30 THE STUDY OF SPEECH CURVES. 



matching curves when the apparatus has been stopped, and it is desirable 

 as a constant test of the rate of rotation. 



It is necessary to know the relations of magnification between the 

 traced curve and the vibration on the disc. For the X-axis of the tracing 

 the distance corresponding to one revolution of the disc is found by measur- 

 ing the distance on the tracing between any two corresponding checks 

 of" the time-marker. If the rate of rotation in the original recording 

 machine is known (it is usually 75 per minute), the "time equation" for 

 the tracing can be found. For example, if the distance between two Uke 

 checks is 2024mm. and the disc made one revolution in 4 of a minute 

 or in 0.83., the time equation for the X-axis will be 



1mm. = 0.8 - 2024 = 0.000357s., 

 or, say, lmm.=0.0004s. It is desirable to sound a tuning-fork just before 

 and just after taking a record on a gramophone; the curve of the fork then 

 gives the time equation directly. 



To obtain the magnification sidewise the tracing point is placed in 

 a groove and the apparatus is moved just enough to make a short line 

 on the drum; the point is then Ufted to the next groove and another short 

 line is made; it is then lifted to the original groove to give a third line. 

 The process is repeated a number of times, giving a series of short hues 

 alternating from left to right. The distances between the successive ends 

 of the lines are measured and averaged, whereby the variations due to 

 the vibration are eliminated. Divided by the distance apart of the gramo- 

 phone grooves this gives the magnification. To obtain the distance apart 

 of the grooves a chalk Une is made radially across the disc after the record 

 has been traced off; it is placed on a gramophone and allowed to turn 

 a large number of times; the distance over which the chalk has l^een 

 scratched off divided by the number of turns gives the distance apart of 

 the grooves. The distance apart can also be measured by a microscope 

 with ocular micrometer. 



The apparatus should be tested thoroughly. It is necessary, however, 

 to first determine how precisely the records can be measured. For 

 this, a piece of tracing has its amplitudes and wave-lengths measured in 

 the manner to be adopted for the whole tracing; for example, with a scale 

 in tenths of a millimeter read by a magnifying glass. The results are 

 placed in the record book that belongs with the apparatus. After these 

 results have been forgotten, the same tracing is again measured in the 

 same way. The two sets of measurements are then compared and the 

 degree of precision computed. 



