have, unless the far end of the wires is terminated 

 by the "surge" impedance of the line, a standing wave system set up. 

 This standing wave will be most pronounced when the outer end is 

 so terminated as to return all the energy arriving there and this 

 requires that the terminating impedance be a pure reactance. If the 

 extreme values of zero or infinite reactance be chosen, a current anti- 

 node or node will respectively occur at the far end. 



If the far end be reactively terminated and we observe the current 

 distribution while moving back towards the generator, we shall find 

 the standing wave persisting up to the generator itself. However, 

 if the line be dissipative, the returned wave will not completely cancel 

 the outgoing wave at phase equality locations and the current maxima 

 and minima will become less contrasty. If the line be practically 

 non-dissipative, the maxima and minima will not deteriorate as we 

 approach the generator. 



The returning wave is re-reflected at the generator end and traveling 

 to the far end returns once more, this process being repeated until 

 its amplitude has faded out. Usually the generator appears as a 

 resistive impedance when viewed from the line so that not much 

 energy survives reflection at this end. In any case, as the generator 

 is the primary energy source, the generator voltage introduced into 

 the line and the voltage of the re-reflected waves add vectorially to 

 give a component just sufficient to maintain the standing wave line 

 current. By an adjustment of the effective line length, either by 

 changing the physical length, the far end reactance, or the generator 

 impedance as viewed from the line, the power input to the line may 

 be maximized for the particular generator used. 



When this state of affairs has been attained, the standing wave 

 may be observed by either a current or voltage operated device 

 moved along the line. (If this device absorbs too much energy, it 

 becomes a source of disturbing reflections itself, complicating matters 

 by superposing on the original standing wave another pair of standing 

 waves. It is not advisable to permit such secondary waves to exist 

 in measurable amplitude.) Necessarily the standing wave is closely 

 sinusoidal and at the maximum values dlfdl = so that locating 

 these current extremes is not an accurate experimental process. The 



