IMAGE TRANSMISSION SYSTEM 459 



parallel. These large tubes are used because their plate impedance 

 is of the same order of magnitude as the impedance of the neon lamp, 

 and because they will supply the necessary direct current to the neon 

 lamp without overheating. 



Figure 8 also shows what may be termed a voltage level diagram for 

 the whole system. Ordinates on this diagram represent voltage 

 amplitudes at the junctions between units of the system, and by 

 themselves tell nothing at all about the power conditions in the system, 

 since the impedances are not specified. It is interesting to observe 

 that the signal voltage produced by the three banks of photoelectric 

 cells has an effective value of about 50 microvolts across the 50,000 

 ohm input resistance; the transmitting amplifier delivers about 1 volt 

 to the 125-ohm cable circuit, and the receiving amplifier delivers 

 about 100 volts to the 1,000 ohm neon lamp circuit. The signal cur- 

 rent through the neon lamp has an effective value about a thous- 

 and million times greater than that of the current variation in one 

 of the photoelectric cells. 



The most outstanding contribution to the development of tele- 

 vision amplifiers is the combination of output and input transformers 

 whose transmission characteristics are shown in Fig. 9, A, and whose 

 impedance characteristics are shown in Fig. 9, B, and C. The ex- 

 ceptionally wide frequency range, corresponding to a ratio of. limiting 

 frequencies of 5,000 to 1, transmitted by these transformers is 

 due largely to the use of chrome permalloy, a recently developed 

 core material having very high permeability. The improved char- 

 acteristics are also the result of refinements in design which involve 

 the use of adjusted capacities and resistances to control the character- 

 istics at the higher frequencies. Due to the fact that each terminated 

 transformer looks like a resistance of 125 ohms over practically the 

 entire frequency range of the image signal, it makes no difference in 

 the form of the overall voltage amplification characteristic of the cir- 

 cuit whether the transformers are connected together directly or by 

 means of the equalized cable circuit whose characteristic in shown in 

 Fig. 10. Advantage of this circumstance is taken in providing switch- 

 ing means whereby each transmitting amplifier may be connected 

 through a resistance pad to its local receiving amplifier, enabling a 

 person to see his own image in the television booth, which is a conven- 

 ience in making apparatus adjustments. 



Transformers of this type must be carefully protected against 

 magnetizing forces which might cause polarization of the core material. 

 In order to keep the plate current of the final tube of the transmitting 

 power amplifier from flowing through the winding of the output 



