Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST centimeters for the cell shown by Ives) and the pick-up of light flux is greater. Four anode voltage-current characteristics are drawn in Fig. 5; these cover the range from 0.05 lumen | to 0.5 lumen. If, now. we take this family of curves and, ! with a fixed anode voltage, such as the cell would have in practice, find the currents cor: responding to various illuminations, we secure j the rectilinear graph of Fig. 6, which shows : that within this range the photo-cell current l is directly proportional to the light falling on ! the cell. The voltage in this case is 70. The slope of the line is a measure of the sensitivity of the cell under the conditions then existing. A steeper curve would show greater sensitivity. The cell under discussion, with a voltage of 70 between the cathode and the anode, has a sensitivity of about 10 microamperes per lumen. The sensitivity of photo-cells is normally expressed in terms of microamperes per lumen, the amount of light used in the measurement being of the order which the cell will receive in use — a value of 0.1 lumen would be about right for Cell No. 2. As explained in the previous article on photo-cell circuits, the cell may be coupled to the associated vacuum-tube amplifier in any of the usual ways, as through a resistance or transformer. If a coupling resistance of 2 megohms is assumed, as in Fig. 7, an instructive calculation of the practical efficiency of the cell, the ratio, that is, of the electrical energy output to the light energy input, may be carried out. Light is known to have a mechanical equivalent, in the region of maximum visibility, of about 1.5 milliwatts per lumen. The sensitivity of the cell described above (No. 2) is about 10 microamperes per lumen in the circuit of Fig. 7. Assuming an input of 1 lumen, or 0.0015 watt, the electrical output energy is that corresponding to 10 microamperes through 2 megohms, which, by the application of 12R, is found to be 0.0002 watt. The electrical fight efficiency of the cell is then 0.0002 divided by 0.0015, or about 13 per cent. This demonstration, however, may be more interesting than rigorous. Testing This and That IN THE old days, when tubes did not oscillate as readily as they do now when you want them to the experimenter would frequently touch the grid terminal with his finger to find out whether the circuit was functioning or not. A dull thud in the phones as the finger was applied to the grid and again when it was removed indicated that oscillations were present. This is one of those simple tests which make laboratory work and trouble shooting less arduous, and it is still used in work on receiving sets. Of course, it is not applicable to transmitters, where the grid bias is often dangerously high. Somewhat later, in broadcast stations, the operators would tap the microphones at the beginning of a transmitting period and listen to the noise in the monitoring speakers for a check on the air. Of course speech input to the microphone gave a better test along the fine to the modulators, but one could not put it out on the air without attracting attention whereas the taps would mean nothing except to the insiders. Now the finger method is used for test purposes in still another application. In sound-movie systems it is necessary to test the apparatus before each performance, in addition 0.5 Lumen Lumen 50 100 + ANODE VOLTAGE Fig. 5 to the usual meter checks, by some simple audio method which will only take a few seconds. Where disc reproduction is used this is accomplished by tapping the needle of the phonograph pick-up gently, with all the amplifiers on and the projection room speaker going, but the house speakers, preferably, cut off. The tapping, amplified, is heard' in the speaker if everything is o.k. The proper loudness with the normal gain setting is soon learned. If the sound is taken off the film the . finger method is equally useful, although the application is somewhat modified. Before film is put in the machine, but with the exciting lamp in the sound head and all the vacuum tubes lighted, the finger is moved up and down through the beam of light where it enters the photo-cell. As it shuts off the fight and lets it through again characteristic clicks are heard in the speaker. The indication in that case is that there is nothing radically wrong between the film and the speakers. Of course if the test is made with the house speakers as to 6 UJ UJ is <c 6 g4 o 2 UJ Q O 0.1 0.2 0.3 0.4 0.5 INCIDENT LIGHT, LUMENS 0.6 Fig. 6 — This curve shows the lightcurrent characteristics of gas-filled photo-electric cell No. 2. The diagrams below show methods of connecting a photo-cell ivith an amplifier Amplifier input. february, 1929 Fig. 8 page 245 • well the security is even greater, but the volume should be kept down. When there is more time and scientific means are available, sound-movie machines are tested with special discs or films supplying tone at constant frequency and amplitude. Some of these provide a number of frequencies within the band which the system is expected to transmit. The overall frequency characteristic, as well as any periodic irregularities, such as flutter in the tones, caused by the gear system — a defect to which some film sound reproducers are liable — may thus be checked. The method corresponds to tone tests on a broadcast wire channel or amplifier system, except that the tone is supplied optically or mechanically instead of with an oscillating tube. But for everyday use nothing beats the linger tests. Photo-Cell Connections FIG. 3 on page 33 of November Radio Broadcast, showing the connection of a photo-cell to the first stage of an amplifier through a resistive coupling, is somewhat awkwardly drawn, and Fig. 8 in the present issue gives a more orthodox and complete picture. The circuit is the same, it will be noted, as a standard resistance-coupled amplifier. The polarizing voltage reaches the photo-cell anode through the resistance Bi, across which the audio voltage is taken off. Ci is a blocking condenser to confine the transfer to this audio variation. R2 is a leak for the first three-element tube, with the Cbattery at its base. R2 is usually several times as large as Ri, which may be of the order of 2 megohms. Philology in the Movies DR. DE FOREST, who got off the famous sneer about GraecoSchenectady designations when the terms "pliotron" and "kenotron" were first offered to the world, should have something to say about the present crop of names for sound devices in the movies. Leaving out the past nomenclature, in which the mortality is rather heavy as the promotors go back to playing the horses, the following synthetics were discovered in a cursory search of two moving picture trade papers: Phototone, Orchestraphone, Orchestrola, Duotone, Electrograph, Duplex-o-phone, Dramaphone, Cortellaphone, Bristolphone, Sonoratone, Phonofilm, BCA Photophone, Movietone, Vitaphone, Vocafilm, Synchraphone, Theatrephone, Moviephone, Tonefilm, Biophone, Cinephone. This leaves out the foreign legion as well as a considerable number of American devices which were not advertising in those issues. A few are big, synchronized outfits, but most of them are theatre photographs intended for use in the smaller houses as an unsynchronized accompaniment to the pictures — simply a twin turntable, switches, a volume control, amplifier, and a cone loud speaker or two. After extended calculation, in which I consulted the works of Poincare', Bertrand Bussell, Lobatchevski, and Weierstrass, I find that 1.363 x 1018 possible combinations of phone, tone, and film with various other words in the English language remain on tap, so that no alarm need be felt by those interested in the continuance of this educational activity.