DRS. GUY BARLOW AND H. B. KEENE ON THE ANALYSIS OF SOUND. 
135 
if it were required to reconstruct tlie wave-form of the vibration. It would not be 
difficult to adapt the method of analysis by interruption for the determination of phase- 
difference, but no experiments in this direction have yet been made. Hence at present 
the method is incomplete in that it fails to take account of wave-form. If the wave¬ 
form were required it would appear simpler to deal with it directly by means of an 
oscillograph method than to build it up from a complete analysis. 
Apparatus. 
Brass 
sUp 
hEbonLte, 
~ Shaft 
The interrupter (fig. 3) consisted of a brass cylinder made up of five discs ; the first 
was complete and served as a slip-ring, the other four contained" ebonite segments giving 
respectively 1, 4, 16 and 64 interruptions per 
revolution of the cylinder. Contact was made / 4'/6 64_ 
by means of two small brushes cut from - 2 \, mm. 
sheet brass, each brush possessing four or five 
separate fingers. One brush pressed lightly on 
the slip-ring, the other on whichever disc was 
the most convenient for the frequency under Bbo/ule 
examination. The electric contact was found Breuss 
to be satisfactory when the surfaces were kept 
clean and well lubricated with machine-oil. The 
cylinder, insulated with ebonite, was mounted 
directly on the shaft of an electric motor the 
speed of which could be regulated over the range 3-30 revs./sec. The four discs gave 
overlapping ranges of interruption frequencies with a total range of 3 to over 2000/sec. 
It was required that the rotation of the interrupter should be extremely uniform and 
perfectly under control. This is especially important for analysis at high frequencies ; 
thus for 1000/sec. an irregularity of rotation of 1 in 2000 would in one second completely 
reverse the phase of the response, and with a galvanometer of 3 sec. period the full 
value of the response would not be obtained. 
Much preliminary work was done in examining the conditions necessary for steadiness 
and smooth running of small motors. Two forms of apparatus have been constructed :—- 
Brass brushes 
Fig. 3. Interrupter. 
(1) A Siemens-Sch ticker t 12-volt i 1 (i h.p. motor was directly coupled to a fly wheel 
(radius 11 cm., mass 7|- kgm.) to prevent sudden changes of speed. This apparatus, 
which was used in nearly all the laboratory experiments, was suitable for exact measure¬ 
ments, as the motor could be made to run very slowly over any required small range 
of speed, and this range could be repeated by using the finger as a brake on the fly¬ 
wheel. Since plain lined bearings were used, there was the disadvantage that the 
ultimate speed attained was limited only by the work done in friction, and this varied 
with the state of the lubrication. The great weight of the flywheel made this frictional 
