24 THE STUDY OF SPEECH CURVES. 



turned in the "revolving barrel" by means of the projecting screw-head 

 in the slot; it feeds through a brass "nut" attached to the frame. The 

 under side of the metal disc carries a hard rubber insulating plate through 

 which 10 sets of metal pins have been driven into the metal disc; the 

 sets of 2, 3, 4 pins are shown in the figure. On the carriage is the hard 

 rubber "contact support" carrying a copper brush which rests against 

 the hard rubber plate. The spring and the frame of the apparatus are 

 placed in circuit with a battery and a magnetic marker (not shown) ; the 

 marker is adjusted to record on the smoked paper beside the lever; as 

 the disc turns, the pins make contact with a copper brush and a series 

 of 1, 2, 3 ... 10 checks is recorded for each revolution. The screw 

 for side movement should have the same number of threads as the gramo- 

 phone spiral; several interchangeable screws with their nuts are provided. 

 Bevel gears can not be used in the rotator, as the unevenness of the teeth 

 causes irregularity in the rotation. An end view of the rotator (figure 20) 

 shows the metal plate with the gramophone disc on it and the rubber 

 disc with the contact pins below. The thoroughly braced supporting 

 arm carries the lever holder so that the tracing point rests in a groove 

 on the gramophone disc. 



A view of the tracing end of the lever is given in figure 21. The 

 rotator has upon it the gramophone disc. The lever is carried by a metal 

 piece containing a fixed steel center below and a pointed steel "center 

 screw" above. The two points are inserted in a little "vertical axle." 

 The center screw is turned until the movement of the lever on the axle 

 is perfectly tight and yet perfectly loose; the screw is fixed by a little 

 "jam nut." The fine "tracing point" rests in the groove; its side move- 

 ments force the " tracing lever" to move. The adjustment of the lever 

 on the vertical axle is a vital matter. The point of the center screw must 

 be exactly in the center of its diameter, and the screw hole must be so 

 placed that its center is exactly opposite the fixed center below. The 

 holes in the vertical axle must also be exactly true. Any deviations of 

 the centers or the holes from an exactly straight line produce differences 

 in friction at different points of the movement. The surfaces of the centers 

 and the holes must he. true cones and rings. Differences in friction pro- 

 duce differences in the movement of the lever for the same movement 

 in the speech groove. Defects in the construction thus falsify the curves. 



The necessity for perfect tightness with perfect looseness arises from 

 the length of the long arm of the lever as compared with that of the short 

 arm. Any looseness allows the tracing point to make movements not 

 recorded on the paper. Any stiffness causes the supports to bend, although 

 they are made with such stoutness that bending seems impossible. 



