168 BELL SYSTEM TECHNICAL JOURNAL 



different companies doing such work, although not in any fundamental 

 manner. The negative electroplate thus made may be used to hot- 

 press a molding compound such as shellac containing a finely ground 

 filler. This first electroplate is called a "master." From it two 

 test pressings are usually made. If satisfactory the matter is then 

 electroplated with a positive, being first treated so that this positive 

 plate may be easily removed. This positive is sometimes called an 

 "original." From this in turn is plated a metal mold or "stamper." 

 From these, duplicate "originals" may be plated and from them, 

 duplicate "molds" or "stampers." These processes involve no 

 measurable injury to the quality of the record and are comparatively 

 simple and extremely safe in practice. By this practice of making a 

 number of duplicates it is possible to safeguard the "master" and 

 insure against any accident which might destroy a valuable record. 

 From a single "stamper" it is not unusual to make a thousand finished 

 pressings. The time required for these operations is such that test 

 pressings are commonly obtained from the "wax" in 12 hours. Recent 

 refinements in the art have reduced the time required so that finished 

 records may, if necessary, be obtained in 3 hours after delivery of 

 the "wax." 



Hard Record or "Pressing" 



Various materials have been used in making the hard record or 

 "pressing." In some cases the material has been made homogeneous 

 and in others the surface is of a different material from that used in 

 the body of the record. Some have used a laminated structure. 

 There has not, however, been much latitude allowed the experimenter 

 concerned with materials for the hard record. The material has had 

 to be quite hard and, in order to show a reasonable life, it has had to 

 contain sufficient abrasive to grind the needle quickly to a good fit. 

 At the beginning of the run of a new needle due to the small bearing 

 surface, the pressures are very high. They rapidly decrease so that 

 with an ordinary loud steel needle after one minute's wear in the 

 ordinary phonograph, the bearing area is increased to such an extent 

 that the pressure is only about 50,000 pounds per square inch. As 

 the needle continues to wear to a larger bearing surface, the pressure 

 obviously continues to decrease. These high pressures and necessary 

 abrasive characteristics of the record have introduced irregularities 

 which are responsible for most of the extraneous noise commonly 

 known as "surface" or "needle scratch." 



The "pressing" copies the "wax" record with a very high degree 

 of accuracy so that if our attention be confined to frequency charac- 

 teristics alone, the "pressing" shows almost complete perfection. 



