When a condenser microphone such as described above is acted upon by sound waves the diaphragm moves in and out at the frequency of the sound waves. A potential of 180 volts DC is impressed across the plates, and a resistor of 20 to 50 million ohms resistance is in series with one of the leads. As the diaphragm is moved in and out by the action of the sound waves, the alternating current set up flows through the resistor and produces an alternating voltage across it. This alternating voltage is impressed on the grid of the first tube of a vacuum tube amplifier. The amplified signals are then used to operate the recording device.

63. Disc Recording.-In disc recording the output of the amplifier actuates a vibrating armature which has attached to it a sapphire stylus (cutting point). This stylus is placed on the surface of a rotating plate and cuts a wavy groove in its surface. This plate, while usually referred to as a wax plate, is in reality made of an insoluble soap. The plate is rotated at an absolutely constant speed of thirtythree and one-third revolutions per minute.

Any number of duplicates can be made from this wax record in the following manner:-The wax is coated with a fine powder of conducting material. It is then electroplated so as to give a metallic negative record called a "master." This master is again electro-plated after the surface has been suitably treated to permit an easy removal of the resultant positive plate. This positive is commonly called an "original." From the positive another negative is made, a metal mold called a "stamper." From it duplicate "originals" can be plated to make duplicate stampers. These successive plating processes involve no measurable injury to the quality of the record. By the custom of making a number of duplicates, the master is protected from accidents and wear to which it would be subject if used to make the finished record.

The stampers are used to press the final product, and as many as a thousand records can be made with one stamper. The material from which the records are made is called "record stock." This material must have a hard surface to resist wear, and must contain enough abrasive (wear producing material) to grind the needle quickly to a good fit. At the beginning of a run of a new needle the pressure on the record is very great because of the small area of the needle point. However, after a minute's wear the needle pressure is reduced to 50,000 pounds per square inch.

64. Synchronizing Sound on Disc. -When the wax record is made, the wax disc is rotated at a uniform speed of thirty-three and one-third r.p.m. and is driven by a synchronous motor. The camera is driven so as to have a film speed of ninety feet per minute by another synchronous motor operating from the same power supply as the motor which drives the wax disc. The recording disc and the camera are started at the same time, and after they get up to speed they are operated simultaneously. Since they are both driven by synchronous motors they will always have the same relative speed. Therefore, the sound and picture will always be in synchronism when reproduced if the film and disc are both started at the proper "start" position, unless there is a mishap such as the needle of the pick-up jumping the groove, or the film breaking and not being properly patched. Should the film break, it is necessary to have the same number of frames in the patched film as it had originally, if the sound and picture are to remain in synchronism. The addition of blank film to replace parts of the film which were torn results in disagreeable breaks in the picture.

3