332 PHYSICS: C. BARUS 
ciable when average currents within the order of milliamperes are treated. 
As soon, however, as approximate synchronism is established, the sensitive- 
ness of the apparatus increases enormously. It is best for this purpose to vary 
the period of the motor of the alternate current generator by the slide rheostat 
If the fringes are horizontal and the objector therefore vibrating horizontally 
across the vertical slit image, the motion of he fringes is vertical. Hence the 
horizontal band of fringes in the absence of current, at once takes the form of 
a given Lissajous succession, with oppositions of rotations quite visible. 
The continuous change of these may be most conveniently accelerated or re- 
tarded by controlling the motor of the alternator with a slide rheostat. It is 
annoying if they leave the field while executing their gyrations, though they 
may always be restored by moving the micrometer. For mean tensions, the 
higher vibration curves 2 : 3,3 : 4 may be obtained, both for the alternator 
moving at smaller and at larger periods than the vibrating mirror. To obtain 
them the motor running at maximum speed is gradually slowed down by means 
of the rheostat, when the forms appear in succession passing through the 
elliptic series at mean speeds. 
The Lissajous curves continue to be very marked when additional resistances 
as high as 10,000 ohms are put into the circuit of the alternator. To obtain 
some idea of the smallest average current appreciable the Siemens dynamo- 
meter was put in circuit. Estimating the average current as i = Cy/i where 
C is the dynamometer constant and <£> the deflection in centimeters, the current 
when 10,000 ohms is inserted and the magneto reduced in speed to T = .25 
second was found to be of the average value of i = 6 X 10" 6 amperes. 
In a later and more refined adjustment the ellipses obtained with an inser- 
tion 10,000 ohms filled (as to their vertical or current axes) fully one-quarter 
of the field of the telescope. As little as one-tenth to one-hundredth of this 
would be easily appreciable with certainty, so that the minimum average cur- 
rent capable of detection may be estimated as well within 10~ 6 ampere. In 
this respect the device was somewhat disappointing. It must be remembered 
however that the above mirrors (»') and appurtenances are unnecessarily 
heavy, and the bifilar too robust. 
If one of the telephones is reversed, the fringes which showed marked vibra- 
tion in the first position, frequently ceases to show any vibration until the ellipses 
in the former case are very large (small resistance in circuit) . The results in such 
a case are uniformly consistent. The reason of this is apparent: For when the 
telephones are joined in series the lever mm' , figure 3, is periodically rotated and 
released around a vertical axis and the displacement of fringes is proportional 
to the small angular amplitude of rotation. If, however, the telephones are 
connected differentially, the mirror mm', if properly adjusted, merely moves 
parallel to itself, fore and aft and the fringes remain stationary. More usually, 
however, there is a difference in the size of ellipses in the two cases. 
It is for the investigation of this question that the adjustment pushing screws 
IV and springs ss' (fig. 4) pulling toward the rear, the telephones being on a 
