214 PHYSICS: C. BARUS 
To measure the v\^idth of the image band of Hght produced by the vibration, 
the ocular T should be on an axle t with slight friction. In my earlier impro- 
vised apparatus, single scale-parts (0.1 mm.) only could be guaranteed; but 
with a perfected optical system there is no reason why tenths of scale-parts 
should not be equally trustworthy. 'This makes a scale of 1000 parts in the 
ocular. 
3. Observations. — As a source or alternating current I selected a small induc- 
tion coil with a mercury break (Kohlrausch's design) to facilitate the initial 
tests. This was put in series with a rheostat (to 30,000 ohms), a Siemens's 
precision dynamometer reading to within milliamperes, an ordinary tele- 
phone to indicate the continuous action of the coil and the vibrator above 
described. 
The Siemens's dynamometer was first standardized with a Clarke cell. 
Accepting the effective current i as being 
2 = c 
where (p is the deflection on a scale at about 1 meter of distance, the constant 
C = 1.12 X 10~^ relative to amperes was found and the'mean resistance of the 
coils included about 250 ohms. Virtual currents of 10~^ amperes would escape 
detection. 
The coil was now started and measurements made simultaneously both at 
the Siemens's dynamometer and at the vibrating telephone (slit distance a = 
35 cm., ocular distance b = 75 cm.), with good results, the virtual currents of 
the dynamometer being compared with the width of the image band (in scale- 
parts) seen in the vibration telescope. To obtain different virtual currents, 
resistances from 10,000 to 2000 ohms were put into the circuit. The dyna- 
mometer showed deflections from 2 to 20 cm. on the scale. For larger deflections 
the coil current would have been too irregular for use. 
The vibrator indicated about 10 scale-parts per milliampere, and readings 
even beyond 5 or 10 milliamperes would be possible. The deflections were 
fairly proportional to the current, indicating that with a good optical system 
virtual currents as small as 10~^ ampere should have been perceptible. The 
apparatus is thus very well worth further development and would be particu- 
larly useful where alternators with a definite period are in question. 
The method of observation consists in gradually increasing the tension of 
the wire from a slightly low value, to beyond the maximum tension. In this 
case the band widens from a relatively small width to the maximum and then 
abruptly falls off to a small value. To repeat the observation the tension must 
often again be reduced to the low value and the whole operation repeated; but 
after some practice maximum may be reached in the reverse direction unless 
the band has become too narrow. 
When the current is broken and thereafter closed, a low width of band is 
obtained which will not widen unless the operation described is carried out from 
low tension. In other words there are often two cases of equilibrium for each 
