Voi,. 6, 1920 
PHYSICS: A. L. FOLEY 
311 
to insure the identification of their shadows on the photographic plate. 
The rotational speed of the disk was measured by means of a calibrated 
Van Sicklen-Elgin tachometer. A light-tight box encloses the spark 
gaps I and S, the dry plate P and the disk D. 
FIG. ] 
When a spark occurs at S a cylindrical sound wave is generated and a 
shadow of the saw teeth is thrown on the photographic plate. Assuming 
that the time interval between S and I has been adjusted properly the 
spark at I casts on the dry plate a shadow of the sound wave and the teeth 
1 — 1 — 1 — 1 — 1 — 1 — 
YsSPACE IN MM 
0 STRONG SPARK 
• VUFAK <>PAPK 
111 
X=TIME X 10" 
1 1 1 1 
X 
0 Z A Of 8 10 IZ 14 le 18 20 21 24 2&28 30 32 34 
FIG. 2 
of the disk, the latter having moved into a new position during the interval 
between the sparks. The true radius of the sound wave is calculated 
from the geometry of the figure and the radius as measured on the photo- 
graphic plate. The time interval is calculated from the measured dis- 
tance through which any given tooth has turned, together with the speed 
