166 



BELL SYSTEM TECHNICAL JOURNAL 



used on ordinary "wax." That this would be so is obvious from the 

 fact that the vertical pressures between the point of the needle and 

 the record in an ordinary phonograph are of the order of 50,000 pounds 

 per square inch. Obviously any such pressures would destroy a 

 groove cut in soft "wax." These high pressures have been necessary 

 in order that the groove might properly drive the needle point of 

 the reproducer. Reduction of this pressure requires reduction of the 

 impedance offered by the needle point to transverse vibration. 



_ ttNR' 

 ""' 24 Vo 

 Tm= MAX. PLAYING TIME IN MINUTES 

 N = GROOVES PER INCH 

 R = OUTSIDE GROOVE RADIUS-INCHES 

 Vo = MIN. LINEAR SPEED-FT PER MINUTE 

 r = INSIDE GROOVE RADIUS-INCHES 



Pig 6 — Relation between playing time and rate of rotation of disc for various 



values of R{,R = 2r). 



The design of a suitable "wax" "playback" requires reduction of 

 both the mass and the stiffness of the reproducing system to a 

 minimum. In the past such "playbacks" have failed to reproduce 

 the higher and lower frequencies with much satisfaction. The device 

 shown in Fig. 7 represents a large advance toward ideal reproduction. 



► •»tSi:^SW!rtfX%it9im 



mtm 



Fig. 7 — Playback pickup for "wax" records. 



The response of such a device when driven by a "wax" record recorded 

 at constant velocity over the frequency range is shown in Fig. 8. 

 This reproduction is not widely dissimilar from that obtained from 

 finished records with the best electric pickups now commercially 

 available and is sufficiently good to serve as a very valuable criterion 

 in judging the quality of the record. The record may be played a 

 number of times without great injury. The extent of the injury is 

 indicated by the frequency characteristics obtained on successive 



