International photographer (Jan-Dec 1934)

October, 1934 The INTERNATIONAL PHOTOGRAPHER Three By CHARLES FELSTEAD, Associate Editoi cated value at all times during the making of sound takes. The Electromagnet The assembly of electromagnet and light valve — the combination which constitutes the light beam modulating device — is mounted between the lamp house and the objective lens system attached to the end of the exposure chamber. Incidentally, it must be explained that this objective lens system serves to focus the light beam onto the sound track strip of the film after the beam has passed through the light valve. In other words, the objective lens system functions in much the same manner as the condensing lens system. But whereas the condensing lenses focus the light from the lamp onto the slit between the ribbons of the light valve, the objective lenses focus the modulated light from the light valve onto the film. The field winding of the electromagnet, commonly known as the magnetizing winding, is a large multilayer coil of wire wound around the center leg of an Eshaped core. The open part of the E faces toward the lamp house. The end of this center leg of the core, which is shorter than the other two legs, is wedge-shaped, with the long way of the wedge on the same plane as the plane of the light valve ribbon. There is an opening in the face of this wedge that is eight mils wide by 256 mils long; and this opening is part of a slot that expands toward the back of the E to form an opening 204 mils wide by 256 mils long facing the objective lens system in the end of the exposure chamber. After the electromagnet assembly has been lined up at the factory in exactly the right position, it is fastened securely in place and the screw heads covered with sealing wax. This is to prevent any shifting in the alignment of the slot through which the light beam shines. The two lens assemblies are likewise se MOINITOR ROOM To output or MONITOQUMO CQUIPMFMT AMPLIFIER poor"! recording iattcnuator; magnetizing winding at the beginning of a take results in sound not being recorded on the film ; so a signal light is often connected across this circuit to serve as an indicator when the key switch is in the on position. If the lamp is not burning during recording, it behooves the operator to check the magnetizing winding circuit and the signal lamp. The PEC Monitoring Circuit The locations of the two PEC {photo-electric cell) amplifiers in the monitoring circuit will be evident from an examination of Figure 2. One of these amplifiers is provided for each recording machine ; and it is mounted directly on the frame of the machine in a metal box supported below the exposure chamber. Inside of each film recording machine, a photoelectric cell is mounted behind the moving film in line with the light beam that shines through the light valve and exposes the sound track on the film. Through the action of the light valve, as we shall see in a later chapter, the beam of light is modulated by the amplified speech current supplied to the light valve by the bridging amplifier. After exposing the sound track on the film in a degree directly proportional to the frequency and volume of the sound being picked up by the microphones, a portion of the light (in the order of four per cent) passes through the film and impinges on the sensitive surface of the photo-electric cell. Only those variations in the recording light that have passed through the film, and so have been recorded on it, the photo [>o^'; » (§, P£C<,Phoh>-el<?ctri(.c»ll) 4 REPfATfOU can thus affect electric cell. j/ 11 \A ^ LTFI ~vv o>— & pECORDINfa Roor-i * *-* DISC* B A -X fej D' PA* EL -X X— Q o * SOUND STACt c -DO Pf LAY NO 1 ' 1 1 " 'Iff 7fc -J X— <f No. 2 Block Schematic Diagram of Recording Channel cured against shifting or tampering by sealing wax. There is no control in the magnetizing winding circuit other than a key switch mounted on the panel on the wall, the winding being connected directly to a twelve-volt battery without a series resistance. Failure on the part of the recording operator to turn on the The Photo-Electric Cell This PE cell comprises an evacuated glass container, resembling an electric light bulb, in which there are two electrodes. Except for a circular window on one side to admit light, the interior surface of the glass is coated with a sensitive photo-electric substance. This forms one electrode, known as the cathode. Potassium was used at first for this coating, but now caesium has almost entirely replaced it. The other electrode is a ring of metal, insulated from the PE coating and supported in the center of the bulb. It is called the anode. The photo-electric cell is connected to the input (the grid-filament) circuit of the first tube in the PEC amplifier in the manner shown in Figure 3. The polarizing voltage provided by battery PB is placed across the two electrodes of the cell through a high resistance (500,000 ohms) marked PR. The positive terminal of the polar Please mention The International Photographer when corresponding with advertisers.