Motion Picture News (Oct-Dec 1930)

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42 Mot, Picture News November 29 , 1930 fTHE" Projectionists' Round Table WHILE it is difficult to imagine such a thing, a hot filament throws off particles of negative electricity and, if we were to set up a measuring station a small distance from the emitter, we would note a cloud of these particles around the emitter. The territory surrounding such an emitter is in a state which is of interest because it has a very definite bearing upon the operation of the three and four-element vacuum tube. The fact that the free electron leaves the filament, or the emitter, is due to sufficient acceleration being provided by the temperature of the filament or the emitter. But where does it go after it leaves the emitter? While sufficient acceleration has been provided for it to leave the filament, the speed of the electron is still not sufficient to cause it to move a great distance from the emitter. At the same time, the greater the number of -i H 0 B FIG. IOO electrons, or charges of negative electricity, that leave the emitter, the greater is the positive charge accumulatd by the emitter. Hence, there is an attraction by the emitter upon some of the electrons. These two actions cause the formation of a cloud of electrons around the emitter. We shall refer to this state a little later. The method used to heat the emitter to the proper temperature is of little consequence. Actually, the activating agent depends solely upon the type of tube used. One general type of vacuum tube employed for amplification and the conversion of alternating currents into direct currents, makes use of a filament which may be heated by direct or alternating current. As far as the actual emission of electrons is concerned, the type of current employed to heat the filament has very little bearing upon the action of the tube. As will be shown later, it does influence the mode of application of the tube, but if the effective value of alternating current is the equal of a certain value of direct current, the rate of emission of electrons remains unchanged. In connection with the two forms of excitation current, it is necessary to mention that certain types of filaments are more suitable for use with alternating current. At the same time, direct current may be applied to these filaments without any change in emissivity. Another form of emitter secures its excitation by virtue of the transmission heat gener By John F. Rider ated in a heater. In other words, indirect heating is employed. This type of structure is known as the heater-cathode type of tube, wherein a filament is employed to heat a cathode sleeve, the former serving purely as a heater and the latter as the electron emitter. In connection with electron emitters in general, the source of electrons is usually referred to as the cathode. Thus, in the filament type of tube, the filament is the cathode and, in the heater type of tube, the sleeve surrounding the heater is the cathode. Examples of filament types of tubes are all Western Electric tubes, the 345, the 350, the 326 and the 371. Examples of indirect heater tvpes of tubes are the 327 and the 324. The Plate. Investigators during the 19th century noted that if an incandescent filament and a plate were located within an evacuated chamber and, if the plate were given a positive charge with respect to the filament or the cathode, current would flow through the external circuit. Such a system is shown in Figure 100, where F is the filament and P is the plate. The filament battery is A and the means of applying a positive charge to the plate P is the plate battery B. The current indicating instrument is G. Further researches showed some very significant facts relating to this circuit arrangement. First, was the polarity of the B battery. With a constant value of filament potential, current was indicated upon G, only when the + end of the B battery was connected to the plate of the tube. When the — side was connected to plate and the + end to the filament, the current meter G indicated zero. Therefore, the relation of the charges upon the plate and the filament became known and definite. Since there was no metallic link between the filament and the plate within the tube and since current flow was indicated through the external circuit, it was evident that current flow was existent between the filament and the plate within the tube; that, under certain conditions when the plate was positive, the electrons emitted by the filament traveled across the same intervening between the filament and plate and moved to the plate, constituting the filament-plate current, generally referred to as the plate current. At the same time certain forms of varying the plate current were noted. Since the attraction of the emitted electron by the plate is a l l l l 5 " ' 5 z> -' / i u r0 / C.iHPCNT LD CWST , J 5 " ' function of the positive charge applied to the plate or the value of the plate voltage, any change in plate voltage will cause a change in the number of electrons which arrive at the plate. This is so because of the change in the attracting power of the plate. On the other hand, any change in the value of the filament battery changes the temperature of the filament. Therefore, its emissivity characteristic reduces or increases the number of electrons available for passage to the plate. Thus, the plate current is not only a function of the plate potential, but also of the filament potential, current or temperature. However, we must recognize certain definite limitations in both instances. Assuming a very definite value of filament current, the emission of electrons at that value of current is very definitely limited. If the emission is little or great, it is still limited. Since the number of emitted electrons per second are limited, the emission current is limited; therefore, the number which may reach the plate per second to constitute the plate current is finite. Starting with zero positive plate voltage, the plate current will be zero. As the plate voltage is increased, the attracting power of the plate is increased and more and more of the electrons emitted by the filament will be pulled toward the plate. Consequently; the plate current will increase. However, one cannot continue increasing the plate voltage to an infinite value and hope to attract an infinite number of electrons. There are two factors which produce two critical values of plate voltage. The lower of these is that which causes plate current saturation. Jn other words, for a certain value of attracting force, or plate voltage, all of the available electrons emitted by the filament per second arrive at the plate. Increasing the plate voltage beyond this point does not increase the plate current because there are no longer any additional electrons available. A fixed amount being emitted by the filament per second and all of these arrive at the plate. This relation may be expressed as the plate voltage-plate current characteristic with constant filament potential and may be illustrated as shown in Figure 101. This curve shows the plate current in milliamperes upon the left-hand ordinate and the plate voltage is indicated upon the abscissa As is evident in the curve, the critical plate voltage for whatever value of filament voltage, current or temperature is being applied, is about 80 volts. This means a plate current of about 24 milliamperes. Increasing the plate voltage to 100 volts produces very little increase in plate current. In other words, the plate current curve flattens out. Now from this curve, we can glean the information that the length of the plate current curve is a definite function of the filament emission. If the emission is reduced by decreasing the filament current to say .5 ampere instead of the 1 ampere originally applied, the point of saturation may be reduced to 35 volts instead This is Lesson 27 in The Rider Series on Sound Projection