PRODUCTION OF TELEVISION SIGNALS 593 



that the space current of the last tube is a function of the alternating 

 current voltage impressed on the first, being roughly proportional to 

 the square of its amplitude. By means of a direct current milli- 

 ammeter, therefore, it is possible to keep a very accurate check on 

 the amplitude of the signal delivered to the line. 



Receiving Circuit 



Coming now to the receiving terminal equipment we find that the 

 signal wave which was delivered to the line at a power level of 10 

 milliwatts may, under some conditions, be reduced to a level 50 T U 

 below this, or to 0.1 microwatt. It is, therefore, necessary first of 

 all to provide amplification to bring the signal to a level where it may 

 operate the circuits controlling the illumination from which the image 

 is to be reconstructed. In view of the fact that several types of 

 receiving equipment are to be operated and also since the signal may 

 be derived from any of several sources, either wire line, radio or local 

 transmitting station, it is desirable to fix some one energy level as a 

 reference point and to bring all signals to this value so that they may 

 be supplied interchangeably to the several receiving systems. A 

 convenient reference level is that already set as the proper input to a 

 telephone line, namely, 10 milliwatts. At the receiving terminal, 

 therefore, amplifiers have been provided which are similar to the final 

 stages used at the transmitting terminal. These include units con- 

 taining the small-sized tubes and terminate in units identical with 

 that supplying current to the line except that the output transformer 

 is omitted. The first stage is, as mentioned in the preceding section, 

 connected to the line through an input transformer. The amplifiers 

 associated with the several incoming signals are each provided with a 

 level indicator of the type already described. These terminal ampli- 

 fiers and the several receiving circuits are all terminated in jacks, 

 exactly like telephone circuits, and it is possible, therefore, to connect 

 any receiving machine to any desired transmitting station simply by 

 patching the proper jacks together, exactly as telephone circuits are 

 connected at the central office. 



Before describing the final stages of the amplifier circuits it is 

 necessary first to examine the properties of the light source which is 

 to be controlled. In the case of the disk receiving machines described 

 in the first section of this paper it is recalled that a single neon lamp 

 is used having a rectangular electrode the entire area of which glows 

 at each instant with an intensity proportional to the intensity of the 

 initial luminous signal. The current voltage characteristic of a typical 

 neon lamp is given in Fig. 25. It will be seen that no current flows 



