334 
PHYSICS: C. BARUS 
obvious. It does not occur in the later work. While the fundamental 
showed a band-width of 5 scale-parts, the same for the octave was 42 
cm, wide, and for the next harmonic 26 cm. wide. 
The remarks made presuppose that the telephones are acting in con- 
cert, on opposite sides of the vibrator. We may refer to this as an 
arrangement in series. When the telephones are acting in opposition, 
the band- width, s, decreases to one or a few scale-parts, depending on 
the symmetry of adjustment, etc., which if perfectly made should throw 
the differential s out entirely. 
4. Observations with the Interferometer. — As has been stated the 
fringes are easily found because the rapid motion of the vibrator im- 
plies considerable damping. The slit image is thus quite stationary and 
the fringes clear and strong. On starting the inductor, the fringes at 
once vanish and after breaking the circuit slowly reappear, unless the 
vibration telescope is used. When the period of the latter differed 
from that of the induction (to be very weak), the even band of fringes 
changed to wave lines travelling in opposed directions and of continu- 
ally increasing amplitude. Eventually the crests or troughs only are 
seen and these but on one side (as some micrometer adjustment for one 
or the other will be necessary to keep them in the field) , again travelling 
in pairs, in opposite directions, through each other. On breaking the 
circuit these pulses, slowly coalesce into the wave bands and finally 
into the even band. 
The case here presented is that of a relatively slowly vibrating tele- 
scopic objective, at V. If the frequency of the latter can be counted, 
the frequency of the alternator may be deduced from the number of 
moving crests in the field. Thus when the frequency of the objective 
was n' = S, there were four crests in motion, implying a frequency of 
n = 20 for the interruptor of the coil. In this respect the case of different 
periods is advantageous. 
The net double amplitude of the waves measured by the ocular 
micrometer was about 4 scale-parts and ^ = 4 X 10~^ amperes came to a 
scale-part, in case of the present small fringes. 
The next step in advance consisted in adding an electromagnet, e; 
figure 2, at the objective of the vibration telescope F, in series with the 
electromagnet e of the interruptor, /. The two springs at V and /, 
moreover, were adjusted to about the same period. The electromagnet 
e' could be rotated on a vertical axis, so that by moving it closer to or 
further from the steel spring of V, any degree of band-widths was ob- 
tainable in the telescope. Magnificent octave fringes were obtained in 
this way. They moved merely on opening and closing the circuit. 
